Our Methodology

This page explains the rules every Countora calculator follows. Each calculator page has its own formula, worked example and sources; this page covers what they have in common.

Where formulas and figures come from

Formulas are standard ones from geometry, algebra and finance, or from the published source named on the page. Figures that a formula depends on are cited in the Sources section at the bottom of each calculator page. These include material coverage rates, tax brackets and official health guidance. Health and finance pages cite government agencies, regulators, professional bodies or peer-reviewed research. Where a value varies, such as paint coverage or a loan's interest rate, the calculator gives you a field to change it, and the default is only a starting point.

Construction and DIY: waste allowances

Real projects lose material to cuts, breakage and spillage, so material calculators add a waste allowance. The default is filled in for you and you can change it.

Rounding rules

• Rounding only happens at the end. Results are worked out at full precision and rounded only for display, so small rounding errors don't add up across steps.
• Material counts always round up. You can't buy 41.3 bags, so the calculator says 42. This applies to bags, boxes, sheets, boards, posts and tiles.
• Ready-mix concrete rounds up to the next quarter cubic yard.
• Paint is shown as whole gallons plus quarts. Three or more extra quarts become one more gallon, since a gallon usually costs less than three quarts.
• Money is shown to the cent. Payments and interest are calculated at full precision first.
• Fractions stay exact. The fraction calculator works with whole numbers and fractions throughout, and the algebra and derivative calculators work symbolically rather than by approximation.

Units

Calculators that take measurements use US customary units by default, and most have a metric option. Conversions use exact definitions (for example, 1 inch = 2.54 cm and 1 pound = 0.45359237 kg). Your choice of units is remembered in your browser only; see our privacy policy.

Finance assumptions

• Loans use the standard amortization formula with a fixed rate, and monthly interest equal to APR ÷ 12.
• Credit card and debt payoff calculators also charge interest monthly at APR ÷ 12. Card issuers usually calculate interest daily, so actual figures can differ slightly.
• The income tax calculator uses the IRS figures for the tax year shown on the page and covers ordinary income only. It doesn't include credits.

Health formulas

Health calculators use published equations, such as the Mifflin–St Jeor equation for BMR and the US Navy method for body fat, and official categories, such as the CDC's adult BMI ranges. These describe population averages, so results for one person can be off, especially for athletes, older adults or people with medical conditions.

Testing and review

Each calculator's formulas have automated tests covering normal values, edge cases and unit conversions. The worked examples on each page are checked against the calculator itself. Each page shows a "Last reviewed" date, and we update it whenever figures or guidance change.

Corrections

If you think a formula, figure or source is wrong, please tell us. We check every report and correct confirmed errors.