Conversion factors on this site come from published definitions rather than from other converters. Where a unit has an exact legal or standards-body definition, that exact value is stored and never rounded.
Linked references
Each document below was opened and its title read off the live page on the date shown. These are the references our guides and core conversion pages cite directly.
| Document | Publisher | Used for | Link checked |
|---|---|---|---|
| 계량에 관한 법률 | 국가법령정보센터 (Korea Ministry of Government Legislation) | The legal status of Korea's traditional 척관법 units: metric units are required for 거래·증명, which is why pyeong and geun are described as everyday rather than legal units. | 30 August 2026 |
| SI Brochure — The International System of Units (SI), 9th edition | BIPM (Bureau International des Poids et Mesures) | Definitions of the SI base and derived units, and the SI prefixes. | 12 August 2026 |
| Special Publication 811 — Guide for the Use of the International System of Units (SI) | NIST (US National Institute of Standards and Technology) | Conversion factors between US customary and SI units, and the rounding and significant-figure conventions this site follows. | 12 August 2026 |
| Special Publication 330 — The International System of Units (SI), 2019 Edition | NIST | The US edition of the SI definitions, including the 2019 redefinition. | 12 August 2026 |
| Definitions of the SI units: The binary prefixes | NIST | The IEC binary prefixes (kibi, mebi, gibi) and why they differ from kilo, mega, giga. | 12 August 2026 |
| Metric (SI) Program | NIST Office of Weights and Measures | US metric usage, including the retirement of the US survey foot. | 12 August 2026 |
| Publications and Documentary Standards (NIST Handbook 44, Handbook 133) | NIST Office of Weights and Measures | US legal metrology — the handbooks that define commercial weighing and measuring requirements adopted by all 50 states. | 12 August 2026 |
| CSS Values and Units Module Level 4 | W3C (World Wide Web Consortium) | The CSS definition that fixes 1 in = 96 px, which is what makes a screen 'inch' different from a printed one. | 12 August 2026 |
| Saffir-Simpson Hurricane Wind Scale | NOAA National Hurricane Center | A worked example of the same wind speed published simultaneously in mph, knots, and km/h. | 12 August 2026 |
| Learn More About the Fuel Economy Label for Gasoline Vehicles | US EPA / US Department of Energy (fueleconomy.gov) | How US fuel economy labels report MPG alongside gallons per 100 miles, and why consumption-per-distance is the fairer comparison. | 12 August 2026 |
| FoodData Central | USDA Agricultural Research Service | Food composition and portion-weight data behind volume-to-weight cooking conversions. | 12 August 2026 |
Other standards relied on
These are consulted for specific unit families. They are named rather than linked because they are paywalled or distributed only as purchasable documents — we would rather name the standard precisely than link to a third-party copy of it.
- The international yard and pound agreement of 1959: 1 yard = 0.9144 m and 1 pound = 0.45359237 kg, both exact. This is the basis for every length and mass conversion between the two systems.
- IEC 80000-13 for the binary prefixes (kibibyte, mebibyte, gibibyte). NIST's linked summary above covers the same definitions.
- ASTM E380 and ASTM E96 for engineering conversion factors and water-vapor permeance (the perm).
- ISO 80000-3 for the neper and bel, and ICRU reports for the roentgen, gray, and sievert.
Selected exact definitions used
| Relationship | Value |
|---|---|
| 1 inch | 0.0254 m (exact) |
| 1 foot | 0.3048 m (exact) |
| 1 mile | 1609.344 m (exact) |
| 1 pound | 0.45359237 kg (exact) |
| 1 US gallon | 3.785411784 L (exact) |
| 1 Imperial gallon | 4.54609 L (exact) |
| 1 acre | 4046.8564224 m² (43,560 ft²) |
| Standard atmosphere | 101,325 Pa (exact) |
| 1 pound-force | 4.4482216152605 N |
| 1 calorie (thermochemical) | 4.184 J (exact) |
| 1 calorie (international table) | 4.1868 J (exact) |
| 1 BTU (international table) | 1055.05585262 J (exact) |
| 1 roentgen | 2.58 × 10⁻⁴ C/kg (exact) |
| 1 oersted | 1000/4π A/m ≈ 79.5774715 A/m |
| 1 board foot | 1/12 ft³ (exact) |
| 1 point (PostScript) | 1/72 inch (exact) |
Approximate values
A few units depend on context and are marked as approximate in the tool. Average calendar month and year lengths use the Gregorian calendar (365.2425 days per year). Sea-level standard values are noted where relevant. These are labeled so you can apply a more precise figure when your application requires it.
Some families are conventional rather than defined: fluid-column pressures (inches of water at 4 °C or 60 °F) assume a stated reference temperature, legacy media capacities are nominal, and the water-vapor perm exists in both 0 °C and 23 °C variants. Where a published value differs between sources, this site uses the exact modern definition and notes the temperature or reference basis in the unit name.