Measure & Convert · Facts & Figures
U.S. Measurement & SI Reference
Explore the measurement system behind everyday U.S. units: the seven SI base units, exact U.S.–SI relationships, temperature formulas, and the standards that keep conversions consistent.
Interactive reference
Common exact length relationships
Switch among common conversion families. Exact relationships are labeled separately from formula-based temperature conversions.
The SI foundation
Seven base units anchor modern measurement
| Quantity | SI base unit | Symbol | Definition anchored to |
|---|---|---|---|
| Time | second | s | Cesium-133 transition frequency |
| Length | meter | m | Speed of light in vacuum |
| Mass | kilogram | kg | Planck constant |
| Electric current | ampere | A | Elementary charge |
| Temperature | kelvin | K | Boltzmann constant |
| Amount of substance | mole | mol | Avogadro constant |
| Luminous intensity | candela | cd | Luminous efficacy of specified radiation |
Since the 2019 SI revision, the base units are defined through fixed numerical values of fundamental constants rather than physical artifacts.
Common U.S. ↔ SI relationships
Useful conversion benchmarks
| From | To | Relationship | Type |
|---|---|---|---|
| 1 inch | centimeters | 2.54 cm | Exact |
| 1 foot | meters | 0.3048 m | Exact |
| 1 yard | meters | 0.9144 m | Exact |
| 1 mile | kilometers | 1.609344 km | Exact |
| 1 pound (avoirdupois) | kilograms | 0.45359237 kg | Exact relationship |
| 1 U.S. gallon | liters | 3.785411784 L | Exact relationship |
| 1 mile/hour | kilometers/hour | 1.609344 km/h | Exact |
| 1 psi | kilopascals | 6.894757 kPa | NIST tabulated factor |
Temperature is different
Temperature conversion requires an offset, not just multiplication
Unlike length or mass, Fahrenheit and Celsius scales have different zero points. A correct converter therefore applies both a scale factor and an offset.
U.S. measurement context
The United States already rests on the SI underneath
NIST states that the SI is the preferred measurement system for U.S. trade and commerce and that U.S. customary units are based on SI units. The international foot has been the sole U.S. foot definition for new applications since January 1, 2023, when the U.S. survey foot was retired.
Metric measurements are also common in U.S. healthcare, nutrition labels, automotive products, sports, beverages, science, engineering, manufacturing, and many consumer goods.
What the reference shows
Conversion factors are standards, not arbitrary calculator constants
A conversion calculator is reliable only when its factors and formulas trace back to stable definitions. NIST’s SI guidance and conversion-factor appendices provide that reference layer. Exact factors should be stored at full precision and rounded only when a result is displayed to a user.
Methodology & source
About these figures
This page uses NIST’s current Metric (SI) Program, current SI base-unit definitions, Metric Policy guidance, and NIST SP 811 conversion-factor tables. Where NIST identifies a relationship as exact, CalculateMeasure preserves that precision. Displayed approximations are labeled accordingly.
Sources: NIST Metric (SI) Program · Definitions of SI Base Units · NIST SP 811 Appendix B.8 · NIST Metric Policy.
CalculateMeasure refresh: Aug. 22, 2026 · Review cadence: Standards-triggered, with an annual source-link and guidance check.
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