How we check our numbers

Every calculator here turns on tax figures that change each year and are easy to get wrong. This page says exactly where ours come from, how often they are re-checked, what an independent audit found when it went looking — and what we have not checked at all.

Figures last checked against the documents that publish them: 2026-09-12.

One file, not fifteen

Every tax figure this site uses — rates, thresholds, dollar limits, bracket tables — lives in a single file that all 15 calculators read from. None of them contains a number of its own.

That sounds like housekeeping. It is not. When a calculator keeps its own copy of a threshold, next year's update moves the shared file and leaves the copy behind, and the tool goes on returning a confident, wrong answer with nothing to signal it. This site shipped exactly that bug once: a retirement contribution limit stayed at the previous year's figure because one tool had its own copy. An automated check now fails the build if any calculator starts carrying its own number again.

Checked against the source, twice over

A calculator agreeing with our figures file proves only that the two agree. It says nothing about whether the file is right. So the file is checked in the other direction, against the documents that actually publish each figure:

We keep the publications themselves, as text, rather than fetching them when needed. It means a claim can be checked against the words as they were on the day it was made, and it means an IRS revision shows up as a visible change rather than quietly moving under us.

An audit that was allowed to disagree with us

A correct number used in the wrong formula is still a wrong answer, and no amount of checking figures catches that. In September 2026 all 15 calculators were audited against IRS publications by sessions working only from those publications, with no access to the code — so that agreement would mean something.

It found 13 defects. All are fixed. Some mattered a great deal:

• Self-employment tax was charging Social Security twice on income already taxed for it — $9,300 a year too much for someone with both a salary and a business.
• Five of the six head-of-household bracket breakpoints were wrong, taxing part of that income at 22% when it belonged at 12%.
• The qualified business income deduction cut off at a cliff instead of tapering: one extra dollar of income could cost $9,842.

One pattern accounted for a third of everything found. Four separate calculators multiplied income by 15.3% and stopped — no 92.35% factor, no wage base, no deductible half. Every one of them passed every test, because each test had been written from the same misunderstanding as the tool it was checking. That is the argument for an audit that is not allowed to look at the code.

When we get something wrong

We fix it, and we say so. The audit above was published in full, including the finding where the audit corrected an earlier fix from the same audit — the first attempt at the depreciation recapture rule was closer but still wrong, and the usual one-line summary of that rule understates the tax by 46%.

If a figure here looks wrong to you, tell us. A correction from a reader who actually files this return is worth more than another automated check.

The documents we check against

These are the publications behind the figures on this site. Each article also lists the ones it relies on, at the bottom of the page.

• Self-Employment Tax (Social Security and Medicare Taxes) IRS — The 15.3% rate, its 12.4% and 2.9% halves, and who owes it.
• Topic no. 554, Self-employment tax IRS — The 92.35% net-earnings factor and the $400 filing threshold.
• Publication 505, Tax Withholding and Estimated Tax IRS — Quarterly payment rules: the $1,000 threshold, the 90% and 100%/110% safe harbours, and the rate schedules.
• Publication 334, Tax Guide for Small Business IRS — Schedule C income and deductions for sole proprietors.
• Publication 535, Business Expenses IRS — What counts as an ordinary and necessary business expense.
• Qualified Business Income Deduction IRS — The section 199A deduction and who can take it.
• Publication 560, Retirement Plans for Small Business IRS — Solo 401(k) and SEP IRA limits, and the circular calculation behind the 20% sole-proprietor rate.
• S Corporation Compensation and Medical Insurance Issues IRS — The reasonable-compensation requirement for S-corp owner-employees.
• About Form 2553, Election by a Small Business Corporation IRS — How and when to make the S-corp election.
• Publication 527, Residential Rental Property IRS — The 27.5-year depreciation life, the 14-day rule, and personal-use day counting for short-term rentals.
• Publication 925, Passive Activity and At-Risk Rules IRS — The $25,000 special allowance, its phase-out between $100,000 and $150,000, and material participation.
• Publication 544, Sales and Other Dispositions of Assets IRS — Gain on sale, depreciation recapture, and the section 121 exclusion.
• Publication 523, Selling Your Home IRS — The $250,000 / $500,000 exclusion and the ownership and use tests.
• Topic no. 409, Capital gains and losses IRS — Long-term rates and the 25% ceiling on unrecaptured section 1250 gain.
• Topic no. 701, Sale of your home IRS — The headline rules for excluding gain on a main home.
• Net Investment Income Tax IRS — The 3.8% rate and the $200,000 / $250,000 thresholds.
• Publication 15, (Circular E), Employer's Tax Guide IRS — Payroll withholding, including Additional Medicare Tax.
• IRS releases tax inflation adjustments for tax year 2026 IRS — Standard deductions and the 2026 rate schedules.
• Cost-of-Living Increase and Other Determinations for 2026 (full text) Social Security Administration, via the Federal Register — The Social Security wage base, the PIA bend points, and the substantial gainful activity amounts behind the SSDI figures.