Methodology and sources

• BIPM, The International System of Units (SI Brochure), 9th edition, 2019

  Definitions of the metre, kilogram, second, kelvin and the coherent derived units (pascal, joule, watt), plus the SI prefixes used for kilo-, mega-, giga- and tera- multiples.

• NIST Special Publication 811, Guide for the Use of the International System of Units, 2008 edition, Appendix B

  Reference table of conversion factors to SI. Every non-SI factor on this site that is not fixed by a treaty or statute below is taken from this appendix.

• International yard and pound agreement, 1 July 1959

  1 yard = 0.9144 m exactly and 1 pound (avoirdupois) = 0.453 592 37 kg exactly. All customary lengths (inch, foot, mile) and masses (ounce, stone, US ton) derive from these two values.

• NIST Handbook 44, Appendix C, General tables of units of measurement

  US customary liquid measures: the US gallon is 231 cubic inches exactly (3.785 411 784 L); the cup, fluid ounce, tablespoon and teaspoon follow from it.

• UK Weights and Measures Act 1985, Schedule 1

  The imperial gallon is 4.546 09 L exactly; the imperial fluid ounce and pint follow from it.

• International Hydrographic Conference, Monaco, 1929

  The international nautical mile is 1 852 m exactly; the knot is one nautical mile per hour (1 852 / 3 600 m/s).

• International Astronomical Union, Resolution B2 (2012) and Recommendations on units (1976)

  The light year is the distance light travels in one Julian year of 365.25 days: 299 792 458 m/s × 31 557 600 s = 9 460 730 472 580 800 m exactly.

• King Arthur Baking, Ingredient Weight Chart; USDA FoodData Central

  Densities of cooking ingredients (grams per US cup) on the /cooking pages. Not physical constants: they describe a cup spooned and levelled, and vary by about ±10 % between cooks. The source is named on each ingredient page.

• IEC 80000-13:2008, Quantities and units, Part 13: Information science and technology

  Decimal prefixes (kB = 1 000 B) are distinct from binary prefixes (KiB = 1 024 B). Bit units (kbit, Mbit) are separate from byte units; 1 byte = 8 bits.

Where a unit has more than one accepted definition, the site uses the following.

Year, month, decade, century

A year is the Julian year of 365.25 days (31 557 600 s), the astronomical convention; a month is one twelfth of it. Calendar years and months vary in length and are not used.

Calorie

The thermochemical calorie, 4.184 J exactly. The kilocalorie used on food labels is 1 000 of these.

British thermal unit

The International Table BTU, 1 055.055 852 62 J exactly (NIST SP 811).

Horsepower

Mechanical horsepower, 550 ft·lbf/s = 745.699 871 582 270 22 W. Metric horsepower (PS) is 75 kgf·m/s = 735.498 75 W exactly.

Atmosphere and torr

The standard atmosphere is 101 325 Pa exactly; the torr is 1/760 of it, which differs from the millimetre of mercury by less than one part in seven million.

Pound per square inch

6 894.757 293 168 Pa, derived from the pound-force (0.453 592 37 kg × 9.806 65 m/s²) over one square inch.

Mach

Speed of sound taken as 343 m/s, the value in dry air at 20 °C. Mach number depends on air temperature and altitude; the site uses this single conventional value.

Cup, tablespoon, teaspoon

US customary measures (1 cup = 236.588 2365 mL). The US legal cup used on nutrition labels (240 mL) and the metric cup (250 mL) are different.

Temperature

°F = °C × 9/5 + 32; K = °C + 273.15; °R = (°C + 273.15) × 9/5. Results below absolute zero are refused. Results are rounded to ten decimals to absorb binary floating-point noise.

Fuel economy

Miles per gallon and litres per 100 km are inversely related, so every conversion goes through kilometres per litre using 1 mi = 1.609 344 km and the US (3.785 411 784 L) or imperial (4.546 09 L) gallon.

Display rounding

The converter shows six significant digits. Tables are computed from the same engine at render time, never copied by hand, so a table and the widget can never disagree. The API returns full double precision.

• 11 August 2026. Knot corrected from 0.5144 m/s to the exact 1 852 / 3 600 m/s; light year corrected to the IAU value; year, month, decade and century aligned on the Julian year; "Mbps" and "Gbps" aliases that pointed to byte units replaced by real bit units.
• 12 September 2026. Temperature results rounded to ten decimals (−459.67 °F gave −5.7 × 10⁻¹⁴ K instead of 0 K). Conversion tables on written pages now computed by the engine instead of stored values (3.785 L was labelled as exactly 1 US gallon).
• 30 October 2026. Size charts rebuilt from named sources (Nike, adidas, ASOS, Mulberry, ISO 8653), each linked on its page. The previous women’s clothing table put European sizes one step too low (UK 10 shown as EU 36; ASOS gives EU 38), and the shoe tables mixed several brands into one column; Nike and adidas are now shown side by side. Ring sizes added.

To report an error, write to [email protected] with the page and the source you compared against.

Exact values used by the engine, expressed in the base unit of each category. Temperature and fuel economy have no fixed factor and use the formulas above.