Accuracy & Methodology

How M-Convert sources, validates, and documents every conversion factor it publishes.

Sources

Every conversion factor on M-Convert is drawn from one of four primary references:

  • BIPM SI Brochure, 9th edition (2019) — the authoritative definition of all SI units, prefixes, and accepted non-SI units. Used for meter, kilogram, second, ampere, kelvin, mole, candela, liter, minute, hour, day, degree, and all SI prefix values.
  • NIST Special Publication 811 — the U.S. National Institute of Standards and Technology's guide to SI. Used for international foot, inch, yard, mile, pound, ounce, gallon, acre, and related customary conversions.
  • IEEE 1541-2002 — the IEEE standard for binary prefixes. Used for kibibyte, mebibyte, gibibyte, tebibyte, pebibyte.
  • CODATA 2018 — the Committee on Data for Science and Technology's recommended values for physical constants. Used for electron mass, proton mass, and other non-fixed constants.

Exact vs. Approximate Factors

Some conversion factors are exact by definition. These are the results of international agreements or SI decisions that fix a value exactly. Examples:

  • 1 inch = 2.54 cm exactly (1959 international yard and pound agreement)
  • 1 foot = 0.3048 m exactly
  • 1 international mile = 1.609344 km exactly
  • 1 pound = 0.45359237 kg exactly
  • 1 US liquid gallon = 3.785411784 L exactly
  • 1 bar = 100,000 Pa exactly
  • 1 standard atmosphere = 101,325 Pa exactly
  • 1 thermochemical calorie = 4.184 J exactly

Other factors are approximate because they depend on measured values or context:

  • The parsec, derived from the astronomical unit and the definition of the arcsecond (a geometric constant involving π and a trigonometric function).
  • Inch of mercury, which depends on the conventional density of mercury at 0 °C.
  • US gallon per minute, derived from the exact gallon-to-liter definition and the exact minute-to-second definition — the decimal representation is what makes it appear approximate.
  • Any conversion involving an ingredient density (cups to grams for flour, for instance).

The Conversion Engine

All linear conversions use a single internal reference: the base unit of each dimension (meter for length, kilogram for mass, liter for volume, and so on). A value is converted from the source unit to the base unit and then from the base unit to the target unit. This design guarantees that any two units in the same dimension convert consistently, and it eliminates the need to store a separate factor for every pair.

Temperature is a special case. Celsius, Fahrenheit, Kelvin, and Rankine have offsets as well as scale ratios, so M-Convert uses explicit forward and reverse functions for each temperature pair. This prevents the common error of multiplying by a scale factor and forgetting the offset.

Numerical Precision

All arithmetic uses IEEE 754 double-precision floating-point. This provides approximately 15–16 significant decimal digits of accuracy, which exceeds the practical precision requirement of nearly every real-world application. The displayed result is rounded to a significant-figure count that scales with the input magnitude, so a small input like 0.5 m shows more decimal places than a large input like 10,000 m.

What We Do Not Do

  • We do not use secondary sources for a factor that a primary source defines.
  • We do not round intermediate results; rounding is applied only to the final display.
  • We do not fabricate precision. If a factor is approximate, the page says so.
  • We do not present a conversion as authoritative in a context where the definition depends on jurisdiction, industry, or specific equipment.

Reporting an Error

If you believe a conversion factor on M-Convert is incorrect, please report it via the Contact page. Include the page URL, the value you entered, the result shown, and a source for the correct value. Verified corrections are prioritized and applied promptly.