How the calculators work

This page explains how every calculator card on keepcalculator.com works, so you know what the numbers mean and when to check them twice.

1. One standard formula per card

Each card uses the standard formula for its question, the one found in a textbook or reference handbook. The card writes it out in words and symbols, defines every variable with its unit and works an example with the default numbers. Constants such as π, e, standard gravity (9.80665 m/s²), the gas constant or a unit factor are stated where they are used, with their source values (CODATA for physical constants, exact definitions for units).

2. Inputs and units

• Every input shows its unit at the end of the box. Enter numbers in that unit, or use the unit converter first.
• Thousands separators (1,250) and decimal points (12.5) are both fine.
• List inputs accept numbers separated by commas, spaces or new lines.
• Drop-down inputs apply a fixed value, such as an ingredient’s density, which is shown in the formula box.

3. How results are calculated and rounded

• Sums run in full double precision and are rounded only for display. Each result keeps the number of decimals shown in the card’s “Rounding” line.
• Durations are shown as m:ss or h:mm:ss where a card says so, rounded to the nearest second.
• Trigonometry uses radians unless the card says degrees.
• Standard deviation is the sample value (dividing by n − 1) unless the card says population.
• If a result cannot be computed (a division by zero, the logarithm of zero, a power factor above 1), the card says “Cannot compute” instead of showing a misleading number.

4. Checking a result

Every card has a worked example that uses the default numbers and shows each step. The “Formula used” box lists the exact lines the calculator runs. A small chart shows how the main result changes when the first input moves between half and one and a half times your value, with a table you can open.

5. Limits

A formula is a model. Air is not always at one temperature, engines lose power in the drivetrain, heaters lose heat to their tank. Each card lists the limits that matter for its formula. Results are estimates, not professional advice; see the disclaimer. There are no cards for loans, tax, investments, medical dosing or safety sizing.

6. Errors and corrections

If a result looks wrong, send the card’s address and the numbers you entered through the contact page. Confirmed errors are fixed and listed on the corrections page. A calculator under review can be switched off while its formula stays visible.