Metric to Imperial: Complete Conversion Guide

What is the difference between metric and imperial?

The metric system is built on powers of ten: 1,000 millimeters make a meter, 1,000 meters make a kilometer, 1,000 grams make a kilogram. Every unit converts by moving a decimal point, which is why nearly every country uses it for science, trade and everyday life. The imperial system, used in the United States and partly in the United Kingdom, grew out of older customary units: 12 inches make a foot, 3 feet make a yard, 1,760 yards make a mile, and 16 ounces make a pound. The factors are irregular, so conversion means multiplying or dividing by a specific number rather than shifting a decimal point.

Because the two systems were defined independently, most conversion factors are not round numbers. The exceptions are the ones fixed by definition: the inch is defined as exactly 2.54 centimeters, and the pound is defined as exactly 0.45359237 kilograms. Every other length and weight factor follows from those two. Volume and temperature are messier — the gallon has two different definitions, and Celsius and Fahrenheit have different zero points as well as different degree sizes — so they get their own sections below.

How do I convert length: meters to feet and inches?

Length is the conversion you will use most often, for heights, room sizes, road distances and screen sizes. The key factors all come from 1 inch = 2.54 cm: 1 foot = 30.48 cm, 1 yard = 0.9144 m, and 1 mile = 1.609344 km. To convert metric to imperial, multiply by the factor in the table; to go the other way, divide by the same factor (or multiply by its reciprocal).

Height deserves special care, because it is written in two notations. A height of 5.905 feet is 5 feet plus 0.905 × 12 = 10.87 inches — 5′11″, not 5′9″. Whenever a conversion gives you a decimal number of feet, split it: the whole number is the feet, and the decimal part times 12 is the inches. A unit converter does this split for you, but knowing the step keeps you from misreading a result.

How do I convert weight: kilograms to pounds?

Weight conversion is simpler than length, because there is only one factor to remember: 1 kg = 2.20462 lb. Multiply kilograms by 2.2 for a quick estimate — the error is under 0.3% — and by 2.20462 when the number matters, for example on a shipping label or a luggage allowance. Going the other way, divide pounds by 2.20462, or multiply by 0.45359237.

Two details catch people out. First, the stone: in the UK, body weight is often quoted in stones and pounds, where 1 stone = 14 lb = 6.35029 kg. A weight of 11.81 stone means 11 stone and 0.81 × 14 = 11.3 lb — 11 st 11 lb. Second, the ounce has two meanings: the avoirdupois ounce (28.35 g) for ordinary weight, and the troy ounce (31.10 g) for precious metals. A recipe or a parcel always means the avoirdupois ounce; only gold and silver use troy.

How do I convert volume: liters to gallons?

Volume is where metric-to-imperial conversion goes wrong most often, because the gallon is not one unit. The US gallon is 3.78541 liters; the UK (imperial) gallon is 4.54609 liters, about 20% larger. The same split runs through pints and fluid ounces, so a "pint" in an American recipe and a "pint" in a British recipe are different amounts. A US pint is 473 ml; a UK pint is 568 ml. If a recipe or a fuel-economy figure does not say which gallon it uses, check the country of origin before you convert — mixing them up throws the answer off by a fifth.

Cooking adds one more trap: the cup. A US cup is 236.59 ml, a metric cup is 250 ml, and a UK cup is 284 ml. Recipes that measure flour or sugar in cups are already approximate — a cup of flour can weigh anywhere from 120 g to 150 g depending on how it is scooped — so for baking, convert cups to grams with the recipe's own weights where possible, and use a unit converter for the volume part.

How do I convert temperature: Celsius to Fahrenheit?

Temperature is the only common conversion that needs addition as well as multiplication, because the two scales start at different points: water freezes at 0 °C but 32 °F. The formula is °F = °C × 9/5 + 32. To go back, °C = (°F − 32) × 5/9. A degree Fahrenheit is smaller than a degree Celsius — 1 °C equals 1.8 °F — so Fahrenheit numbers change faster than Celsius numbers for the same change in warmth.

For a fast estimate, double the Celsius value and add 30: 20 °C becomes roughly 70 °F, against the exact 68 °F. The shortcut is good enough for weather and conversation, but use the real formula for cooking — an oven set 10 °F off can be the difference between baked and burnt. The two scales agree at exactly one point: −40 °C is −40 °F.

Worked example: convert a 10 km run, a height and a weight

Take a runner who is 180 cm tall, weighs 75 kg, and has just run 10 km in 20 °C weather, and convert every number to imperial, step by step.

Step 1 — distance. 10 km × 0.621371 = 6.21371 miles, so the run was about 6.21 miles. A 10 km race is often called a 6.2-miler for exactly this reason.

Step 2 — height. 180 cm ÷ 2.54 = 70.866 inches. Divide by 12: 70.866 ÷ 12 = 5.9055 feet. The whole feet are 5; the remainder is 0.9055 × 12 = 10.87 inches. The runner is 5 feet 10.9 inches, or 5′11″ rounded.

Step 3 — weight. 75 kg × 2.20462 = 165.3465 lb, about 165.3 lb. In stones, 75 ÷ 6.35029 = 11.81 stone, which is 11 stone and 0.81 × 14 = 11.3 lb — 11 st 11 lb.

Step 4 — temperature. 20 °C × 9/5 = 36, plus 32 = 68 °F. Check it backwards: (68 − 32) × 5/9 = 36 × 5/9 = 20 °C. The round trip confirms the arithmetic.

Practical tips

• Memorize four anchors, not forty factors. 1 in = 2.54 cm, 1 kg = 2.2 lb, 1 km = 0.62 mi, and °F = °C × 1.8 + 32 cover most everyday conversions; look up the rest.
• Always ask which gallon. US and UK gallons differ by 20%, and the same split applies to pints and fluid ounces. Check the country before converting any volume.
• Never read decimal feet as feet and inches. 5.9 feet is 5 feet 11 inches, not 5 feet 9 inches. Convert the decimal part back to inches by multiplying by 12.
• Round to the precision of the original. A height given as 180 cm (three significant figures) deserves 5′11″, not 5′10.866″. Extra digits are noise, not accuracy.
• Use the estimate, then check it. Double-and-add-30 for temperature, divide-by-2.5 for centimeters, and multiply-by-0.6 for kilometers give fast sanity checks; run the exact factor before you act on the number.
• Keep units in the math. Writing "10 × 0.621371 = 6.21 miles" instead of just "6.21" stops the classic error of converting twice or converting in the wrong direction.

Frequently asked questions

How do I convert cm to inches?

Divide the centimeter value by 2.54, because one inch is exactly 2.54 cm. So 180 cm ÷ 2.54 = 70.87 inches. For a rough answer in your head, divide by 2.5 instead — the error is under 2%. The same rule works in reverse: multiply inches by 2.54 to get centimeters. A unit converter handles the division for you when you have a long list of measurements to convert.

How many pounds is 1 kg?

One kilogram equals 2.20462 pounds, so multiply kilograms by 2.2 for a quick estimate and by 2.20462 for an exact figure. For example, 75 kg × 2.20462 = 165.35 lb. Going the other way, one pound is 0.45359237 kg, so divide pounds by 2.20462 to get kilograms. Remember that the pound used here is the avoirdupois pound — the standard pound for body weight and everyday goods in the US and UK.

What is the difference between a US gallon and a UK gallon?

A UK (imperial) gallon is 4.54609 liters, while a US gallon is 3.78541 liters — the UK gallon is about 20% larger. The same split applies to pints and fluid ounces: a UK pint is 568 ml and a US pint is 473 ml. This is the most common conversion trap in cooking and fuel economy. Always check which gallon a recipe or a miles-per-gallon figure uses before you convert, because mixing them up throws the answer off by a fifth.

How do I convert Celsius to Fahrenheit?

Multiply the Celsius temperature by 9/5 (1.8) and add 32. For example, 20 °C × 9/5 = 36, plus 32 = 68 °F. To go from Fahrenheit to Celsius, subtract 32 and multiply by 5/9: (68 − 32) × 5/9 = 20 °C. The two scales meet at −40, which is −40 in both. A quick estimate is to double the Celsius value and add 30 — 20 °C gives roughly 70 °F, close enough for weather but not for cooking or science.

How tall is 180 cm in feet and inches?

180 cm is 5 feet 10.9 inches, usually rounded to 5′11″. The math: 180 ÷ 2.54 = 70.87 inches total. Divide by 12 to get 5.905 feet, so the whole feet are 5, and the remainder is 0.905 × 12 = 10.87 inches. The common mistake is to write 5.9 feet and read it as 5 feet 9 inches — it is not. Decimal feet and feet-plus-inches are different notations, so always convert the decimal part back into inches.

Why does the UK use miles but sell petrol in litres?

The UK runs a mixed system. Road distances and speed limits stayed imperial when the country metricated in the 1970s, so signs show miles and miles per hour. Petrol, food, and most everyday goods moved to metric, so fuel is sold in litres and groceries are weighed in grams and kilograms. People also still quote body weight in stones and pounds and height in feet and inches. Knowing both systems — and which one a given number uses — is the only way to read British measurements correctly.

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