How We Test Our Calculators

Money is counted in cents

Calculations keep money as whole cents and use exact decimal arithmetic rather than floating point, so a value like $0.10 never drifts. Loan interest is computed each month on the balance and rounded to the cent, half up, the way lenders post it, and the last payment of a schedule clears any leftover cents so balances end at exactly $0.00. Growth calculations, such as savings and compound interest, keep full precision and round only for display.

Expected values come from a second, independent model

A test is only as good as its expected answer, so we never take expected values from the code being tested. For each calculator we write a separate model in Python, working from the primary source rather than from our TypeScript: the IRS Schedule D Tax Worksheet line by line for capital gains, HUD's handbook and mortgagee letters for FHA rules, VA circulars and regulations for VA assumptions, and each state's statute or revenue department guidance for state taxes. The model's results become the expected values in our automated tests, and both must agree to the cent before a calculator is published.

The tests also cover the cases that break calculators: zero interest rates, payments larger than the balance, losses instead of gains, holding periods right at the one-year line, and income levels where brackets, exclusions or surtaxes start.

Primary sources, cited on every page

Rates, limits and rules come from the agency that sets them: the IRS, HUD, the Department of Veterans Affairs, the Consumer Financial Protection Bureau and state legislatures and revenue departments. State income tax brackets come from the Tax Foundation's annual table, with laws passed later in the year applied on top and cited on the state's page. Each calculator lists its sources at the end of the article, and when we can't confirm a rule from a primary source, we leave it out rather than guess.

When the data is updated

• October and November: the IRS inflation adjustments for the next tax year (brackets, standard deduction, capital gains thresholds) and the Social Security wage base.
• December and January: FHA loan limits and mortgage insurance changes, and the switch to the new tax year.
• February: the Tax Foundation's state income tax table.
• After state legislative sessions: mid-year state law changes, including how states tax capital gains.
• Monthly: the IRS applicable federal rates used by the seller financing calculator.

What the "Updated" date means

The date under each calculator's introduction is the last time the page's content or data changed, or the last time we reviewed it against its sources. The same date appears in the page's structured data and in our sitemap; we don't change it to make a page look fresher than it is.

Corrections

If you find a number that doesn't match a source, tell us. We check it against the source, fix the code and the independent model together, and update the page's date when the fix changes what it says. Our editorial guidelines cover how we write, review and separate content from advertising.