Conversion Methodology

Where our numbers come from, and what we round.

One stored factor per unit

Every unit on this site stores its size in a base unit: kilograms for weight, liters for volume, meters for length, square meters for area, kilometers per hour for speed and degrees Celsius for temperature. A conversion is always value → base unit → target unit. Because a pair and its reverse read the same two stored numbers, pages such as pounds to kilograms and kilograms to pounds cannot disagree.

Unit standards we follow

• Pound and foot: the international yard and pound agreement of 1959 — 1 lb = 0.45359237 kg and 1 ft = 0.3048 m, both exact by definition.
• Ounce (weight): exactly 1/16 of a pound.
• US liquid gallon: exactly 231 cubic inches = 3.785411784 liters. The quart is a quarter of it, the pint an eighth, the fluid ounce 1/128 of it, and the US customary cup is 8 fluid ounces.
• US tablespoon and teaspoon: the tablespoon is exactly half a US fluid ounce (14.78676478125 mL) and the teaspoon a third of that (4.92892159375 mL). The 15 mL and 5 mL metric spoons, and the 20 mL Australian tablespoon, are different sizes and are named on the pages that use spoons.
• Metric units: SI prefixes — 1 g = 0.001 kg, 1 mL = 0.001 L, 1 mm = 0.001 m, 1 cm = 0.01 m, 1 km = 1000 m. 1 mL is also exactly 1 cm³, and 1 hectare is exactly 10,000 m².
• Square foot: 0.3048 × 0.3048 = 0.09290304 m², the length factor applied twice.
• Acre: the international acre, exactly 43,560 square feet = 4046.8564224 m². The US survey acre, still found in older land records, is about 4 parts per million larger and is not converted here.
• Inch: exactly 2.54 cm, from the same 1959 agreement. The inch is defined in metric terms, which is why inches to centimeters is exact and the reverse is not.
• Stone: exactly 14 pounds, so exactly 6.35029318 kg.
• Mile: the international mile, 5280 ft = 1609.344 m exactly. The nautical mile (1852 m) and the retired US survey mile are different units and are not converted here.
• Mile per hour: the international mile is 5280 ft = 1609.344 m exactly, so 1 mph = 1.609344 km/h exactly. 1 knot is one nautical mile (1852 m) per hour = 1.852 km/h, and 1 m/s = 3.6 km/h.
• Celsius, Fahrenheit and Kelvin: 0 K = −273.15 °C exactly, and the Fahrenheit degree is exactly 5/9 of a Celsius degree with its zero 32 °F below freezing.

Temperature is the one exception

Every other unit here sits on a ratio scale: zero pounds and zero kilograms are the same nothing, so converting is one multiplication. The temperature scales put their zero in three different places — 0 °C is freezing water, 0 °F is a cold winter day, 0 K is absolute zero — so a temperature conversion has to shift the number as well as scale it.

The engine stores that shift as an offset alongside the scale, and converts as value × scale + offset into the base unit and back out again. The offset is zero for every other unit on the site, so the same code path serves both without special cases.

One consequence is worth stating plainly, because it is where temperature converters usually mislead: a reading and a step are different questions. The reading 1 °C is 33.8 °F, but a rise of 1 °C is a rise of 1.8 °F. Our temperature pages give both numbers and say which is which.

Where a unit has more than one meaning, the page says which one it uses. US liquid units are the default on volume pages, and the Imperial difference is stated on the page rather than left for you to discover.

Reference sources

The factors below are checked against standards publications, not copied from another converter. These primary sources let you check the definitions and the rounding approach for yourself:

• NIST Handbook 44, Appendix C (2026) gives tables for customary and metric units, including volume, area and the international acre. NIST identifies exact conversion factors in the tables.
• The BIPM International System of Units (SI) Brochure defines SI units and prefixes, including the kelvin and the relationship between kelvin intervals and degrees Celsius.
• NIST Special Publication 447, Weights and Measures Standards of the United States documents the 1959 international yard and pound definitions used for the foot, inch, pound and derived units on this site.
• NIST's note on the U.S. survey foot explains why current conversions use the international foot and distinguish older survey measurements.

The factor table below shows the value used for each published converter. Exact definitions are kept at full precision in the calculation; only the displayed answer is rounded. If a cited standard changes or a reader reports a mismatch, we check the unit definition and the independently validated output before changing a factor.

Factors used by the published converters

Rounding

Results show four decimal places by default, and you can change that on any converter. We round only for display: the calculation itself keeps full precision, and trailing zeroes are removed so an answer reads the way a person would say it. Where a factor does not terminate — kilograms to pounds, for example — the page says so instead of implying the result is exact.

Corrections

If a number here looks wrong, it matters more to us than any other bug. Tell us what you entered and what you expected via the contact page. We check the reported value against the defining standard, fix the stored factor if it is wrong, and the fix reaches every page that uses it at once.

We do not add invented review dates or fake author credentials to these pages. The definitions above are fixed by international agreement — most of them since 1959 — and pretending they change would be dishonest.