Unit Conversion Guide: Methods, Formulas, and Common Conversions

Unit conversion is one of the most practical mathematical skills you can have. Whether you are cooking with a recipe from another country, travelling abroad, or working on a science problem, knowing how to convert between units quickly and accurately is essential.

This guide covers the most common conversions across length, weight, volume, and temperature, with clear worked examples and the conversion factors you need.

The Conversion Factor Method

The most reliable approach to unit conversion is multiplying by a conversion factor, which is a fraction equal to 1. For example, since 1 kilometre = 1000 metres:

Choose the fraction that cancels the unit you want to remove and introduces the unit you want.

Length Conversions

Key conversion factors for length:

• 1 inch = 2.54 cm
• 1 foot = 12 inches = 30.48 cm
• 1 mile = 1.609 km
• 1 metre = 100 cm = 1000 mm
• 1 kilometre = 1000 m

Example 1: Miles to Kilometres

Convert 26.2 miles (a marathon) to kilometres.

Example 2: Centimetres to Inches

Convert 175 cm (a person's height) to feet and inches.

First convert to inches:

Then convert to feet and inches:

Weight (Mass) Conversions

Key conversion factors:

• 1 pound (lb) = 0.4536 kg
• 1 kilogram = 2.205 pounds
• 1 stone = 14 pounds = 6.35 kg
• 1 ounce = 28.35 grams
• 1 kilogram = 1000 grams

Example 3: Kilograms to Pounds

Convert 80 kg to pounds.

Volume Conversions

Key conversion factors:

• 1 litre = 1000 millilitres
• 1 US gallon = 3.785 litres
• 1 imperial gallon = 4.546 litres
• 1 cup (US) = 236.6 ml
• 1 fluid ounce (US) = 29.57 ml

Example 4: Litres to US Gallons

Convert 50 litres to US gallons.

Temperature Conversions

Temperature conversions are different because they involve a shift, not just a scaling factor.

For Kelvin:

Example 5: Celsius to Fahrenheit

Convert 37 degrees Celsius (body temperature) to Fahrenheit.

Example 6: Fahrenheit to Celsius

Convert 350 degrees Fahrenheit (a common oven temperature) to Celsius.

Metric Prefixes

The metric system uses standard prefixes to indicate powers of 10. Once you know these, converting within the metric system is straightforward:

• kilo (k) = = 1000
• centi (c) = = 0.01
• milli (m) = = 0.001
• micro (u) = = 0.000001
• nano (n) =

Example 7: Metric Prefix Conversion

Convert 4500 millilitres to litres.

Tips for Accurate Conversions

1. Write out the units. Always include units in your calculation so you can verify that they cancel correctly.
2. Know the difference between US and imperial. US and imperial gallons, pints, and fluid ounces are different sizes.
3. Round appropriately. Use the same level of precision as your original measurement. Converting 5.0 miles does not require 8 decimal places.

Try It Yourself

Convert between hundreds of units instantly with our free Unit Converter. It covers length, weight, volume, temperature, area, speed, and more.