How accurate is Retire Plan?

Last checked September 2026, against 2026 tax law.

A retirement planner is only worth using if its arithmetic is right and its rules match the law. This page is the record of how I check both. It shows what our planner was tested against, works a few of the sums out by hand, and says plainly what our planner still leaves out.

How it is checked

Most of the checking is against work that other people published.

• Official worked examples. The IRS, the Social Security Administration and state tax departments publish examples that give the inputs and the answer. I ran our planner on 55 of them.
• An outside tax calculator. PolicyEngine¹ is an open-source tax and benefits model built with no connection to this project. I ran 122 households through it and through Retire Plan.
• Published research and another planner. The Trinity study² tables, and cFIREsim³, a planner many people in the FIRE community use, whose open-source code I ran on the same plans.
• Independent market data. Retire Plan’s returns are compared year by year with Aswath Damodaran’s series⁴.

On top of those, one check of my own: a second calculator I wrote from the tax law that shares no code with Retire Plan.

The review before release

Before releasing this version of the engine I ran an initial review of it, rule by rule, against the primary sources. The arithmetic held up: the brackets, the distribution tables and the Social Security formula already matched the government figures. What it found were rules that were missing or squeezed into a single number where the law uses a formula. State tax used a top rate where states use brackets. Social Security was always taxed at the maximum share. Capital gains paid one flat rate. The full plan left payroll tax off pay. Couples shared limits the law gives each person.⁵ A few edge cases, like the year a plan runs out and the year someone turns 59, were counted the wrong way.

Those issues are resolved, and every fix is covered by the checks on this page. The things Retire Plan still does not model are listed at the bottom.

Official worked examples

Of the 55 published examples, 44 match Retire Plan exactly. Five do not apply because they cover things Retire Plan has no input for, such as married filing separately and self-employment income. Six differ, and none of them is an error in Retire Plan. Three are the publisher rounding a figure. Two are examples that skip a step in their own worksheet; Retire Plan follows the worksheet. One is the Social Security earnings test in the year someone reaches full retirement age, where Retire Plan spreads pay evenly across the year because it does not know the birth month. Given the month, it matches that example too.⁶

Examples from 2025 editions are run with that year’s published amounts where a figure is indexed. Rules fixed in law, like the Social Security thresholds and the investment income tax, are compared directly. The state examples also include California’s tax table, Virginia’s rate schedule and Oregon’s.^20,21,22 The full list of 55 is kept with Retire Plan’s code.

Checked against PolicyEngine

I ran 122 households through PolicyEngine¹ and through Retire Plan for tax year 2026. They cover single and married filers, ages 45 to 72, Social Security that crosses both taxable thresholds, gains in each of the 0%, 15% and 20% bands, incomes across the senior deduction phase-out, investment income over the 3.8% tax threshold, working couples for payroll tax, and retirees and workers in 17 states.

The one federal difference is a household with $600,000 of capital gains in a single year, where the alternative minimum tax applies. Retire Plan does not model it yet, and it is on the list below.

The state differences mostly come from things PolicyEngine includes and Retire Plan leaves out, such as Oregon’s deduction for federal tax paid and Colorado’s refundable credits, or from 2026 amounts that PolicyEngine had not updated yet. In the four largest, New York, Illinois and Michigan retirees with IRA withdrawals, PolicyEngine gives the retirement exclusion only to pensions. The state laws cover IRA withdrawals too, and Retire Plan follows them. One is a real gap: a Virginia adjustment for married couples worth up to $259, which Retire Plan is missing.

Against the research and another planner

The best-known test of withdrawal rates is the Trinity study² (Cooley, Hubbard and Walz, 1998). Its Table 3 counts the 30-year periods from 1926 to 1995 that never ran out, with the withdrawal raised each year for inflation. That is 41 cases, the ones starting 1926 to 1966, and Retire Plan runs the same 41 below.

Percent of cases that never ran out. Trinity rounds to whole percents. The one 4% failure at 75/25 in Retire Plan started in 1966. At the rates people actually plan with, 3% and 4%, the two agree. The gap opens at 5% and above with more bonds, because Trinity held long-term corporate bonds and Retire Plan holds 10-year Treasuries, and corporate bonds paid more over those decades.

cFIREsim³ is a backtesting planner many people in the FIRE community use. Its open-source version’s own code ran unchanged on the same plans, over every case its data allows, the ones starting 1871 to 1985.

Retire Plan comes out a little higher every time, and the whole gap has two causes. cFIREsim takes the year’s spending on January 1, and Retire Plan draws it monthly the way most retirees are paid, so on average more of the money stays invested longer. cFIREsim also adds stock dividends once a year where Retire Plan reinvests them monthly. With the timing matched, a separate model reproduces cFIREsim’s results case for case, and no case fails in our planner that survives in cFIREsim.

Stock returns come from Robert Shiller’s monthly series²³, which starts in 1871, and Retire Plan’s yearly returns are rebuilt from his raw file to within a millionth. Bond returns come from the 10-year Treasury yield, buying the bond in January and selling it a year later. The stock and bond series correlate 0.97 and 0.96 with Aswath Damodaran’s independent series⁴ over 1928 to 2025.

Five sums done by hand

Each of these is a single year, worked by hand from the published tables, next to what Retire Plan gives for the same inputs.

Federal tax on a retiree with Social Security

A single person aged 67, with $30,000 of Social Security and a $40,000 IRA withdrawal.

Other income plus half the Social Security

$55,000

Taxable Social Security, from the IRS worksheet

$22,350

Adjusted gross income

$62,350

Standard deduction, extra amount at 65, senior deduction

−$24,150

Taxable income

$38,200

10% of the first $12,400, 12% of the next $25,800

$1,240 + $3,096

Federal tax, by hand

$4,336

Federal tax, Retire Plan

$4,336

Federal tax, PolicyEngine

$4,336

The worksheet taxes up to 85% of the benefit, and less at lower incomes: 85% of the $21,000 above $34,000, plus $4,500 from the band between $25,000 and $34,000. The deductions are $16,100, $2,050 and $6,000.^7,24,25

Capital gains inside the 0% band

A single person aged 50 who sells $40,000 of long-term gains from a brokerage account and has no other income.

Long-term gains

$40,000

Standard deduction

−$16,100

Taxable income, all of it gains

$23,900

Top of the 0% gains band for a single filer

$49,450

Federal tax, by hand

$0

Federal tax, Retire Plan

$0

Gains sit on top of ordinary income and are taxed at 0%, 15% or 20% depending on where they land.²⁴

California tax on an IRA withdrawal

A single person aged 65 in California who takes $50,000 out of a traditional IRA.

Withdrawal

$50,000

California standard deduction

−$5,706

Taxable income

$44,294

1%, 2%, 4% and 6% brackets

$1,192.53

Personal and senior exemption credits

−$306

State tax, by hand

$886.53

State tax, Retire Plan

$886.53

California has not published its 2026 schedules yet, so this uses the 2025 ones.¹⁷ Retire Plan carries a table like this for every state with an income tax, each citing that state’s own tax department.

A required minimum distribution at 75

A $1,000,000 IRA on December 31, and an owner who turns 75 this year.

Divisor from the Uniform Lifetime Table

24.6

$1,000,000 ÷ 24.6, by hand and in Retire Plan

$40,650

Divisor if the spouse is 60 (Joint Life table)

28.3

$1,000,000 ÷ 28.3, by hand and in Retire Plan

$35,336

The joint table applies when a spouse is the only beneficiary and more than ten years younger. All 49 rows of the uniform table were checked against the publication.⁸

Payroll tax for a couple

A married couple who each earn $150,000.

Social Security, 6.2% of each person’s pay up to $184,500

$18,600

Medicare, 1.45% of all pay

$4,350

Additional Medicare, 0.9% of joint pay over $250,000

$450

Payroll tax, by hand and in Retire Plan

$23,400

The wage cap applies to each person separately, which matters for couples with two high earners.^26,11

Whole plans in a second calculator

A single year can be right while a 30-year plan goes wrong, so I also run whole plans through a second calculator. I wrote it from the tax law and the IRS tables, not from Retire Plan’s code, and it imports nothing from Retire Plan. Its tax rules are the same ones checked against the official examples and PolicyEngine above.

Both run six households year by year from 2026. Each grows at a steady 4% a year after inflation, with inflation set to zero, so every year uses 2026 tax law and nothing about markets can differ. Money moves in and out monthly in both, and the year someone turns 59 carries the early withdrawal penalty on half the year, since the plan does not know the birth month. The totals below are summed over every year of the plan.

Five of the six agree to the dollar over the whole plan. The sixth differs because of a real gap: when one spouse dies, the survivor files as single and keeps only the larger Social Security check. Retire Plan does not model that yet, so it keeps both checks and the married brackets, and it ends about $429,000 too high.

What it still leaves out

Any of these can matter more to your plan than the things our planner does model.

• Healthcare costs before Medicare, Medicare premiums and the high-income surcharge, and long-term care.
• The years after a spouse dies: single filing and the survivor keeping one Social Security check.
• Roth conversions and 72(t) payments, the two usual ways to reach traditional money before 59 and a half.
• The alternative minimum tax, which mostly matters in a year with very large capital gains.
• Tax each year on dividends and interest inside a brokerage account.
• Health savings accounts, 457(b) plans and the 403(b) 15-year catch-up.
• City and county income taxes, and a handful of smaller state credits and deductions.

References

1. PolicyEngine, open-source tax and benefits model for the US (source code), run for tax year 2026 in September 2026.
2. Cooley, Hubbard and Walz, “Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable”, AAII Journal, February 1998, Table 3.
3. cFIREsim, open-source version, run from its own code and data.
4. Aswath Damodaran, historical returns on stocks, bonds and bills.
5. IRS Notice 2025-67, 2026 401K and IRA limits, catch-ups and phase-outs.
6. Social Security Administration, receiving benefits while working.
7. IRS Publication 915, Social Security and Equivalent Railroad Retirement Benefits (2025).
8. IRS Publication 590-B, Distributions from Individual Retirement Arrangements (2025).
9. IRS Publication 590-A, Contributions to Individual Retirement Arrangements (2025).
10. IRS, Questions and Answers on the Net Investment Income Tax.
11. IRS, Instructions for Form 8959, Additional Medicare Tax (2025).
12. IRS Publication 550, Investment Income and Expenses (2025).
13. Social Security Administration, benefit examples for workers with maximum-taxable earnings.
14. Social Security Administration, the primary insurance amount formula and bend points.
15. Social Security Administration, early retirement reduction, born 1960 or later.
16. Social Security Administration, delayed retirement credits, born 1960 or later.
17. California Franchise Tax Board, 2025 tax rate schedules.
18. New York Department of Taxation and Finance, information for seniors.
19. New Jersey Division of Taxation, NJ-1040 instructions and tax table (2025).
20. California Franchise Tax Board, 2025 Form 540 booklet and tax table.
21. Virginia Tax, 2026 Form 760ES rate schedule.
22. Oregon Legislative Revenue Office, Basic Facts 2026. Every other state is cited in Retire Plan’s tax table.
23. Robert Shiller, market data.
24. IRS Revenue Procedure 2025-32, 2026 brackets, standard deduction and capital gains bands.
25. IRS, Tax deductions for working Americans and seniors, the 2025 to 2028 senior deduction.
26. Social Security Administration, 2026 fact sheet: wage base, earnings test, bend points.

If a number looks wrong

Write to support@retire-plan.app with the plan and the number you were looking at. If you work the sum a different way and get a different answer, I want to see it. I am not a finance professional, Retire Plan shows projections from historical data, and you should talk to a fiduciary before you do anything drastic with your money.