Metric vs Imperial Units: A Practical Conversion Guide

What is the difference between metric and imperial?

The metric system, formally the International System of Units (SI), was developed in France in the 1790s. Its units scale in powers of ten, and prefixes tell you the multiplier: a kilometer is 1,000 meters, a centimeter is a hundredth of a meter and a milligram is a thousandth of a gram. Converting within the system means moving a decimal point.

Imperial and U.S. customary units grew out of older English measures based on everyday references such as feet, thumbs and grains of barley. The relationships are irregular: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound and 14 pounds in a stone. That makes mental conversion within the system harder, though the units are familiar and convenient at human scale.

Which countries use the imperial system?

The United States is the only large country that uses customary units for most everyday purposes, alongside Liberia and, partly, Myanmar. Even in the U.S., science, medicine, the military and many industries work in metric.

The United Kingdom officially uses metric for most trade, but imperial units survive in daily life: road signs show miles and miles per hour, beer and milk are sold in pints, and many people give their height in feet and inches and their weight in stones. Canada is similar, with metric officially but feet and pounds still common in conversation.

Exact conversion factors worth knowing

Since the international yard and pound agreement of 1959, imperial length and mass units have been defined exactly in metric terms. That means these factors are not approximations; they are the definitions, and every other conversion follows from them.

• 1 inch = 2.54 centimeters exactly.
• 1 foot = 30.48 centimeters, or 0.3048 meters.
• 1 yard = 0.9144 meters.
• 1 mile = 1.609344 kilometers.
• 1 pound = 0.45359237 kilograms exactly; 1 ounce ≈ 28.35 grams.
• 1 stone = 14 pounds ≈ 6.35 kilograms.

Worked examples: height, distance and body weight

To convert a height of 180 cm to feet and inches, divide by 2.54 to get about 70.87 inches. Twelve inches go into that 5 times with 10.87 left over, so 180 cm is about 5 ft 10.9 in. Running distances work the same way: a 10 km race is 10 ÷ 1.609344 ≈ 6.21 miles, and a marathon of 42.195 km is about 26.22 miles.

For weight, 70 kg ÷ 0.45359237 ≈ 154.32 lb. Going the other way, someone who weighs 11 stone 4 pounds has (11 × 14) + 4 = 158 lb, which is about 71.7 kg. The weight converter handles kilograms, pounds, ounces and stones directly.

Health formulas are a common reason to convert. BMI is defined in kilograms and meters, so imperial measurements need either converting or the 703 multiplier. Our BMI calculator accepts both systems.

If you are unsure how to read the result, our guide on what BMI is and what your number means explains the result.

How to convert Celsius and Fahrenheit

Temperature is different from length and weight because the two scales have different zero points as well as different step sizes. To go from Celsius to Fahrenheit, multiply by 9/5 (or 1.8) and add 32: 25°C × 1.8 + 32 = 77°F. To go back, subtract 32 and multiply by 5/9: (98.6 − 32) × 5/9 = 37°C.

Oven temperatures are a frequent source of kitchen mistakes. 180°C is exactly 356°F, but recipes usually round it to 350°F, which is about 176.7°C. The two scales meet at −40, where −40°C equals −40°F. Scientists also use kelvin, which has the same step size as Celsius but starts at absolute zero, so 0°C is 273.15 K. Our temperature converter switches between all three.

Quick mental shortcuts for metric and imperial

When you just need a rough idea, such as reading a road sign or a weather forecast, these shortcuts are close enough. Use exact factors for anything that matters, like medication doses, engineering or paperwork.

• Kilometers to miles: multiply by 0.6 (10 km ≈ 6 miles; exact is 6.21).
• Miles to kilometers: multiply by 1.6.
• Kilograms to pounds: multiply by 2.2 (70 kg ≈ 154 lb).
• Pounds to kilograms: halve, then subtract 10% (150 lb ≈ 67.5 kg; exact is 68.04).
• Celsius to Fahrenheit: double and add 30 (10°C ≈ 50°F; accuracy drops at higher temperatures, so 25°C gives 80 rather than 77).
• Feet: one foot is a little over 30 cm, and one meter is about 3 feet 3 inches.

Common conversion mistakes to avoid

The U.S. and UK systems are not identical. A U.S. gallon is about 3.785 liters while an imperial (UK) gallon is 4.546 liters, and a U.S. fluid ounce (about 29.57 ml) is slightly larger than a UK fluid ounce (about 28.41 ml). Fuel economy figures and recipes from the two countries are therefore not directly comparable.

Unit mix-ups can be costly. In 1999 NASA lost the Mars Climate Orbiter because one team's software produced thrust data in pound-force seconds while another expected newton-seconds. Always label units, keep full precision until the final step and round only the answer. You can find all of our unit tools on the unit converters page.