Methodology

1. How calculators are built

Each calculator answers one question. Inputs are limited to what the formula actually needs, and every calculation runs in your browser, so nothing you type is sent to a server. Where a result depends on a judgement call (waste allowance, rounding to whole bags, safety margins) the assumption is stated next to the result.

2. How formulas are sourced

Formulas come from primary or widely accepted references: textbooks, standards bodies, building codes, government agencies, university extension services and peer-reviewed papers. Where a source is cited it is listed under "Formula source" in the About this calculator box at the bottom of the page. Estimating tools that depend on local prices or regional rules are labelled as estimates.

3. How results are tested

Before a calculator goes live it is checked against worked examples calculated by hand or taken from the source material, including edge cases such as zero, very large values and unit conversions. The worked example shown on each page is produced by the same code that runs the calculator, so what you read is what it computes. When a calculator is re-checked, its "Last reviewed" date is updated.

4. How errors are reported and fixed

Every calculator has a "Report an error" link that opens an email with the calculator name filled in. Reports are checked against the source formula; confirmed errors are fixed and the review date is updated.

5. Limits

Results are for general information and are not medical, legal, tax, structural or financial advice.

Contact

Email hello@calculationmachine.com or read more about who builds the calculators.