Unit Converter
Convert between any units of measurement: length, weight, volume, temperature and more.
The SI System — Why Unit Conversion Works
Every SI unit is defined against seven base units (metre, kilogram, second, ampere, kelvin, mole, candela). Converting is a ratio of definitions: since 1 mi = 1609.344 m exactly (by the 1959 international yard agreement), the conversion is exact, not approximate.
The ratio method also handles chained conversions: 3 ft/s → km/h = 3 × 0.3048 (m per ft) ÷ 1000 × 3600 (s per h) = 3.292 km/h. Keeping units in the numerator/denominator ('dimensional analysis') catches flipped ratios — if units don't cancel, the ratio is upside down.
Worked Example: 55 mph → m/s
55 mi/h × 1609.344 m/mi = 88,513.9 m/h
÷ 3600 s/h = 24.59 m/s
Shortcut: mph × 0.44704 = m/s (the combined exact constant)
Check: 60 mph ≈ 26.8 m/s — highway speeds are ~25-30 m/s
Learn the handful of exact anchors: 1 in = 2.54 cm (exact), 1 mi = 1609.344 m, 1 lb = 0.45359237 kg. Everything else chains from them.
Frequently Asked Questions
Which conversions are exact vs approximate?
By international agreement: inch-cm (2.54 exact), mile-metre (1609.344 exact), pound-kg (0.45359237 exact). Approximate ones come from measurement: litre-gallon (US 3.785412 L), and older units like the nautical mile (1852, adopted by convention).
Why does the US still use customary units?
Inertia plus infrastructure: every road sign, recipe, and tool dimension would change. US law has defined customary units in metric terms since 1893 (Mendenhall Order) — a US inch IS 2.54 cm by definition. Science, medicine, and the military in the US already run metric.
What is dimensional analysis?
Multiplying by ratios that equal 1 written with units (60 min/1 h). Units cancel algebraically like variables, catching errors: if 'mi' doesn't cancel when converting speed, a ratio is flipped. It is the fastest error-check in all of physics.
Are US and UK gallons the same?
No — the last trap. US gallon = 3.785 L; imperial (UK) gallon = 4.546 L (20% bigger). Same for pints and fluid ounces. Always check which gallon a recipe or fuel spec means.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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