Methodology

This page documents how Calc Hammer's calculators are designed, which formulas they use, how we verify accuracy, and how often we review the underlying assumptions. We publish this so that you can make an informed decision about whether to rely on a given result — and so that anything we get wrong is auditable.

Editorial principles

Three rules govern every calculator on the site:

1. Show the formula. If a calculator hides the math, the result is opaque. Every calculator page includes the underlying equation in plain text and a worked example.
2. State the assumptions. A "monthly mortgage payment" assumes a fixed rate, fully amortizing schedule, and no escrow. A "BMI" assumes the standard adult-WHO scale. We name those assumptions on each page so you can decide whether they apply to your situation.
3. Refuse false precision. A 30-year financial projection rounded to the cent is theatre. We round results to a precision that reflects the underlying uncertainty in the inputs.

Formula sources

We use industry-standard formulas from primary references, not aggregated content from other calculator sites. Specifically:

• Finance. Standard amortization formula (M = P·r·(1+r)ⁿ / ((1+r)ⁿ − 1)) for mortgage and loan payments; compound interest A = P(1 + r/n)^nt for savings and investment projections; ROI and IRR per the standard textbook definitions (Bodie, Kane, Marcus, Investments).
• Health. Basal metabolic rate uses Mifflin–St Jeor (Mifflin et al., 1990, American Journal of Clinical Nutrition 51(2)) by default, with Harris–Benedict and Katch–McArdle available as alternatives. Body fat estimation uses the U.S. Navy circumference method (Hodgdon & Beckett, 1984). BMI uses the WHO adult classification ranges. TDEE applies standard activity multipliers (1.2–1.9) over BMR.
• Physics. SI-unit equations from standard university references (Halliday, Resnick & Walker, Fundamentals of Physics). Constants (gravitational acceleration, speed of light, etc.) are sourced from CODATA 2018 values.
• Math. Operations follow standard arithmetic and trigonometric definitions. Where multiple conventions exist (e.g., quadrant sign convention for inverse trig), we name the convention used.
• Construction. Volume and area formulas are standard geometry; waste-factor adjustments use industry-typical ranges (concrete: 5–10%, paint: 10%, flooring: 5–10%) which we list on the page.

Verification and testing

Every calculator on Calc Hammer is implemented as a small, self-contained Ruby object with a unit-test suite. For each calculator we test:

• Known-answer cases from textbook examples and authoritative published worked examples (e.g., the SEC's investor.gov compound-interest example).
• Edge cases: zero inputs, very large inputs, very small inputs, and inputs at the boundary of validity (e.g., 0% interest, 100-year loan term).
• Sanity ranges: outputs must fall inside plausible ranges (e.g., a 30-year mortgage payment at 5% must be between 1/240th and 1/120th of principal).

Every change to a calculator passes through the full automated test suite before deploying. Test coverage is publicly verifiable: the calculator classes live under app/calculators/ and the tests under test/calculators/ in the public codebase.

Limits of these calculators

These calculators are designed for fast estimation, not for regulated financial, medical, or engineering advice. Important limits:

• Financial calculators do not account for taxes specific to your jurisdiction, fees, insurance, prepayment penalties, variable rates, or inflation adjustments unless explicitly noted.
• Health calculators use population-level equations. Individual results vary significantly with body composition, genetics, and medical conditions. Nothing on this site is a substitute for advice from a licensed clinician.
• Construction calculators give material estimates, not load-bearing or structural calculations. Engaging a licensed engineer is required for anything load-bearing.

See our Disclaimer for the full statement of limitations.

Review cadence

We review the formulas and assumptions on every calculator at least once per year, and immediately when:

• A reader reports a discrepancy — please contact us if you find one.
• An underlying standard changes (e.g., updated tax brackets, revised WHO ranges, new CODATA constants).
• A calculator is meaningfully restructured in code.

The site-wide editorial last-reviewed date is shown at the bottom of every calculator page; individual calculators may have been reviewed more recently.

Corrections

If you find a result that you believe is wrong, please tell us. Include the calculator URL, the inputs you used, the output you got, and the value you expected (with the source of that expectation, if possible). We will respond, investigate, and — if the calculator is wrong — fix it and credit the reporter.

Independence

Calc Hammer is supported by advertising through Google AdSense. Advertisers have no influence over which calculators we build, which formulas we use, or which results we return. The calculators run entirely in your browser; your inputs never reach our servers, and certainly never reach an advertiser.

Methodology last updated May 14, 2026. Maintained by Goran Arsov.