How We Check Our Math

Last updated: August 13, 2026

Who checks the math

The PiggyMath Editorial Desk owns calculator accuracy. We are practitioners who ship educational tools, not a licensed CPA firm issuing opinions on your return. Authorship on tool pages points here so readers can see the verification standard behind a number.

How we verify (checklist)

1. Primary build — implement the user-facing calculator from IRS/SSA instructions or standard finance formulas (assets/app.js / TAX2026).
2. Shadow build — a second implementation written from the same instructions without copying the first.
3. Scenario battery — ordinary cases plus edges (zero/low profit, wage-base spillover, high earners, Additional Medicare, QBI phase-outs, head of household and married filing separately vs single/MFJ).
4. Source cross-check — each tax-year constant confirmed in at least two publications before it enters TAX2026 / related tables.
5. Ship gate — both implementations must agree within ±$1 on covered fields; material misses are fixed before release and noted when relevant.

Federal dual-implementation battery

For the shared federal core (self-employment tax, federal income tax, simple and advanced §199A, and the set-aside stack used by quarterly / set-aside / related tools), we maintain a runnable dual-implementation check in tools/dual_impl_federal_tax.mjs. It loads the live formulas from app.js, runs an independently written shadow engine against the same inputs, and fails if any compared dollar amount differs by more than $1.

Current coverage (80 scenarios, all within ±$1 as of this update):

• Filing status — Single, Married filing jointly, Head of household (standard deduction $24,150), and Married filing separately (standard deduction $16,100; brackets match Single until the 37% band at $384,350), including side-by-side band checks at $40k–$180k profit.
• Zero / low profit — $0, sub-$400, and small side-hustle cases.
• Additional Medicare — high earners ($250k–$400k), including the MFS $125,000 threshold, with confirmation that the deductible half of SE tax excludes the 0.9% Form 8959 amount.
• W-2 Social Security spillover — Schedule SE wage-base room after W-2 box 3 wages, including partial room and full-base offset.
• QBI (§199A) — simple 20% path, taxable-income cap, advanced wage/UBIA limit, SSTB phase-out, and in-range phase-in (including HoH / MFJ / MFS thresholds; MFS threshold is $201,775).

Non-federal calculators (mortgage, compound interest, and similar) still use the same dual-build checklist before ship; the Node battery above is the automated gate for the 2026 federal tax core.

Prep, not file

Our estimators intentionally omit full return machinery: we do not compute a complete Form 6251 (AMT), we do not e-file, and we do not replace tax software. High-income users may see an on-page AMT awareness note pointing to the IRS — that is a flag to check elsewhere, not an AMT calculator.

Every calculator on this site is built twice. The version you use is written first; then a second, independent implementation is written from the underlying formula or IRS instruction — deliberately not by copying the first — and the two are run against each other across dozens of scenarios, including the awkward ones: zero interest rates, income above the Social Security wage base, expenses larger than revenue, a 100% down payment. A calculator ships only when both implementations agree and every edge case behaves sensibly.

That process is not decoration. When we ran it in August 2026 it caught five real errors in our own tools before they reached anyone: a freelance rate formula that taxed deductible business expenses, a quarterly tax estimate that ignored the taxable-income ceiling on the qualified business income deduction, a 1099-versus-W-2 comparison that counted paid time off twice and failed to cap the employer Social Security contribution at the wage base, and a mortgage payment that omitted private mortgage insurance entirely. Each of those would have sent someone away with a materially wrong number.

The process keeps catching things. A later run flagged a discrepancy above $200,000 of net earnings, where our tools were folding the Additional Medicare Tax into the deduction for one-half of self-employment tax. They should not: that 0.9% tax is computed on Form 8959, and the Schedule SE instructions confirm it is not figured on Schedule SE at all — so the deduction, which is half of the Schedule SE amount, cannot include it. Three calculators shared the error and all three were corrected. It only affected higher earners, which is exactly the sort of edge a casual check misses.

The August 11, 2026 federal battery (after Married filing separately shipped) re-ran those edges plus HoH / MFS brackets, the MFS $125k Additional Medicare floor, and W-2 spillover; primary and shadow agreed on every scenario within ±$1, so no new production bug was opened from that pass.

The same day, a state-bracket dual-impl battery checked CA, NY, NJ, OR, MN, and DC (exact statutory schedules by filing status, including California’s 1% Mental Health Services Tax above $1M) against an independent progressive calculator; every scenario agreed within ±$1.

Where the tax figures come from

The 2026 numbers used across the site come from the Social Security Administration's annual announcement and IRS Revenue Procedure 2025-32 (and related IRS retirement / HSA notices). We check each figure against at least two independent publications before using it. Tax rules change every year; when the IRS publishes 2027 figures, these tools are updated and this page records the date.

Sourced constants live in data/tax-constants-2026.json and are listed with live citations on the Tax Constants 2026 reference page (each entry has value, source, source_url, effective_from). Runtime calculators still read the mirrored values in assets/app.js; node tools/verify_tax_constants.mjs checks they stay aligned. Inline examples (values hydrate from the JSON when available):

• Social Security wage base: $184,500
• Standard deduction: $16,100 single / MFS · $24,150 HoH · $32,200 MFJ
• SE net-earnings factor: 92.35%
• Solo 401(k) elective deferral: $24,500 · annual additions $72,000 · compensation cap $360,000
• Mileage: 72.5¢ (Jan–Jun) → 76¢ (Jul–Dec)

Head of household and married filing separately are available on the flagship freelancer tax estimator and the main federal tax tools.

What we deliberately do not do

We do not ask who you are, and nothing you type into a calculator ever leaves your browser — there is no server to send it to. We do not display user ratings or review counts, because we do not collect any and inventing them would be dishonest. And we do not claim professional credentials we do not hold: these tools produce careful estimates, not tax advice. For a filing decision, a return, or anything with real money at stake, talk to a qualified accountant.

Limits of the model (honest gaps)

• AMT — not computed (no Form 6251). High-income pages may show an awareness note only.
• NIIT — light 3.8% estimate on investment income above MAGI thresholds where a tool exposes it; not a full Form 8960. MFS uses the $125,000 MAGI floor.
• State tax — flat / no-income-tax states use the published rate (or $0) on a profit-after-SE-half proxy. Exact brackets for CA, NY, NJ, OR, MN, and DC use statutory multi-bracket schedules by filing status (CA includes the 1% Mental Health Services Tax above $1M). Other graduated states remain multi-bracket or coarse approximations. None of these are a complete state return (state standard deduction, exemptions, and credits are omitted).
• MFS Child Tax Credit — phaseout uses the $200,000 MAGI threshold, but we do not model who may claim a qualifying child when spouses file separately, or other MFS-only credit restrictions.
• Credits / e-file — most credits beyond light CTC / other-$ estimates are out of scope; we do not e-file.

Assumptions worth knowing

Some simplifications are unavoidable in a general-purpose tool. State income tax is estimated by applying your state's rate schedule to profit after the self-employment tax adjustment, because every state defines its own deductions — that errs slightly high, which is the safer direction for a set-aside figure. For CA, NY, NJ, OR, MN, and DC we apply exact published brackets by filing status; elsewhere flat rates or compact bracket tables still apply. Investment calculators compound monthly and ignore inflation, fees and taxes on gains unless stated. Mortgage results exclude closing costs. Where an assumption materially changes the answer, the calculator's own page says so.

Citeable data pages

Sourced constants: Tax Constants 2026. For journalists and educators: 2026 set-aside by income band, SE tax vs employee FICA, Solo 401(k) 20% vs 25%, S-Corp payroll vs lost QBI, and Form 2210 annualized method. Brand assets: Press & brand.

Found a mistake?

Tell us. Corrections are the most useful message we can receive, and we would rather fix an error quickly than defend it. Write to contact@piggymath.com with the numbers you entered and what you expected — we reply to these first.