Unit Converter Guide: Metric, Imperial, and the Quirky Conventions in Between
From recipe ingredients to flight altitudes to fabric measurements, the world runs on a tangled mix of unit systems. A unit converter handles the math, but knowing which units are appropriate where — and the gotchas that catch people off guard — is the harder part. This guide covers it.
Most of the world uses metric. The US, Liberia, and Myanmar use imperial (or close variants) for most everyday measurements. The UK uses metric officially but imperial colloquially — pints in pubs, miles on roads, kilograms at the doctor. Aviation uses feet for altitude, nautical miles for distance, and knots for speed, regardless of country. The result is that international communication often requires conversion — and getting it wrong has real consequences.
The Mars Climate Orbiter (1999) crashed because one team used metric Newtons and another used pounds-force. The Gimli Glider (1983) ran out of fuel because of a kilograms-vs-pounds mixup. These are spectacular examples of an everyday problem: every kitchen, gym, and hardware store deals with this on a smaller scale.
Quick reference: most-used conversions
| From | To | Multiply by | Example |
|---|---|---|---|
| Inches | Centimeters | 2.54 | 5 in = 12.7 cm |
| Centimeters | Inches | 0.3937 | 10 cm = 3.94 in |
| Feet | Meters | 0.3048 | 6 ft = 1.83 m |
| Miles | Kilometers | 1.609 | 60 mi = 96.6 km |
| Pounds | Kilograms | 0.4536 | 150 lb = 68.0 kg |
| Ounces | Grams | 28.35 | 8 oz = 226.8 g |
| Fahrenheit → Celsius | — | (°F − 32) × 5/9 | 70°F = 21.1°C |
| Celsius → Fahrenheit | — | °C × 9/5 + 32 | 20°C = 68°F |
| Fluid ounces (US) | Milliliters | 29.57 | 16 oz = 473 mL |
| Cups (US) | Milliliters | 236.6 | 1 cup = 237 mL |
| Gallons (US) | Liters | 3.785 | 10 gal = 37.85 L |
| Acres | Square meters | 4047 | 1 ac = 4,047 m² |
| MPG (US) → L/100km | — | 235.21 ÷ MPG | 30 MPG = 7.84 L/100km |
The four traps that catch everyone
First: imperial volume units differ between the US and UK. A US gallon is 3.785 L; a UK 'imperial' gallon is 4.546 L. The same 'gallon' word can mean either depending on context. UK pints are 568 mL; US pints are 473 mL. Always check which system a recipe or specification means.
Second: weight vs mass vs force. A kilogram is mass; a Newton is force; a pound can mean either depending on context (pound-mass vs pound-force). For everyday weighing on Earth they're effectively interchangeable, but in physics and engineering the distinction matters.
Third: temperature is offset, not just scaled. You don't multiply Celsius by some factor to get Fahrenheit — there's an addition step. 0°C = 32°F, but 0°F = -17.8°C. The intuition that 'twice as hot' is meaningful in F or C is wrong; only Kelvin is a true ratio scale.
Fourth: cooking measurements are sometimes by volume (US) and sometimes by weight (Europe). A cup of flour is between 120g and 150g depending on how packed it is. Recipe accuracy is much better when you weigh dry ingredients in grams instead of measuring in cups.
Categories this calculator handles
- Length / distance
- mm, cm, m, km, in, ft, yd, mi, nautical mi
- Weight / mass
- mg, g, kg, t (metric ton), oz, lb, st (stone)
- Volume
- mL, L, m³, US fl oz, UK fl oz, US cup, US pt, US qt, US gal, UK gal
- Temperature
- °C, °F, K, °R (Rankine)
- Speed
- m/s, km/h, mph, knots, ft/s
- Area
- mm², cm², m², ha, km², in², ft², yd², ac, mi²
- Energy
- J, kJ, kcal (Cal), kWh, BTU
- Pressure
- Pa, kPa, bar, psi, atm, mmHg
When precision actually matters
- •Medication dosing — never round; use the exact converted dose your prescriber specified.
- •Engineering tolerances — converted values inherit the precision of the source. 1 inch is exactly 25.4 mm — not 25 mm.
- •Cooking high-stakes baking — gram measurement always beats cups for repeatability.
- •Aviation and marine navigation — wrong unit can be life-threatening; standards exist for a reason.
- •International shipping — declared weight in pounds vs kilograms changes customs forms.
Extended FAQ
Why are temperatures different between Celsius and Fahrenheit by an offset, not just a multiplier?
Celsius was designed around water's freezing (0°C) and boiling (100°C) points at standard pressure. Fahrenheit was designed around brine (0°F) and a flawed estimate of body temperature (96°F, later refined to 98.6°F). The two scales align at -40° (the same temperature in both) but diverge in opposite directions from there.
Why do US and UK gallons differ?
Historical accident — different definitions of the gallon based on different reference volumes (US wine gallon vs UK imperial gallon, set in 1824). Both still in use; both 'official' for their respective countries.
Is the calculator's conversion to many decimal places correct?
Conversions use exact factors where they exist (1 inch = 25.4 mm exactly) and high-precision values elsewhere. Display rounds to a sensible number of digits — internally the math is precise to ~15 significant figures.
What about archaic or local units?
We support common international units. For specialized units (Korean pyeong, Japanese tsubo, sailing fathoms, etc.) you may need a specialty calculator. The most-used 50 units cover ~99% of everyday queries.
Are my conversion inputs saved?
No. The calculator runs entirely in your browser.