Skip to content
Menu

How we check values

Where the numbers come from, how they're computed, and what happens when something is wrong.

Last updated

Sources

Every unit is defined from a published standard, never copied from another converter:

  • Length: the international inch (exactly 25.4 mm) and foot, as set out by NIST Handbook 44 and the 1959 international yard and pound agreement.
  • CSS and screen units: the W3C CSS Values and Units specification, which defines 1 in = 96 px and 1 pt = 1/72 in.
  • Paper sizes: ISO 216 for A and B sizes, and the North American sheet sizes defined in inches (used by the paper size calculator).
  • Weight, volume and other customary units: the international pound (exactly 0.45359237 kg), and the US and UK gallons, cups, fluid ounces and spoons, from NIST Handbook 44 and NIST SP 811. US and UK measures are kept as separate units because they differ.
  • SI units: the BIPM SI Brochure for the metric units and prefixes, and for temperature (0 °C is exactly 273.15 K).
  • Data sizes: decimal prefixes (1 GB = 1,000,000,000 bytes) and the binary prefixes KiB, MiB and GiB from IEC 80000-13.
  • Time zones: the IANA time zone database, as built into your browser, including daylight saving rules.

Each guide lists the specific sources it relies on at the end of the page.

How values are computed

  • Exact definitions. Factors are stored as exact decimal strings (for example, 0.0254 for the inch), not rounded approximations.
  • Decimal arithmetic. Every calculation uses a decimal math library with 40 significant digits, so results never carry the floating-point errors of ordinary JavaScript numbers.
  • Stated assumptions. Where an answer depends on context, such as DPI for pixels or root font size for rem, the assumption is shown next to the result and can be changed.
  • Rounding only for display. Values are rounded to the precision you choose (6 significant figures by default) only at the very end. Whole pixel counts round half up, the way design tools do.

How values are tested

The conversion engine is covered by nearly 3,000 automated test cases that run on every change:

  • Reference values checked against their standards, for example 1 in = 2.54 cm and 16 px = 1 rem.
  • Round trips for every pair of units in every category: converting A → B → A must return the original value.
  • Formula checks confirming that the step-by-step working shown on each page produces the same answer as the converter.
  • Calculator cases, including zero, negative, very large and invalid input.
  • Content checks confirming that every page has a guide, valid related links and a description.
  • Fact checks on the guides: every statement in the form “12 in = 30.48 cm” is recomputed by the engine, so a wrong number in the text fails the build.

Worked examples, tables and the “1 unit =” lines on each page are generated from the same engine at build time, so the text and the tool cannot disagree.

Review

Before a guide is published, it is read and approved by the person who maintains TidyUnits, and every value in it is recomputed by the conversion engine and checked against its source standard. The date of the last review is shown at the bottom of the page. Guides are reviewed again when a source standard changes or when a correction is reported.

Corrections

If you think a value, explanation or source is wrong, please use the contact form and include the page address and what you expected. Confirmed errors are fixed as a priority, and the page’s review date is updated when the fix is published.

Limits

TidyUnits is a reference and a convenience. For measurements where an error could cause harm or significant cost, such as engineering, medical or legal work, confirm critical values with the governing standard or a qualified professional.