The Direct Answer
Unit conversion means expressing the same quantity in a different unit. A mile and 1.60934 kilometers are the same distance written two ways. For almost every unit pair, the conversion is a single multiplication: your number times the conversion factor. Temperature is the one famous exception, because Celsius and Fahrenheit do not share a zero point, so it needs a formula instead of a plain multiplier. Learn the handful of factors you use weekly and hand everything else to a converter.
How to Convert Units: The One Rule
Every unit has a conversion factor relative to a base unit. Meters are the base for length, kilograms for weight, liters for volume. To convert, multiply your value by the factor that takes you from the from-unit to the to-unit. Direction matters, which is where most mistakes happen: going from a big unit to a small unit multiplies up, and going from a small unit to a big unit divides down.
Kilometers to miles: 15 km x 0.621371 = 9.32 mi
Miles to kilometers: 15 mi x 1.60934 = 24.14 km
Pounds to kilograms: 150 lb x 0.453592 = 68.04 kg
Sanity check every answer. Converting miles to kilometers should give a bigger number, because a kilometer is shorter than a mile. Converting kilograms to pounds should also give a bigger number. If your result went the wrong direction, you multiplied by the reciprocal of the right factor. Flipping the direction check catches the error in seconds.
Length Conversion Chart
The anchors worth memorizing: a meter is roughly a yard, a kilometer is about 0.62 miles, an inch is exactly 2.54 cm.
| 1 unit | Equals |
|---|---|
| 1 kilometer | 0.621371 miles |
| 1 mile | 1.60934 kilometers |
| 1 meter | 3.28084 feet |
| 1 foot | 30.48 centimeters |
| 1 inch | 2.54 centimeters |
| 1 yard | 0.9144 meters |
| 1 nautical mile | 1.852 kilometers |
Weight Conversion Chart
The anchor: a kilogram is 2.20462 pounds. Kitchen math uses the 2.2 shortcut, but shipping and postage need the full factor.
| 1 unit | Equals |
|---|---|
| 1 kilogram | 2.20462 pounds |
| 1 pound | 0.453592 kilograms |
| 1 ounce | 28.3495 grams |
| 1 metric ton | 2,204.62 pounds |
| 1 US ton | 907.185 kilograms |
| 1 stone | 6.35029 kilograms |
Temperature Conversion Chart
Temperature conversions need formulas because the scales start at different places. Celsius to Fahrenheit: multiply by 9/5 and add 32. Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Kelvin is Celsius plus 273.15.
F to C: (F - 32) x 5/9
C to F: (C x 9/5) + 32
Quick trick: double the Celsius, subtract 10% of that, then add 32. (20 C x 2 = 40, minus 4 = 36, plus 32 = 68 F.)
| Fahrenheit | Celsius | What it is |
|---|---|---|
| 212 °F | 100 °C | Water boils |
| 98.6 °F | 37 °C | Body temperature |
| 68 °F | 20 °C | Room temperature |
| 32 °F | 0 °C | Water freezes |
| -4 °F | -20 °C | Home freezer |
Volume Conversion Chart
These are US customary units. A US gallon is smaller than an imperial gallon, which is why recipes from UK sites can go sideways.
| 1 unit | Equals |
|---|---|
| 1 US gallon | 3.78541 liters |
| 1 liter | 0.264172 US gallons |
| 1 US cup | 8 US fluid ounces |
| 1 US fluid ounce | 29.5735 milliliters |
| 1 US tablespoon | 14.7868 milliliters |
| 1 cubic meter | 1,000 liters |
The Mistakes That Catch People Out
Fluid ounces vs. ounces. A fluid ounce measures volume, an ounce measures weight. A cup is 8 fluid ounces, but how many ounces a cup weighs depends on the ingredient: water is about 8.3, honey about 12, flour about 4.4. Volume-to-weight conversion needs the ingredient's density, which no plain factor can give you.
US gallons vs. imperial gallons. A US gallon is 3.785 liters; an imperial gallon is 4.546 liters. British recipes and older fuel-economy figures use the imperial one. If a conversion looks 20% off, this is usually why.
Tons come in three sizes. A metric ton is 1,000 kg, a US (short) ton is 907 kg, and a UK (long) ton is 1,016 kg. Shipping paperwork always specifies which one; assume metric for international freight.
Kilobytes: 1,000 or 1,024? Drive manufacturers use 1,000 bytes per kilobyte (decimal), while operating systems use 1,024 (binary). That is why a "1 TB" drive shows up as about 931 GB on your computer. Our converter uses the 1,024 convention, which matches what your devices report.
Frequently Asked Questions
How do I convert kg to lbs?
Multiply kilograms by 2.20462. For example, 70 kg x 2.20462 = 154.32 lb. To convert pounds back to kilograms, divide by 2.20462.
How many kilometers are in a mile?
One mile is exactly 1.60934 kilometers. Multiply miles by 1.60934 to get kilometers, or multiply kilometers by 0.621371 to get miles.
What is 32 degrees Fahrenheit in Celsius?
32 F equals 0 C, the freezing point of water. Use (F - 32) x 5/9 for any Fahrenheit to Celsius conversion.
How many ounces are in a cup?
A US cup holds 8 US fluid ounces. Keep volume ounces and weight ounces separate: a cup of water weighs about 8.3 ounces, but a cup of flour weighs about 4.4 ounces.
What is the formula for Celsius to Fahrenheit?
Multiply Celsius by 9/5 (1.8) and add 32. For example, 20 C x 1.8 + 32 = 68 F. Reverse it with (F - 32) x 5/9.
Keep Reading: Everyday Math Guides
Conversions are one kind of everyday arithmetic. How to calculate percentages covers the other one you use constantly: discounts, tips, and percent change. And when the conversion is money per hour, the salary converter flips hourly, weekly, monthly, and yearly pay instantly.