Methodology & Privacy

MMult publishes free finance calculators with one rule: show the math and protect the user. This page lists every formula we use, every official data source, and exactly what happens to the numbers you type (short answer: nothing).

Privacy: what "100% client-side" means

Every calculation on this site runs as JavaScript inside your browser. The numbers you enter — salary, savings, loan amounts — are never transmitted to MMult or any third party, never stored, and never sold. There is no account system and no analytics tied to your inputs.

The site is supported by unobtrusive display advertising. Ad networks (e.g. Google AdSense) use cookies for ad personalization; users in the EU/UK are asked for consent first via a consent banner (GDPR/ePrivacy). Your calculator inputs are not part of that data.

Formulas

Mortgage & Loan calculators

M = P × [ r(1+r)ⁿ ] / [ (1+r)ⁿ − 1 ]

Standard fixed-rate amortization. The mortgage calculator adds escrow items (property tax = price × rate ÷ 12, insurance = premium ÷ 12, PMI = loan × rate ÷ 12 when down payment < 20%). The loan calculator simulates extra payments month by month and reports the interest saved versus the base schedule.

Compound Interest calculator

FV = P(1 + i)ⁿ + PMT × [ ((1 + i)ⁿ − 1) / i ]

Simulated month by month: contributions are added monthly, interest is credited at the chosen compounding frequency (monthly / quarterly / annually).

Paycheck calculator

Federal income tax: bracket-by-bracket on taxable income = gross salary − pre-tax 401(k) − pre-tax health premiums − standard deduction. Figures from IRS Rev. Proc. 2025-32 (tax year 2026).
Standard deduction 2026: $16,100 single / MFS · $32,200 MFJ · $24,150 HoH.
Social Security: 6.2% up to the 2026 wage base of $184,500 (SSA). Traditional 401(k) deferrals do not reduce FICA wages.
Medicare: 1.45% on all wages + 0.9% additional above $200,000.
State tax: simplified single effective rate (zero for states without a wage income tax). State deductions, credits and local taxes are not modeled.

401(k) calculator

Balance(t) = [ Balance(t−1) + Contribution + Match ] × (1 + r)

Employee contribution is capped at the IRS elective deferral limit ($24,500 for 2026). Employer match = your contribution (up to the match limit % of salary) × match rate. "Today's dollars" divides the nominal balance by (1 + inflation)^years. Catch-up contributions (age 50+) are not modeled.

Known limitations

Paycheck results are estimates — actual withholding varies with W-4 settings, credits, local taxes and benefits.
Constant-return assumptions simplify real markets, which fluctuate and can lose value.
State tax is modeled as a flat effective rate.
Figures reflect tax year 2026 and are updated annually as the IRS and SSA publish new numbers.

Disclaimer

MMult provides tools for informational and educational purposes only. Nothing on this site is financial, tax, investment or legal advice, and it does not create a professional relationship. For decisions that matter, verify the numbers with a qualified professional or the official sources below.

Primary sources

Questions or corrections? Found a tax figure out of date? Corrections are welcome — accurate numbers are the whole point.