Trust
How we verify Runway's numbers
A retirement plan is only useful if the arithmetic under it is right. Runway's planning engine does a lot of it: federal tax, the taxable share of Social Security, required minimum distributions, Medicare surcharges and benefit claiming. You shouldn't have to take our word that it's correct, so this page shows the checking.
Quick answer. 157 automated checks across 10 areas compare the engine's answers with the
official published numbers (IRS, CMS, the Social Security Act and the federal regulations). All 157 match as of
October 5, 2026. The checks are part of Runway's test suite, which is run before changes are released, and this page is generated from them. Below you can see
every check, its source, the official number and what Runway produced. After the results is a plain list of what this
does not prove.
How the checks work
Each expected number comes from a primary source: the IRS revenue procedure with the 2026 inflation adjustments, IRS Publication 915, the Treasury regulation that holds the Uniform Lifetime Table, the CMS 2026 Medicare fact sheet and the statute that sets Social Security's early-claiming reductions and delayed credits. We type in the published figures (or read them from the source file), then work out the official answer with a short separate calculation, such as "$11,600 plus 22% of the excess over $100,800", that shares no code with the engine. The engine is then asked the same question. The two must agree to the cent. No expected value is ever copied from what the engine says.
The checks are chosen to catch the mistakes that actually happen: values at the edge of a bracket or threshold, the dollar on either side of it, and the unusual rows at the ends of a table. This approach already found two real errors before this page was published. The engine's age-116-to-120 divisors in the RMD table were missing, and the highest Medicare surcharge tier began one dollar late at the $500,000 and $750,000 lines. Both are fixed and are now permanent checks.
Federal income tax rates (2026)Standard deductionLong-term capital gains rates (0% / 15% / 20%)Net Investment Income TaxTaxable Social Security (Pub. 915)Required minimum distributionsMedicare Part B premiumMedicare IRMAA surcharge (2026 tiers)Social Security claiming-age adjustmentsConsistency (the engine against itself)
Federal income tax rates (2026) 25 checks
The official rate tables for married filing jointly and single filers, tested at the bottom, top and every bracket edge.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Married filing jointly: tax on $10,000 of taxable income | IRS Rev. Proc. 2025-32 | $1,000 | $1,000 | ✓ |
| Married filing jointly: tax on $24,800 of taxable income | IRS Rev. Proc. 2025-32 | $2,480 | $2,480 | ✓ |
| Married filing jointly: tax on $24,801 of taxable income | IRS Rev. Proc. 2025-32 | $2,480.12 | $2,480.12 | ✓ |
| Married filing jointly: tax on $60,000 of taxable income | IRS Rev. Proc. 2025-32 | $6,704 | $6,704 | ✓ |
| Married filing jointly: tax on $100,800 of taxable income | IRS Rev. Proc. 2025-32 | $11,600 | $11,600 | ✓ |
| Married filing jointly: tax on $150,000 of taxable income | IRS Rev. Proc. 2025-32 | $22,424 | $22,424 | ✓ |
| Married filing jointly: tax on $211,400 of taxable income | IRS Rev. Proc. 2025-32 | $35,932 | $35,932 | ✓ |
| Married filing jointly: tax on $300,000 of taxable income | IRS Rev. Proc. 2025-32 | $57,196 | $57,196 | ✓ |
| Married filing jointly: tax on $403,550 of taxable income | IRS Rev. Proc. 2025-32 | $82,048 | $82,048 | ✓ |
| Married filing jointly: tax on $500,000 of taxable income | IRS Rev. Proc. 2025-32 | $112,912 | $112,912 | ✓ |
| Married filing jointly: tax on $768,700 of taxable income | IRS Rev. Proc. 2025-32 | $206,583.50 | $206,583.50 | ✓ |
| Married filing jointly: tax on $1,000,000 of taxable income | IRS Rev. Proc. 2025-32 | $292,164.50 | $292,164.50 | ✓ |
| Single: tax on $8,000 of taxable income | IRS Rev. Proc. 2025-32 | $800 | $800 | ✓ |
| Single: tax on $12,400 of taxable income | IRS Rev. Proc. 2025-32 | $1,240 | $1,240 | ✓ |
| Single: tax on $30,000 of taxable income | IRS Rev. Proc. 2025-32 | $3,352 | $3,352 | ✓ |
| Single: tax on $50,400 of taxable income | IRS Rev. Proc. 2025-32 | $5,800 | $5,800 | ✓ |
| Single: tax on $80,000 of taxable income | IRS Rev. Proc. 2025-32 | $12,312 | $12,312 | ✓ |
| Single: tax on $105,700 of taxable income | IRS Rev. Proc. 2025-32 | $17,966 | $17,966 | ✓ |
| Single: tax on $150,000 of taxable income | IRS Rev. Proc. 2025-32 | $28,598 | $28,598 | ✓ |
| Single: tax on $201,775 of taxable income | IRS Rev. Proc. 2025-32 | $41,024 | $41,024 | ✓ |
| Single: tax on $250,000 of taxable income | IRS Rev. Proc. 2025-32 | $56,456 | $56,456 | ✓ |
| Single: tax on $256,225 of taxable income | IRS Rev. Proc. 2025-32 | $58,448 | $58,448 | ✓ |
| Single: tax on $400,000 of taxable income | IRS Rev. Proc. 2025-32 | $108,769.25 | $108,769.25 | ✓ |
| Single: tax on $640,600 of taxable income | IRS Rev. Proc. 2025-32 | $192,979.25 | $192,979.25 | ✓ |
| Single: tax on $900,000 of taxable income | IRS Rev. Proc. 2025-32 | $288,957.25 | $288,957.25 | ✓ |
Standard deduction 5 checks
The 2026 standard deduction, including the extra amount for age 65 and over.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Single, under 65: tax on $60,000 of income | IRS Rev. Proc. 2025-32 | $5,020 | $5,020 | ✓ |
| Single, 65 or older: tax on $50,000 of income | IRS Rev. Proc. 2025-32 | $3,574 | $3,574 | ✓ |
| Married filing jointly, both under 65: tax on $100,000 of income | IRS Rev. Proc. 2025-32 | $7,640 | $7,640 | ✓ |
| Married filing jointly, one 65 or older: tax on $100,000 of income | IRS Rev. Proc. 2025-32 | $7,442 | $7,442 | ✓ |
| Married filing jointly, both 65 or older: tax on $100,000 of income | IRS Rev. Proc. 2025-32 | $7,244 | $7,244 | ✓ |
Long-term capital gains rates (0% / 15% / 20%) 8 checks
Gains are stacked on top of ordinary income, so the same gain can be taxed at 0%, 15% and 20% in one year.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Married filing jointly: $70,000 of gains on top of $20,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $0 | $0 | ✓ |
| Married filing jointly: $60,000 of gains on top of $50,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $1,665 | $1,665 | ✓ |
| Married filing jointly: $100,000 of gains on top of $500,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $15,000 | $15,000 | ✓ |
| Married filing jointly: $50,000 of gains on top of $600,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $9,315 | $9,315 | ✓ |
| Single: $40,000 of gains on top of $30,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $3,082.50 | $3,082.50 | ✓ |
| Single: $30,000 of gains on top of $10,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $0 | $0 | ✓ |
| Single: $60,000 of gains on top of $520,000 taxable ordinary income | IRS Rev. Proc. 2025-32 | $10,725 | $10,725 | ✓ |
| Single: $49,450 of gains on top of $0 taxable ordinary income | IRS Rev. Proc. 2025-32 | $0 | $0 | ✓ |
Net Investment Income Tax 5 checks
The 3.8% surtax that applies once income passes $200,000 (single) or $250,000 (married filing jointly).
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Married filing jointly: $200,000 ordinary income + $100,000 gains | IRS Topic 559 | $1,900 | $1,900 | ✓ |
| Single: $150,000 ordinary income + $100,000 gains | IRS Topic 559 | $1,900 | $1,900 | ✓ |
| Married filing jointly: $150,000 ordinary income + $50,000 gains | IRS Topic 559 | $0 | $0 | ✓ |
| Single: $100,000 ordinary income + $120,000 gains | IRS Topic 559 | $760 | $760 | ✓ |
| Single: $300,000 ordinary income + $40,000 gains | IRS Topic 559 | $1,520 | $1,520 | ✓ |
Taxable Social Security (Pub. 915) 10 checks
Includes the worked examples in IRS Publication 915 and the worksheet's 50% and 85% tiers.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Pub. 915 Example 1 (single): $5,980 of benefits, $28,990 other income → $2,990 taxable | IRS Publication 915 | $2,990 | $2,990 | ✓ |
| Pub. 915 Example 2 (joint): $5,600 of benefits, $29,750 other income, $1,000 adjustment → $0 taxable | IRS Publication 915 | $0 | $0 | ✓ |
| Single: $24,000 of benefits, $30,000 other income (Worksheet 1) | IRS Publication 915 | $11,300 | $11,300 | ✓ |
| Single: $24,000 of benefits, $18,000 other income (Worksheet 1) | IRS Publication 915 | $2,500 | $2,500 | ✓ |
| Single: $36,000 of benefits, $60,000 other income (Worksheet 1) | IRS Publication 915 | $30,600 | $30,600 | ✓ |
| Single: $30,000 of benefits, $5,000 other income (Worksheet 1) | IRS Publication 915 | $0 | $0 | ✓ |
| Married filing jointly: $40,000 of benefits, $40,000 other income (Worksheet 1) | IRS Publication 915 | $19,600 | $19,600 | ✓ |
| Married filing jointly: $40,000 of benefits, $20,000 other income (Worksheet 1) | IRS Publication 915 | $4,000 | $4,000 | ✓ |
| Married filing jointly: $50,000 of benefits, $90,000 other income (Worksheet 1) | IRS Publication 915 | $42,500 | $42,500 | ✓ |
| Married filing jointly: $20,000 of benefits, $60,000 other income (Worksheet 1) | IRS Publication 915 | $17,000 | $17,000 | ✓ |
Required minimum distributions 59 checks
Every divisor in the IRS Uniform Lifetime Table from age 72 to 120, read from the regulation, plus the start age by birth year.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Uniform Lifetime Table divisor at age 72 | 26 CFR § 1.401(a)(9)-9 | 27.4 | 27.4 | ✓ |
| Uniform Lifetime Table divisor at age 73 | 26 CFR § 1.401(a)(9)-9 | 26.5 | 26.5 | ✓ |
| Uniform Lifetime Table divisor at age 74 | 26 CFR § 1.401(a)(9)-9 | 25.5 | 25.5 | ✓ |
| Uniform Lifetime Table divisor at age 75 | 26 CFR § 1.401(a)(9)-9 | 24.6 | 24.6 | ✓ |
| Uniform Lifetime Table divisor at age 76 | 26 CFR § 1.401(a)(9)-9 | 23.7 | 23.7 | ✓ |
| Uniform Lifetime Table divisor at age 77 | 26 CFR § 1.401(a)(9)-9 | 22.9 | 22.9 | ✓ |
| Uniform Lifetime Table divisor at age 78 | 26 CFR § 1.401(a)(9)-9 | 22.0 | 22.0 | ✓ |
| Uniform Lifetime Table divisor at age 79 | 26 CFR § 1.401(a)(9)-9 | 21.1 | 21.1 | ✓ |
| Uniform Lifetime Table divisor at age 80 | 26 CFR § 1.401(a)(9)-9 | 20.2 | 20.2 | ✓ |
| Uniform Lifetime Table divisor at age 81 | 26 CFR § 1.401(a)(9)-9 | 19.4 | 19.4 | ✓ |
| Uniform Lifetime Table divisor at age 82 | 26 CFR § 1.401(a)(9)-9 | 18.5 | 18.5 | ✓ |
| Uniform Lifetime Table divisor at age 83 | 26 CFR § 1.401(a)(9)-9 | 17.7 | 17.7 | ✓ |
| Uniform Lifetime Table divisor at age 84 | 26 CFR § 1.401(a)(9)-9 | 16.8 | 16.8 | ✓ |
| Uniform Lifetime Table divisor at age 85 | 26 CFR § 1.401(a)(9)-9 | 16.0 | 16.0 | ✓ |
| Uniform Lifetime Table divisor at age 86 | 26 CFR § 1.401(a)(9)-9 | 15.2 | 15.2 | ✓ |
| Uniform Lifetime Table divisor at age 87 | 26 CFR § 1.401(a)(9)-9 | 14.4 | 14.4 | ✓ |
| Uniform Lifetime Table divisor at age 88 | 26 CFR § 1.401(a)(9)-9 | 13.7 | 13.7 | ✓ |
| Uniform Lifetime Table divisor at age 89 | 26 CFR § 1.401(a)(9)-9 | 12.9 | 12.9 | ✓ |
| Uniform Lifetime Table divisor at age 90 | 26 CFR § 1.401(a)(9)-9 | 12.2 | 12.2 | ✓ |
| Uniform Lifetime Table divisor at age 91 | 26 CFR § 1.401(a)(9)-9 | 11.5 | 11.5 | ✓ |
| Uniform Lifetime Table divisor at age 92 | 26 CFR § 1.401(a)(9)-9 | 10.8 | 10.8 | ✓ |
| Uniform Lifetime Table divisor at age 93 | 26 CFR § 1.401(a)(9)-9 | 10.1 | 10.1 | ✓ |
| Uniform Lifetime Table divisor at age 94 | 26 CFR § 1.401(a)(9)-9 | 9.5 | 9.5 | ✓ |
| Uniform Lifetime Table divisor at age 95 | 26 CFR § 1.401(a)(9)-9 | 8.9 | 8.9 | ✓ |
| Uniform Lifetime Table divisor at age 96 | 26 CFR § 1.401(a)(9)-9 | 8.4 | 8.4 | ✓ |
| Uniform Lifetime Table divisor at age 97 | 26 CFR § 1.401(a)(9)-9 | 7.8 | 7.8 | ✓ |
| Uniform Lifetime Table divisor at age 98 | 26 CFR § 1.401(a)(9)-9 | 7.3 | 7.3 | ✓ |
| Uniform Lifetime Table divisor at age 99 | 26 CFR § 1.401(a)(9)-9 | 6.8 | 6.8 | ✓ |
| Uniform Lifetime Table divisor at age 100 | 26 CFR § 1.401(a)(9)-9 | 6.4 | 6.4 | ✓ |
| Uniform Lifetime Table divisor at age 101 | 26 CFR § 1.401(a)(9)-9 | 6.0 | 6.0 | ✓ |
| Uniform Lifetime Table divisor at age 102 | 26 CFR § 1.401(a)(9)-9 | 5.6 | 5.6 | ✓ |
| Uniform Lifetime Table divisor at age 103 | 26 CFR § 1.401(a)(9)-9 | 5.2 | 5.2 | ✓ |
| Uniform Lifetime Table divisor at age 104 | 26 CFR § 1.401(a)(9)-9 | 4.9 | 4.9 | ✓ |
| Uniform Lifetime Table divisor at age 105 | 26 CFR § 1.401(a)(9)-9 | 4.6 | 4.6 | ✓ |
| Uniform Lifetime Table divisor at age 106 | 26 CFR § 1.401(a)(9)-9 | 4.3 | 4.3 | ✓ |
| Uniform Lifetime Table divisor at age 107 | 26 CFR § 1.401(a)(9)-9 | 4.1 | 4.1 | ✓ |
| Uniform Lifetime Table divisor at age 108 | 26 CFR § 1.401(a)(9)-9 | 3.9 | 3.9 | ✓ |
| Uniform Lifetime Table divisor at age 109 | 26 CFR § 1.401(a)(9)-9 | 3.7 | 3.7 | ✓ |
| Uniform Lifetime Table divisor at age 110 | 26 CFR § 1.401(a)(9)-9 | 3.5 | 3.5 | ✓ |
| Uniform Lifetime Table divisor at age 111 | 26 CFR § 1.401(a)(9)-9 | 3.4 | 3.4 | ✓ |
| Uniform Lifetime Table divisor at age 112 | 26 CFR § 1.401(a)(9)-9 | 3.3 | 3.3 | ✓ |
| Uniform Lifetime Table divisor at age 113 | 26 CFR § 1.401(a)(9)-9 | 3.1 | 3.1 | ✓ |
| Uniform Lifetime Table divisor at age 114 | 26 CFR § 1.401(a)(9)-9 | 3.0 | 3.0 | ✓ |
| Uniform Lifetime Table divisor at age 115 | 26 CFR § 1.401(a)(9)-9 | 2.9 | 2.9 | ✓ |
| Uniform Lifetime Table divisor at age 116 | 26 CFR § 1.401(a)(9)-9 | 2.8 | 2.8 | ✓ |
| Uniform Lifetime Table divisor at age 117 | 26 CFR § 1.401(a)(9)-9 | 2.7 | 2.7 | ✓ |
| Uniform Lifetime Table divisor at age 118 | 26 CFR § 1.401(a)(9)-9 | 2.5 | 2.5 | ✓ |
| Uniform Lifetime Table divisor at age 119 | 26 CFR § 1.401(a)(9)-9 | 2.3 | 2.3 | ✓ |
| Uniform Lifetime Table divisor at age 120 | 26 CFR § 1.401(a)(9)-9 | 2.0 | 2.0 | ✓ |
| RMD on a $1,000,000 traditional IRA at 73 (balance ÷ 26.5) | 26 CFR § 1.401(a)(9)-9 | $37,735.85 | $37,735.85 | ✓ |
| RMD on a $500,000 traditional IRA at 75 (balance ÷ 24.6) | 26 CFR § 1.401(a)(9)-9 | $20,325.20 | $20,325.20 | ✓ |
| RMD on a $750,000 traditional IRA at 80 (balance ÷ 20.2) | 26 CFR § 1.401(a)(9)-9 | $37,128.71 | $37,128.71 | ✓ |
| RMD on a $250,000 traditional IRA at 90 (balance ÷ 12.2) | 26 CFR § 1.401(a)(9)-9 | $20,491.80 | $20,491.80 | ✓ |
| Age RMDs begin for someone born in 1951 | IRS | 73 | 73 | ✓ |
| Age RMDs begin for someone born in 1955 | IRS | 73 | 73 | ✓ |
| Age RMDs begin for someone born in 1959 | IRS | 73 | 73 | ✓ |
| Age RMDs begin for someone born in 1960 | IRS | 75 | 75 | ✓ |
| Age RMDs begin for someone born in 1961 | IRS | 75 | 75 | ✓ |
| Age RMDs begin for someone born in 1975 | IRS | 75 | 75 | ✓ |
Medicare Part B premium 1 checks
The 2026 standard Part B premium.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Standard 2026 Part B premium, $/month × 12 (engine constant) | CMS | $2,434.80 | $2,434.80 | ✓ |
Medicare IRMAA surcharge (2026 tiers) 24 checks
The income-related surcharge on Parts B and D, tested just below, at and just above each threshold.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Single: income of $100,000, annual Part B + Part D surcharge | CMS | $0 | $0 | ✓ |
| Single: income of $109,000, annual Part B + Part D surcharge | CMS | $0 | $0 | ✓ |
| Single: income of $109,001, annual Part B + Part D surcharge | CMS | $1,148.40 | $1,148.40 | ✓ |
| Single: income of $137,000, annual Part B + Part D surcharge | CMS | $1,148.40 | $1,148.40 | ✓ |
| Single: income of $137,001, annual Part B + Part D surcharge | CMS | $2,884.80 | $2,884.80 | ✓ |
| Single: income of $171,000, annual Part B + Part D surcharge | CMS | $2,884.80 | $2,884.80 | ✓ |
| Single: income of $171,001, annual Part B + Part D surcharge | CMS | $4,620 | $4,620 | ✓ |
| Single: income of $205,000, annual Part B + Part D surcharge | CMS | $4,620 | $4,620 | ✓ |
| Single: income of $205,001, annual Part B + Part D surcharge | CMS | $6,355.20 | $6,355.20 | ✓ |
| Single: income of $499,999, annual Part B + Part D surcharge | CMS | $6,355.20 | $6,355.20 | ✓ |
| Single: income of $500,000, annual Part B + Part D surcharge | CMS | $6,936 | $6,936 | ✓ |
| Single: income of $800,000, annual Part B + Part D surcharge | CMS | $6,936 | $6,936 | ✓ |
| Married filing jointly (both on Medicare): income of $200,000, annual Part B + Part D surcharge | CMS | $0 | $0 | ✓ |
| Married filing jointly (both on Medicare): income of $218,000, annual Part B + Part D surcharge | CMS | $0 | $0 | ✓ |
| Married filing jointly (both on Medicare): income of $218,001, annual Part B + Part D surcharge | CMS | $2,296.80 | $2,296.80 | ✓ |
| Married filing jointly (both on Medicare): income of $274,000, annual Part B + Part D surcharge | CMS | $2,296.80 | $2,296.80 | ✓ |
| Married filing jointly (both on Medicare): income of $274,001, annual Part B + Part D surcharge | CMS | $5,769.60 | $5,769.60 | ✓ |
| Married filing jointly (both on Medicare): income of $342,000, annual Part B + Part D surcharge | CMS | $5,769.60 | $5,769.60 | ✓ |
| Married filing jointly (both on Medicare): income of $342,001, annual Part B + Part D surcharge | CMS | $9,240 | $9,240 | ✓ |
| Married filing jointly (both on Medicare): income of $410,000, annual Part B + Part D surcharge | CMS | $9,240 | $9,240 | ✓ |
| Married filing jointly (both on Medicare): income of $410,001, annual Part B + Part D surcharge | CMS | $12,710.40 | $12,710.40 | ✓ |
| Married filing jointly (both on Medicare): income of $749,999, annual Part B + Part D surcharge | CMS | $12,710.40 | $12,710.40 | ✓ |
| Married filing jointly (both on Medicare): income of $750,000, annual Part B + Part D surcharge | CMS | $13,872 | $13,872 | ✓ |
| Married filing jointly (both on Medicare): income of $900,000, annual Part B + Part D surcharge | CMS | $13,872 | $13,872 | ✓ |
Social Security claiming-age adjustments 18 checks
The statutory reduction for claiming early and the credit for waiting, month by month.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| Full retirement age 67: monthly benefit claimed at 62, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 0.7 × | 0.7 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 63, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 0.75 × | 0.75 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 64, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 0.8 × | 0.8 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 65, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 0.8667 × | 0.8667 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 66, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 0.9333 × | 0.9333 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 67, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 1 × | 1 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 68, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 1.08 × | 1.08 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 69, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 1.16 × | 1.16 × | ✓ |
| Full retirement age 67: monthly benefit claimed at 70, as a share of the benefit at 67 | 42 U.S.C. § 402(q) and | 1.24 × | 1.24 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 62, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 0.75 × | 0.75 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 63, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 0.8 × | 0.8 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 64, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 0.8667 × | 0.8667 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 65, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 0.9333 × | 0.9333 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 66, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 1 × | 1 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 67, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 1.08 × | 1.08 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 68, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 1.16 × | 1.16 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 69, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 1.24 × | 1.24 × | ✓ |
| Full retirement age 66: monthly benefit claimed at 70, as a share of the benefit at 66 | 42 U.S.C. § 402(q) and | 1.32 × | 1.32 × | ✓ |
Consistency (the engine against itself) 2 checks
Not an outside source: two separate paths through the engine that must give the same answer.
| What was checked | Official source | Official | Runway | |
|---|---|---|---|---|
| First-year cash flow of the sample couple: withdrawals − tax = spending + premiums | Runway's own engine — two independent code paths compared | $0 | $0 | ✓ |
| Monte Carlo with volatility set to (nearly) zero reproduces the year-by-year projection's ending balance | Runway's own engine — two independent code paths compared | $1,850,006.29 | $1,850,005.86 | ✓ |
What this does not prove
Matching the official tables proves the building blocks are right. It does not prove that every plan is "right", because a plan also depends on assumptions about the future and on simplifications we chose. Here is what to keep in mind.
- Returns are assumptions, not predictions. The chance your money lasts depends on the return, inflation and volatility assumptions you set. No test can verify the future. The historical backtest uses Robert Shiller's public dataset, and its result depends on that data.
- 2026 rules only. Tax law, thresholds and premiums are the 2026 values, held constant in today's dollars. Future law changes are not modeled, including the scheduled end of some current provisions.
- State income tax is a flat rate. If you enter your state's rate, Runway applies it to income. It does not reproduce each state's deductions, exemptions or retirement-income exclusions.
- Life expectancy is modeled, not looked up. Mortality uses a smooth curve fitted to published tables, not the exact Social Security period life table.
- Dividends and interest are not taxed separately. Taxable-account growth is treated as return on the account, and gains are taxed when you sell.
- Health coverage is simplified. Before Medicare the plan adds a flat benchmark premium rather than quoting your local marketplace plans, and Medicare covers Part B and income-related surcharges only. Part D plan premiums, Medigap, deductibles and out-of-pocket costs are yours to add to spending.
- HSA, long-term care and cash reserve are assumptions. The HSA pays only Medicare premiums and long-term care in the plan, long-term care uses a national median cost, and the cash reserve earns 0% above inflation.
- Not yet independently checked. The year-by-year withdrawal order and the Monte Carlo results are tested by Runway's own test suite and by the two consistency checks above, but they have not yet been compared with another planner's output or reviewed by an outside CPA or CFP. We will add that here when it happens.
If you find a number that disagrees with an official source, tell us and we'll add a check for it.
Where this fits with the rest of Runway
Every figure in Runway's blog posts is computed by this same engine, and the posts say how. See the step-by-step tutorial to run your own plan, and the blog for the research behind each result.