Unit Converter guide
Convert length, weight, and US customary volume between metric and imperial units using exact definitions. Pick a unit and the target list shows only units that make sense, so you never convert cups to feet by accident.
How conversions are done
Every unit is converted to a base unit first (meters for length, kilograms for weight), then to the target unit. Converting 10 inches to centimeters is 10 × 0.0254 ÷ 0.01 = 25.4 cm. Routing through one base unit keeps every pair consistent and avoids compounding rounding errors.
Length and weight cannot be mixed. Converting feet to pounds has no meaning, so pick units from the same group.
Exact definitions worth knowing
Since the 1959 international yard and pound agreement, US customary units are defined in metric terms. One inch is exactly 2.54 centimeters. One foot is exactly 0.3048 meters. One mile is exactly 1,609.344 meters. One pound is exactly 0.45359237 kilograms. One ounce is 1/16 of a pound, about 28.35 grams.
Those exact values are why a converter can be precise. Rounded shortcuts like "a kilo is 2.2 pounds" are fine for groceries but off by about 0.2 percent.
Worked examples
Height: 5 feet 10 inches = 5 × 0.3048 + 10 × 0.0254 = 1.524 + 0.254 = 1.778 m, or 177.8 cm. Enter feet and inches separately and add the results, or convert 70 inches directly.
Running: a marathon is 26.2 miles. 26.2 × 1.609344 = 42.16 km. The official distance is 42.195 km, because the true figure is 26.219 miles. A 5K is 3.107 miles.
Body weight: 150 lb × 0.453592 = 68.04 kg. Baking: 12 oz of chocolate chips = 12 × 28.35 = 340 g.
Mental shortcuts
Kilometers to miles: multiply by 0.6 (or by 5 and divide by 8). 10 km ≈ 6.2 miles. Meters to feet: multiply by 3.3. Kilograms to pounds: double and add 10 percent. 70 kg ≈ 140 + 14 = 154 lb, very close to the exact 154.3.
Centimeters to inches: divide by 2.5 for a quick estimate. 180 cm ≈ 72 inches, which is 6 feet.
Metric prefixes decoded
Once you know the prefixes, metric is effortless. Kilo means 1,000, centi means 1/100, milli means 1/1,000. A kilometer is 1,000 meters, a centimeter is 1/100 of a meter, a millimeter is 1/1,000. The same prefixes apply to grams and liters, so a milligram is 1/1,000 of a gram and a kilogram is 1,000 grams.
US customary units have no such pattern: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile, 16 ounces to a pound. That is exactly why a converter helps, and why every US engineering and science field works in metric behind the scenes.
Where precision matters
For cooking and fitness, one decimal place is fine. For construction, machining, and medicine, it is not. A 0.2 percent error on a kilogram-to-pound shortcut is harmless at the gym, but in medication dosing by weight, always use exact conversion and follow the prescriber's units. Aviation famously learned this the hard way when a fuel calculation mixed pounds and kilograms.
Round only at the end. Converting 5 feet 10 inches to centimeters step by step with rounded intermediate values can drift by a centimeter. Converting 70 inches directly gives exactly 177.8 cm.
Common gotchas
Fluid ounces are volume, not weight. A cup of flour weighs about 4.25 oz, while a cup of water is 8 fluid ounces and weighs about 8.3 oz. For baking by weight, use a kitchen scale.
US and imperial units differ for volume: a US gallon is 3.785 liters and an imperial gallon is 4.546 liters. Tons differ too: a US short ton is 2,000 lb, a metric tonne is 1,000 kg (2,204.6 lb). Results are rounded to six decimal places, which is more precision than any tape measure. Everything runs in your browser.
How we calculate: sources
Frequently asked questions
Which units can I convert?
Length: mm, cm, m, km, in, ft, yd, mi. Weight: g, kg, oz, lb, st. Volume: tsp, tbsp, fl oz, cup, pint, quart, gallon, mL, L. The target list only shows units from the same category as your starting unit.
How many milliliters are in a cup?
A US customary cup is 8 US fluid ounces, about 236.6 mL. US nutrition labels round a cup to 240 mL, and a metric cup used in Australia and Canada is 250 mL.
How many liters are in a gallon?
Exactly 3.785411784 liters in a US gallon. An imperial (UK) gallon is larger, about 4.546 liters, and this converter uses the US gallon.
How many tablespoons are in a cup?
16. A cup is 16 tablespoons, a tablespoon is 3 teaspoons, and 2 tablespoons make 1 fluid ounce.
How many pounds are in a stone?
14 pounds, about 6.35 kg. Stone is still common for body weight in the UK and Ireland.
How much is a pint in the US?
A US pint is 16 fluid ounces, about 473.2 mL. Two pints make a quart, and four quarts make a gallon.
Is my data kept private?
Everything runs in your browser. Nothing you enter is uploaded to a server or stored by us.
How many centimeters are in an inch?
Exactly 2.54 centimeters. It has been defined that way since 1959.
How do I convert kg to lbs quickly?
Multiply by 2.2046. For a mental estimate, double the kilograms and add 10 percent.