How we check the math
Retirement calculators often disagree, and you can’t trust a number you can’t check. Below are worked examples from IRS and SSA rules that you can verify by hand, with what our engine gets for each. They run as automated tests on every change to the code, and the code itself is open source.
Checked against IRS and SSA rules
15 of 15 match exactly, using 2026 figures (checked November 3, 2026). See the test · See every case and its working
$150,000 − $32,200 standard deduction = $117,800 taxable. 10% × $24,800 + 12% × $76,000 + 22% × $17,000 = $2,480 + $9,120 + $3,740.
IRS Rev. Proc. 2025-32 (2026 brackets and standard deduction)$250,000 − $16,100 = $233,900 taxable. 10% × $12,400 + 12% × $38,000 + 22% × $55,300 + 24% × $96,075 + 32% × $32,125.
IRS Rev. Proc. 2025-32 (2026 brackets and standard deduction)Provisional income = $30,000 + half of $36,000 = $48,000. Taxable = 85% × ($48,000 − $34,000) + the lesser of $4,500 or half the benefit = $11,900 + $4,500.
IRS Pub. 915 — Social Security benefits worksheetIncome $30,000 + $16,400 taxable Social Security = $46,400. Deductions $16,100 + $2,050 (age 65+) + $6,000 senior deduction = $24,150. Taxable $22,250: 10% × $12,400 + 12% × $9,850.
IRS Rev. Proc. 2025-32 (2026 brackets and standard deduction)60 months early: 36 × 5/9 of 1% + 24 × 5/12 of 1% = 30% reduction.
SSA — Benefit reduction for early retirement36 months early × 5/9 of 1% = 20% reduction.
SSA — Benefit reduction for early retirement36 months of delayed credits × 2/3 of 1% = 24% increase.
SSA — Delayed retirement credits36 × 25/36 of 1% + 24 × 5/12 of 1% = 35% reduction, i.e. 32.5% of the worker’s benefit.
SSA — Benefit reduction for early retirement$500,000 ÷ 26.5 (Uniform Lifetime Table, age 73).
IRS Pub. 590-B — Uniform Lifetime Table$500,000 ÷ 16.0 (Uniform Lifetime Table, age 85).
IRS Pub. 590-B — Uniform Lifetime TableThe $16,100 standard deduction absorbs all wages and $6,100 of gains, leaving $53,900 taxable. Gains fill the 0% bracket to $49,450; the remaining $4,450 is taxed at 15% = $667.50.
IRS Rev. Proc. 2025-32 (2026 brackets and standard deduction)MAGI $280,000 is $80,000 over the (never-indexed) $200,000 threshold. NIIT = 3.8% x the lesser of $50,000 investment income or the $80,000 excess = 3.8% x $50,000.
IRS Topic 559 - Net investment income taxSSA: "$500,000 or more" is the top tier, so exactly $500,000 pays the full $487.00/month surcharge = $5,844 a year.
SSA POMS HI 01101.031 - 2026 IRMAA tablesProvisional income = $50,000 + half of $40,000 = $70,000, above the $44,000 upper threshold. Taxable = the lesser of 85% of the benefit ($34,000) or 85% x ($70,000 - $44,000) + the lesser of $6,000 (half the tier width) or $20,000 (half the benefit) = $22,100 + $6,000 = $28,100.
IRS Pub. 915 — Social Security benefits worksheetDeductions: $32,200 standard + $1,650 x 2 (65+) + the $12,000 senior deduction reduced by 6% per spouse of the $40,000 MAGI over $150,000 ($12,000 - $4,800 = $7,200) = $42,700. Taxable $147,300: 10% x $24,800 + 12% x $76,000 + 22% x $46,500.
OBBBA sec. 70103 - senior deductionWhy our answer may differ from another tool
If another calculator gives you a different answer, it’s usually one of these choices. Neither has to be wrong — knowing which assumption differs tells you what to look at.
- Taxes, every year
- We work out each year’s federal and state tax, how much Social Security is taxed and required withdrawals, and charge Medicare surcharges (IRMAA) from your income two years earlier, the way Medicare does. Many calculators use one flat tax rate, or none, which usually makes plans look better than they are.
- What your return means
- We treat your expected return as a long-run growth rate, so a steady 7% line is the middle outcome. Tools that treat 7% as the average of yearly returns show lower odds for the same number, because ups and downs drag compound growth below the yearly average.
- How much markets swing
- Your plan stores only an expected return, so we read the size of yearly swings off typical stock/bond mixes: 7% is treated like a roughly 50/50 mix swinging about 9% a year. A tool that assumes an all-stock portfolio shows a much wider range.
- Healthcare and long-term care
- Healthcare is included and inflates faster than other prices, and long-term care is added in the last years of the plan ($3,600/month in today’s dollars by default — you can change or remove it under Expenses). Many tools leave long-term care out.
- Spending over time
- Retirement spending follows go-go, slow-go and no-go phases (100%, 85%, 70% of discretionary spending). Tools with flat inflation-adjusted spending show more spending late in life.
- What counts as failing
- A market counts as failing if any year runs short of money — not only if the balance ends at $0.
- Real history
- Our history check replays every real run of years since 1928 in its true order, but centered on your expected return. Tools such as FIRECalc and cFIREsim use the actual returns, which averaged more than 7% for most periods, so they often look kinder.
- Same answer every time
- 500 markets with a fixed random seed: the same plan always gets the same percentage. Sampling alone moves the result by about ±3 points; your return, spending and retirement-age assumptions move it far more.
Every assumption and data source is listed on How the math works. Found a case where the engine is wrong? Open an issue with the inputs and the answer you expected.
Educational planning estimates, not financial advice.