How the math works

Rounding

• Money is rounded to the cent where it would be in practice — each payment, each period’s interest, each line of a tax form — using round-half-away-from-zero, the same rule as Excel’s ROUND function.
• Rates, ratios, and percentages are carried at full precision and rounded only for display.
• Totals are sums of the rounded amounts, so schedules foot. The final payment of a loan absorbs any rounding difference so the balance ends at exactly zero.
• Negative amounts are shown in parentheses, on the page and in the workbooks.

Dates

• Dates are calendar dates with no time zone: the date you enter is the date you get, wherever you are.
• Adding months keeps the day of the month and moves to the month’s last day when needed — January 31 plus one month is February 28 (29 in a leap year).
• Unless you enter a first payment date, the first payment is one period after the start date. A payoff date is the date of the last payment.

Loans and interest

• Level payments use the standard annuity formula at the periodic rate (the annual rate ÷ payments per year) — the same result as Excel’s PMT.
• Payments are at the end of each period unless a calculator lets you choose payments in advance.
• Effective interest rates are the rate that discounts the actual cash flows back to the amount received.

Tax years and sources

• Every tax figure — wage bases, limits, percentages, due dates — lives in a table for its tax year, with the source it came from: the Internal Revenue Code, Treasury regulations, and IRS forms, instructions, publications, and revenue procedures.
• Tax calculators let you choose a supported tax year (currently 2025 and 2026). Each year’s table has a review-by date, and the site’s automated checks fail once it passes, so outdated limits can’t linger unnoticed.
• The calculators cover the federal rules they describe and say what they leave out, such as state taxes and special elections.

Accounting treatment

• Journal entries are suggestions for your records. Every entry is checked to balance before it is shown.
• Where an accounting standard applies, the calculator names it — for example ASC 842 for leases, ASC 835-30 for debt issuance costs, and ASC 410-20 for asset retirement obligations.

The Excel workbooks

• Workbooks are built in your browser. Inputs are yellow cells with names (like LoanAmount) that the formulas refer to, so changing an input recalculates everything that depends on it.
• Every workbook opens with a Read Me sheet and includes the calculator’s assumptions and sources. Where a sheet holds values rather than formulas, it says so.
• In testing, every formula in every workbook is recalculated by a spreadsheet engine across several scenarios and compared with the website. They must agree to within half a cent, and no formula may return an error.

Worked examples

The “Check my math” example on each calculator is produced by that calculator from its example inputs when the site is built. If the math changes, the example changes with it.

Testing

Expected answers come from authoritative sources wherever they exist — IRS forms, instructions, and tables (such as Schedule SE, Forms 4562, 6252, and 8959, and Publication 946), the Department of Labor’s overtime regulations (29 CFR Part 778), and Excel’s financial functions. Schedules are also checked for consistency: they foot, balances end at zero, and journal entries balance.

Found something that doesn’t add up? It goes in the changelogonce it’s fixed.