Methodology

This page states, in one place, the general assumptions and conventions shared across the calculators. Each calculator also lists the specific assumptions that apply to it.

Compounding

Growth and savings calculators compound monthly, because most real accounts credit growth more often than once a year. Regular contributions are treated as arriving at the end of each month (an ordinary annuity). A few calculators built on present-value math, such as Coast FIRE, use annual compounding, and say so on the page.

Rates and returns

Any return or interest rate you enter is a steady annual assumption, not a prediction. Real markets are volatile, and past averages do not guarantee future results. Where it helps, try a range of rates rather than a single figure.

Inflation

Where a result is shown "in today's dollars," the future amount is discounted by the inflation rate you provide, compounded annually, to reflect purchasing power today.

Tax figures

Tax brackets and contribution limits come from the official IRS Revenue Procedure for the relevant year and are stored as versioned, dated data. The current tax year is 2026. These figures are reviewed and updated once a year when the IRS publishes new numbers. Tax calculators cover United States federal figures only and do not include state taxes.

Testing

Before a calculator ships, its formula is tested against values we can verify independently. This is a standing part of how the site is built, not an afterthought, and it is why we are comfortable saying the numbers are trustworthy.

Estimates, not advice

Every result is an estimate based on the inputs you provide and the assumptions above. It is general educational information, not personalized financial, tax, or investment advice. See the full disclaimer for details.