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Convert Calc

How We Verify Our Conversion Factors

By Marcus Thompson · Published

Every conversion factor on Convert Calc — from the common (centimeters to inches) to the obscure (grains to milligrams) — comes from a small set of authoritative published standards, not from copying figures off other websites or rounding “close enough” numbers. This page explains where those figures come from, how they’re checked, and how we handle the occasional case where a “standard” conversion factor isn’t actually a single fixed number.

Primary sources

The majority of conversion factors used across this site trace back to one of these references:

  • NIST Special Publication 811 (“Guide for the Use of the International System of Units”), published by the US National Institute of Standards and Technology — particularly Table B.8, which lists exact conversion factors between US customary/imperial units and SI units. This is the primary source for length, mass, area, volume, and most other linear conversions.
  • The SI Brochure (published by the BIPM, the International Bureau of Weights and Measures) — the definitive reference for the definitions of SI base and derived units, used to verify the exact relationships between metric prefixes and units.
  • IEC 80000 — the international standard defining the binary prefixes (kibi-, mebi-, gibi-) used for data storage units, which is the basis for the distinction covered in our MB vs. MiB guide.

For category-specific or less common units, we cite the specific source on each converter page (visible below the formula), so you can verify the figure independently rather than taking it on trust.

How we handle units without a single “exact” definition

Most SI-derived conversions (meters to feet, kilograms to pounds, liters to gallons) have a single internationally agreed exact value, because the underlying units are formally defined. But a few categories on this site involve units that aren’t governed by a single international standard, and we handle these differently:

  • Shoe and clothing sizes: these are brand- and region-specific conventions, not formally defined units. Our size charts represent commonly cited correspondences compiled from multiple manufacturer size charts, and we explicitly note that individual brands vary — see the shoe size conversion guide.
  • Cooking measurements (cups to grams for specific ingredients): these depend on ingredient density, which varies by brand, moisture content, and how the ingredient is measured (packed vs. loose). Our ingredient conversion figures are sourced from food-science references and manufacturer nutrition data, and we recommend a kitchen scale for precision — see the cups to grams guide.
  • Fuel economy: the conversion between MPG and L/100km is a fixed mathematical (reciprocal) relationship once the gallon definition is fixed, but US and imperial gallons differ, so we specify which gallon definition applies — see the MPG vs L/100km guide.

In each of these cases, the relevant guide or page explains the source of variability so the figure can be understood as a reference point rather than an exact universal constant.

The “lastReviewed” date

Every converter page with rich content (formula, examples, FAQ) displays a “Last reviewed” date. This date indicates when we last checked that page’s conversion factor and supporting content against its cited source. For SI-derived conversions, the underlying factors essentially never change — SI unit definitions are extremely stable — but we periodically re-review pages to:

  • Confirm the cited source is still accurate and the link still resolves.
  • Check that worked examples and reference points remain realistic and useful.
  • Update any content that referenced figures, prices, or context that may have become outdated.

Rounding and display precision

Conversion results on this site are calculated using full-precision factors internally, then displayed rounded to a sensible number of significant figures for readability — typically 4–6 decimal places for everyday conversions. The full, unrounded factor is what’s used in the calculation; only the displayed result is rounded. If you need maximum precision for a technical application, the formula shown on each page gives the exact factor used.

Corrections

If you find a conversion factor that appears incorrect, or a source link that’s broken or outdated, please get in touch via the contact page. We treat factual accuracy in conversion figures as the highest-priority type of correction, and will verify and update the relevant page promptly.

For more on how specific calculation types work — including the special formulas used for temperature (non-linear) and fuel economy (inverse) conversions — see our methodology page, which covers the underlying math in more detail than this sourcing-focused overview.