Methodology

TL;DR: Every conversion factor on CoolConversion is verified against NIST (US National Institute of Standards and Technology) and, for SI units, the BIPM SI Brochure. Rounding rules depend on the use case — engineering pages keep more decimal places; cooking pages round to practical values. Last reviewed: April 2026.

1. Primary sources

All conversion factors on CoolConversion are derived from, and verified against, authoritative references:

  • US customary units (cup, pint, quart, gallon, fl oz, tablespoon, teaspoon, pound, ounce by weight, inch, foot, mile, …): NIST Handbook 44, Appendix C — “General Tables of Units of Measurement.” This is the legal-metrology reference used by every US weights-and-measures authority.
  • SI / metric units (meter, kilogram, liter, Celsius, pascal, …): the BIPM SI Brochure (9th edition) and the NIST Reference on Constants, Units, and Uncertainty.
  • Imperial (UK) units (imperial pint, imperial gallon, imperial fl oz, stone): the UK Weights and Measures Act 1985 as preserved in the current Schedule, plus NIST Handbook 44 Appendix C which cross-lists Imperial equivalents.
  • Nutrition-label conversions (where a recipe follows US food labeling): the FDA’s 21 CFR 101.9, which defines the labeling fluid ounce as exactly 30 mL (a rounded value used on nutrition panels, not in recipes).
  • Density data (for volume↔weight conversions in cooking and chemistry): derived from peer-reviewed literature and USDA FoodData Central, consolidated in /includes/chemistry-densities.php (192 substances as of April 2026).

2. Rounding rules

CoolConversion rounds differently depending on the page’s audience and use case.

Cooking and everyday pages

Cooking pages (/cooking/, /cooking-volume-weight/, …) round to practical kitchen precision:

  • Milliliters: rounded to the nearest whole mL for values < 100 mL, nearest tenth for 100–999 mL, nearest whole mL again for ≥ 1,000 mL (e.g. 473.2 mL, not 473.176 mL).
  • Cups, tablespoons, teaspoons: rounded to standard kitchen fractions (¼, ⅓, ½, ⅔, ¾) when the exact decimal is close to one.
  • Fluid ounces: rounded to whole numbers when within 0.1 fl oz of an integer.
  • Weight (grams) in recipes: rounded to the nearest whole gram, or nearest 5 g for values ≥ 100 g.

These choices prioritize measurable accuracy — values a home cook can actually hit with a standard measuring cup or kitchen scale — over decimal precision that looks scientific but doesn’t change the outcome.

Engineering and science pages

Engineering pages (chemistry, physics, unit-conversion tables) preserve 6 significant figures by default, which exceeds what most measurement instruments can distinguish. For very small or very large values we use scientific notation.

Exact factors

Some conversions are exact by definition, not approximations. Examples:

  • 1 US fluid ounce = 29.5735295625 mL (exact)
  • 1 inch = 2.54 cm (exact, since 1959)
  • 1 pound = 0.45359237 kg (exact, since 1959)
  • 1 US gallon = 231 cubic inches = 3.785411784 L (exact)

These are marked with an exact-tag on conversion-table pages and are never rounded in the underlying data — only in the displayed output.

3. Unit system conventions

When a unit name is ambiguous (for example, “gallon” can mean three different things), CoolConversion defaults to the most common modern interpretation and notes alternatives clearly.

  • “Cup,” “pint,” “quart,” “gallon” without a qualifier default to the US customary liquid version. Dry-measure variants are noted on each relevant page.
  • “Fluid ounce” without a qualifier defaults to the US fluid ounce (29.57 mL), not the Imperial fl oz (28.41 mL) or the FDA labeling fl oz (30 mL).
  • “Ounce” without “fluid” always means the avoirdupois ounce (28.3495 g, weight), unless the page is explicitly about troy ounces (precious metals).
  • “Ton” defaults to the US short ton (2,000 lb) on US pages and the metric tonne (1,000 kg) on metric pages. The UK long ton (2,240 lb) is available as an explicit option.
  • “Pint,” “gallon,” “stone” on explicitly Imperial/UK pages use the UK Imperial definitions and say so in the page body.

4. Verification and review

Every conversion page is verified through several independent mechanisms before deployment:

  1. Roundtrip testing. Automated tests convert a value from unit A to unit B and back, asserting that the result matches the original within the rounding tolerance. Failing roundtrips block deployment.
  2. Cross-reference against NIST and BIPM. New pages are spot-checked against at least two authoritative external sources. Any disagreement is escalated and resolved before publication.
  3. AI-assisted fact-checking. Since April 2026, high-traffic pages are additionally run through an automated fact-checker (see fact-check-*.js tooling) that reads the rendered page and cross-references the numerical claims. Scores below 90/100 trigger manual review.
  4. Periodic re-review. Each page carries a “Last reviewed” date in its trust-signal block. Pages are re-reviewed at least yearly, or sooner when a source publication (e.g. NIST Handbook 44) is updated.

5. Corrections and feedback

If you find a conversion that looks wrong, or a rounding choice that causes practical problems in a recipe or engineering calculation, please tell us.

Use the contact page and include: the URL of the page, the value you entered, the value we displayed, and (ideally) the value you expected with its source. We acknowledge corrections within two business days and publish fixes as soon as they’re verified.

6. Limitations

Honest caveats about what CoolConversion does not do:

  • Not a legal-metrology authority. CoolConversion is a reference tool for cooks, students, engineers, and curious readers. For legally binding measurements in trade, regulatory filings, or scientific publication, consult the primary sources directly (NIST, BIPM, your national metrology institute).
  • Density data for volume↔weight conversions is approximate. Ingredient densities vary with humidity, temperature, packing technique, and brand. For baking precision, a kitchen scale beats any generic density table.
  • Historical and non-SI units may be approximate. Medieval or regional units (barrel-of-wine, hogshead, Chinese li, old Russian pood) are served for reference but may have multiple historical values; the most widely cited modern equivalence is used.

Last reviewed: April 2026. This page is re-reviewed whenever the underlying sources (NIST Handbook 44, BIPM SI Brochure) publish a new edition, or at least annually.