Converting Units: Legal Units and Exact Factors
Inches to cm, miles to km, Fahrenheit to Celsius: the exact conversion factors, the legal units in Germany and how prefixes such as kilo and milli work.
Unit Converters3 linked calculatorsReading time: 7 min.
If you convert measurements from recipes, assembly instructions or travel guides, you need factors you can rely on. Many Anglo-American units are now defined exactly in terms of the metre and the kilogram – an inch is exactly 2.54 centimetres. This overview shows the legal units, the prefixes and the most important conversions.
Key takeaways
- In official and commercial dealings in Germany, legal units are mandatory (section 1(1) Units and Time Act, EinhZeitG).
- They are based on the International System of Units (SI) with seven base units, including the metre, kilogram and second.
- Prefixes such as kilo (10³) and milli (10⁻³) may only be used one at a time (section 1(4) Units Ordinance, EinhV).
- Inch, foot, mile and pound are defined exactly via the metre and kilogram: 1 inch = 2.54 cm, 1 pound = 0.45359237 kg.
- Celsius to Fahrenheit: °F = °C × 1.8 + 32; Celsius to kelvin: K = °C + 273.15.
Legal units in Germany
In official and commercial dealings in Germany, quantities must be stated in legal units (section 1(1) EinhZeitG). The Units Ordinance defines them: the units in its Annex 1 – from ampere through hectare and litre to watt – and all units derived from them with the factor 1 (section 1(1) EinhV).
The units are based on the International System of Units (SI) of the Metre Convention (section 3(1) EinhZeitG). Its seven base units are:
- second (s) – time
- metre (m) – length
- kilogram (kg) – mass
- ampere (A) – electric current
- kelvin (K) – thermodynamic temperature
- mole (mol) – amount of substance
- candela (cd) – luminous intensity
Who provides the units
The Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute, realises and disseminates the legal units and legal time (section 6(2) EinhZeitG). The time signals of radio-controlled clocks in Germany, for example, go back to it.
Prefixes: from nano to giga
Multiples and fractions of a unit are formed with prefixes. The Units Ordinance lists them in Annex 2; the most common ones:
| Prefix | Symbol | Factor |
|---|---|---|
| giga | G | 109 = 1,000,000,000 |
| mega | M | 106 = 1,000,000 |
| kilo | k | 103 = 1,000 |
| hecto | h | 102 = 100 |
| deci | d | 10-1 = 0.1 |
| centi | c | 10-2 = 0.01 |
| milli | m | 10-3 = 0.001 |
| micro | µ | 10-6 = 0.000001 |
| nano | n | 10-9 = 0.000000001 |
Only one prefix may be used at a time (section 1(4) EinhV): so nanometre, not "millimicrometre". No prefixes are applied to the hour, day, minute, degree Celsius or kilogram (section 1(3) EinhV) – multiples of the kilogram are formed from the gram, such as the milligram.
Common conversions with exact factors
The following factors are exact, not rounded: inch, foot, yard, mile and pound have been defined via the metre and the kilogram since an international agreement of 1959, and the nautical mile as 1,852 metres.
| Unit | Value | equals |
|---|---|---|
| Inch | 1 in | 2.54 cm |
| Foot | 1 ft | 0.3048 m |
| Yard | 1 yd | 0.9144 m |
| Mile | 1 mi | 1.609344 km |
| Nautical Mile | 1 nmi | 1.852 km |
| Ounce | 1 oz | 28.349523125 g |
| Pound (lb) | 1 lb | 0.45359237 kg |
| Fluid Ounce (US) | 1 fl oz | 29.5735295625 ml |
| Gallon (US) | 1 gal | 3.785411784 l |
| Gallon (UK) | 1 gal (UK) | 4.54609 l |
| Acre | 1 ac | 4,046.8564224 m² |
| Miles per hour | 1 mph | 1.609344 km/h |
| Knots | 1 kn | 1.852 km/h |
Values from our converters, not rounded. Definitions according to NIST Special Publication 811 (Appendix B); the gallon and pint differ between the US and the UK.
Convert lengths
Inches, feet, miles, nautical miles and metric lengths.
Convert weights
Ounces, pounds, carats and metric masses.
Temperature: Celsius, Fahrenheit, kelvin
The kelvin is the SI unit of thermodynamic temperature; the degree Celsius is a legal unit as well. Both scales have steps of the same size: K = °C + 273.15. Fahrenheit is not a legal unit in Germany; convert with °F = °C × 1.8 + 32 and °C = (°F − 32) ÷ 1.8.
| °C | °F | K |
|---|---|---|
| -40 | -40 | 233.15 |
| 0 | 32 | 273.15 |
| 20 | 68 | 293.15 |
| 37 | 98.6 | 310.15 |
| 100 | 212 | 373.15 |
Values to remember
At −40 degrees, Celsius and Fahrenheit show the same number. 0 °C (freezing point of water) is 32 °F, 100 °C is 212 °F.
Convert temperature
Celsius, Fahrenheit and kelvin in both directions.
Frequently asked questions
How many centimetres are in an inch?
How do I convert miles to kilometres?
How do I convert Fahrenheit to Celsius?
Is a pound 500 grams?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- Units and Time Act (EinhZeitG), section 1 (use of legal units) (accessed September 30, 2026)
- EinhZeitG, section 3 (SI as the basis) (accessed September 30, 2026)
- EinhZeitG, section 6 (tasks of the PTB) (accessed September 30, 2026)
- Units Ordinance (EinhV), section 1 (accessed September 30, 2026)
- EinhV, Annex 1 (units) (accessed September 30, 2026)
- EinhV, Annex 2 (prefixes) (accessed September 30, 2026)
- International Bureau of Weights and Measures: SI Brochure (accessed September 30, 2026)
- National Institute of Standards and Technology: Special Publication 811 (conversion factors) (accessed September 30, 2026)
- Physikalisch-Technische Bundesanstalt (PTB) (accessed September 30, 2026)
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