Methodology

Our principles

• Standard formulas. We use the formulas found in standard math, statistics and finance references, and we show them on every page.
• Exact where it matters. Whole-number calculations, such as the greatest common factor and least common multiple, use exact integer arithmetic, so very large numbers don’t lose accuracy.
• Round only for display. Results are calculated at full precision and rounded only when shown, usually to the cent for money and up to four decimal places otherwise.
• State the assumptions. Where a result depends on assumptions, such as compounding frequency or growth rates, we say so next to the result.
• On your device. Calculations run in your web browser. Your inputs aren’t sent to our servers.

How we test

Before a calculator is published, we check its results against worked examples calculated independently, including edge cases such as zero, negative numbers, decimals and very large values. We also test that each page works on phones and desktops, in light and dark modes, with a keyboard alone and with screen readers in mind.

Calculator-specific methods

• GCF calculator: Euclidean algorithm, prime factorization by trial division, and LCM from a × b ÷ GCF(a, b).
• Average calculator: arithmetic mean, median after sorting, mode by frequency and range.
• Weighted average calculator: Σ(value × weight) ÷ Σ(weight).
• CD calculator: compound growth from APY, or from an interest rate with the chosen compounding frequency.
• Dividend calculator: income from shares × dividend per share, and a payment-by-payment reinvestment (DRIP) projection.
• Paycheck calculators: federal income tax withholding using the IRS Publication 15-T percentage method (Worksheet 1A) for Forms W-4 from 2020 or later; Social Security and Medicare at the current rates and wage base; and any employee payroll taxes set by the state.
• Overtime calculator: time and a half and double time by the hour, and the federal qualified overtime premium (hours over 40 × rate × 0.5) with the yearly limit.

Tax figures and yearly updates

Paycheck and tax calculators use figures published by government agencies: the IRS (Publication 15-T and Form W-4), the Social Security Administration, and state revenue and labor departments. Each page shows the tax year it covers and the date it was last checked. Federal withholding tables change every January, so we review and update these calculators when the IRS releases the new year’s Publication 15-T. If a law changes partway through the year, we update the affected pages and their dates.

Limitations

Finance results are estimates before taxes and fees unless stated otherwise. Paycheck results estimate withholding, which is not the same as the tax you will owe for the year. Real-world products can use different conventions, such as day-count rules or rounding at each step, so a bank or broker’s figures may differ slightly. Our calculators provide general information only and are not financial, tax or legal advice.

Found an error?

Please email [email protected] with the calculator, the numbers you entered and what you expected. We investigate every report and correct confirmed errors promptly. See our editorial standards for how corrections work.