Unit Converter
Convert between length, weight, temperature, volume, area, and speed units. Metric ↔ imperial. All conversions instant.
The two unit systems
The world uses two main systems: metric (SI) — used by ~95% of countries — and imperial / US customary — used by the United States, Liberia, Myanmar, and a few specific contexts in the UK. Most science, medicine, and engineering uses metric globally; US consumer products, real estate, and weather forecasts typically use imperial.
Conversions matter because mixing systems causes real errors. NASA famously lost the $327M Mars Climate Orbiter in 1999 because of a metric/imperial unit mismatch in spacecraft software. For everyday use, the stakes are lower — but recipes don't scale right, foreign-spec parts don't fit, and travel decisions go wrong without conversions.
Length: meters and beyond
Useful conversion anchors to remember:
- 1 inch = 2.54 cm exactly (defined in 1959).
- 1 foot = 30.48 cm; 1 meter ≈ 3.28 feet.
- 1 mile = 1.609 km; 1 km ≈ 0.621 miles.
- 1 yard = 0.914 m (a yard is slightly less than a meter).
- 1 nautical mile = 1.852 km (used in aviation and shipping).
Weight (mass)
Useful conversions:
- 1 lb = 453.592 g; 1 kg ≈ 2.205 lb.
- 1 oz = 28.35 g; 1 g ≈ 0.035 oz.
- 1 stone = 14 lb = 6.35 kg (UK body weight).
- 1 US ton = 2,000 lb = 907 kg; 1 metric tonne = 1,000 kg ≈ 2,205 lb.
Note: “ton” is ambiguous. US short ton = 2,000 lb. UK long ton = 2,240 lb. Metric tonne = 1,000 kg. They're all different by 5–12%.
Temperature
Three scales in common use:
- Celsius (°C): water freezes at 0°C, boils at 100°C at sea level. Used by most countries.
- Fahrenheit (°F): water freezes at 32°F, boils at 212°F. Used in the US for weather and cooking.
- Kelvin (K): same scale as Celsius but starts at absolute zero (−273.15°C). Used in science. There's no “degrees” for Kelvin.
Quick mental conversion: °F ≈ 2 × °C + 30. Not exact, but close enough for everyday weather. Real formula: °F = °C × 9/5 + 32.
Volume — the recipe trap
US recipes use cups (236.6 mL), tablespoons (14.8 mL), teaspoons (4.93 mL), and fluid ounces (29.6 mL). UK recipes use Imperial cups and ounces, which are slightly different sizes. Continental European recipes use grams or milliliters directly.
For baking, weighing in grams is more accurate than measuring in cups — volumetric measurements vary with how packed the flour is. Most modern baking recipes provide both for that reason.
Area for real estate and land
US real estate uses square feet for homes and acres for land. Metric uses square meters (m²) for homes and hectares (ha) for land. Quick anchors:
- 1 m² ≈ 10.76 ft²; 1,000 ft² ≈ 92.9 m².
- 1 acre = 43,560 ft² ≈ 0.405 hectare.
- 1 hectare = 10,000 m² ≈ 2.47 acres.
- 1 km² = 100 hectares ≈ 247 acres.
Speed
Most useful conversions:
- 1 mph ≈ 1.609 km/h. Quick mental: mph × 1.6 = km/h.
- 1 m/s = 3.6 km/h ≈ 2.237 mph.
- 1 knot (nautical mile/hour) ≈ 1.852 km/h ≈ 1.151 mph.
- Speed of sound (Mach 1) ≈ 343 m/s ≈ 1,235 km/h ≈ 767 mph at sea level.
- Speed of light ≈ 299,792,458 m/s — about 1,000,000× the speed of sound.