Most of the world measures in metric, while a few countries — the United States chief among them — still use imperial units in daily life. If you cook, travel, shop online or read a spec sheet, sooner or later you have to convert between them. Here is the short version.
Why two systems?
The metric system (officially the SI) is built around powers of ten: a kilometer is 1,000 meters, a kilogram is 1,000 grams. That makes it easy to scale up and down. The imperial system grew out of older customary units — inches, feet, pounds and gallons — with less tidy relationships between them (12 inches to a foot, 16 ounces to a pound).
The conversions you actually need
You don’t have to memorise everything. These handful of factors cover the vast majority of everyday conversions:
From
To
Multiply by
Inches
Centimeters
2.54
Feet
Meters
0.3048
Miles
Kilometers
1.60934
Pounds
Kilograms
0.453592
Ounces
Grams
28.3495
Gallons (US)
Liters
3.78541
To go the other way, divide instead of multiply. Rather than doing it by hand, drop the numbers into our unit converter, which covers length, weight, temperature, volume, area, speed and digital storage.
Temperature is the exception
Length and weight are linear — one factor does the job. Temperature is different because the scales don’t share a zero point, so you use a formula: °F = (°C × 9/5) + 32, and to reverse it, °C = (°F − 32) × 5/9.
Common mistakes to avoid
Mixing up ounces. A fluid ounce measures volume; a weight ounce measures mass. They are not the same thing.
US vs UK gallons. A US gallon (3.785 L) is smaller than a UK/imperial gallon (4.546 L).
Multiply the kilograms by 2.20462 to get pounds. For example, 10 kg is about 22.05 lb.
Why two systems still exist
The metric system was designed once, deliberately, on a decimal base, with each unit defined against a physical constant. Imperial units accumulated over centuries from practical trade measures — the foot, the grain, the barrel — and were only later pinned to metric definitions. Today the inch, pound and gallon are all defined in metric terms, so every conversion between them is exact rather than approximate.
Three countries have not adopted metric as their primary system: the United States, Liberia and Myanmar. Even in the US, science, medicine, the military and most manufacturing run metric; the imperial layer is retail and everyday speech.
Exact conversions, by family
Quantity
Conversion
Exact?
Length
1 in = 25.4 mm
Yes
Length
1 ft = 0.3048 m
Yes
Length
1 mi = 1,609.344 m
Yes
Mass
1 lb = 0.45359237 kg
Yes
Mass
1 oz = 28.349523125 g
Yes
Volume
1 US gal = 3.785411784 L
Yes
Volume
1 imp gal = 4.54609 L
Yes
Area
1 acre = 4,046.8564224 m²
Yes
Temperature
°F = °C × 9/5 + 32
Exact formula
Mental arithmetic that is close enough
km to miles: multiply by 0.6 (5 km ≈ 3 mi). Or use consecutive Fibonacci numbers — 8 km ≈ 5 mi, 13 km ≈ 8 mi — which is accurate to under 1%.
°C to °F: double and add 30. 20 °C → 70 (true value 68).
kg to lb: double and add 10%. 70 kg → 154 (exact).
cm to inches: divide by 2.5 and take off 2%.
Litres to US gallons: divide by 4 and add 5%.
The famous failures
Unit confusion is not a trivia topic. NASA lost the $125 million Mars Climate Orbiter in 1999 because one team supplied thruster impulse in pound-force seconds while the navigation software expected newton-seconds. Air Canada Flight 143 became the “Gimli Glider” in 1983 after a fuel load was computed in pounds instead of kilograms, leaving the aircraft with roughly half the fuel it needed and forcing an unpowered landing. Both cases were competent people using correct arithmetic on mislabelled numbers.
The defence is boringly simple: carry the unit with the number everywhere it goes, and state it explicitly at every interface between two systems or two teams.
Significant figures
Converting introduces false precision. “About 5 miles” is not 8.04672 km — it is about 8 km. Round the answer to the precision of the input, and carry full precision only through intermediate steps. Our unit converter computes at full precision and lets you round at the end, which is the right order.
