How to convert units like a pro: a practical guide

Unit conversion is one of those skills you use more often than you realize. Cooking a recipe from another country, calculating distance for a road trip, converting a temperature for travel, or comparing product sizes at the store all require unit conversions. Knowing how to do them quickly saves time and prevents mistakes.

The basic principle

Every unit conversion follows the same principle: multiply by a conversion factor equal to 1. Since 1 foot equals 12 inches, the fraction 12 inches/1 foot equals 1. Multiply any measurement in feet by this fraction, and the feet cancel out, leaving inches.

This method works for every conversion. Write the starting measurement, multiply by conversion factors until the units you want to cancel are canceled, and the remaining unit is your answer.

Length conversions

The most common length conversions use these factors:

Inches to centimeters: 1 inch = 2.54 centimeters. To convert 10 inches to cm: 10 × 2.54 = 25.4 cm.

Feet to meters: 1 foot = 0.3048 meters. To convert 6 feet to meters: 6 × 0.3048 = 1.8288 meters.

Miles to kilometers: 1 mile = 1.609 kilometers. To convert 5 miles to km: 5 × 1.609 = 8.045 km.

A quick mental shortcut: miles to kilometers is roughly multiply by 1.6. Kilometers to miles is roughly divide by 1.6. It is not exact, but it is close enough for most situations.

Weight conversions

Pounds to kilograms: 1 pound = 0.4536 kilograms. To convert 150 pounds to kg: 150 × 0.4536 = 68.04 kg.

Ounces to grams: 1 ounce = 28.35 grams. To convert 8 ounces to grams: 8 × 28.35 = 226.8 grams.

For mental math, remember that 1 kilogram is about 2.2 pounds. If something weighs 50 kg, that is about 110 pounds. If it weighs 200 pounds, that is about 91 kg.

Volume conversions

Gallons to liters: 1 gallon = 3.785 liters. To convert 5 gallons to liters: 5 × 3.785 = 18.925 liters.

Quarts to liters: 1 quart = 0.946 liters. To convert 4 quarts to liters: 4 × 0.946 = 3.784 liters.

Cups to milliliters: 1 cup = 236.6 milliliters. To convert 2 cups to ml: 2 × 236.6 = 473.2 ml.

Cooking conversions are especially common. A European recipe might call for 500 ml of milk, which is about 2.1 cups. An American recipe calling for 1 cup of flour is about 236 ml.

Temperature conversions

Temperature uses a different formula because the scales start at different points.

Fahrenheit to Celsius: (°F - 32) × 5/9 = °C. 68°F to Celsius: (68 - 32) × 5/9 = 36 × 5/9 = 20°C.

Celsius to Fahrenheit: (°C × 9/5) + 32 = °F. 20°C to Fahrenheit: (20 × 9/5) + 32 = 36 + 32 = 68°F.

Mental shortcut: for typical weather temperatures, each 10°C change is about 18°F. 0°C is freezing (32°F), 10°C is cool (50°F), 20°C is room temperature (68°F), and 30°C is hot (86°F).

Converting between metric and imperial systems

The metric system is based on powers of 10, making conversions within the system simple. 1 kilometer = 1,000 meters = 100,000 centimeters = 1,000,000 millimeters. Moving the decimal point is all you need.

The imperial system (inches, feet, yards, miles) requires memorizing conversion factors. The most useful ones to remember are 12 inches in a foot, 3 feet in a yard, and 1,760 yards in a mile.

Common conversion mistakes

The most common mistake is dividing when you should multiply. If you are converting from a larger unit to a smaller one (feet to inches), you multiply. If you are converting from a smaller unit to a larger one (inches to feet), you divide.

Another common mistake is forgetting that temperature conversions are not proportional. 100°C is not twice as hot as 50°C because the zero points are different. Only Kelvin and Rankine scales have true zero points.

Use the Unit Converter to quickly convert between any units for length, weight, volume, temperature, and more.