How We Calculate

Every Plainsum calculator follows a published formula or official rule, explains it on the page, and shows its sources. This page describes how we build, check, and update them.

Where our numbers come from

  • Federal taxes and limits: the IRS (tax brackets, standard deduction, credits, 401(k) and IRA limits) and the Social Security Administration (wage base).
  • State taxes: state income tax brackets and sales tax rates from the Tax Foundation, cross-checked with state revenue departments when a state changes its law during the year.
  • Loans and savings: the standard amortization and compound interest formulas used by lenders and described by the Consumer Financial Protection Bureau and the SEC.
  • Work and pay: the Fair Labor Standards Act rules published by the U.S. Department of Labor.
  • Health: peer-reviewed equations and guidance from public health and medical organizations, such as the CDC and the Mifflin-St Jeor equation.

How we check results

Before a calculator goes live, we work through examples by hand and compare them with the calculator's output, including edge cases such as zero balances, very high incomes, and income-based phase-outs. Tax calculators are also checked against the official tables for several states and filing statuses.

How we keep rates current

Tax rules change every year. We review federal figures when the IRS announces new amounts (usually in October and November), state income tax brackets each February, and sales tax rates each July. Each calculator shows the date it was last reviewed.

What our results are, and aren't

Results are estimates for planning. They follow the common case and state their assumptions, but they can't cover every rule or personal situation. They aren't financial, tax, legal, or medical advice.

Independence

Plainsum may show ads to pay for the site. Advertisers have no say in our formulas, results, or articles, and we don't accept payment to recommend products.

Corrections

Spot a problem? Email support@plainsumcalc.com with the calculator and the numbers you used. We fix confirmed errors and update the review date.