To convert between units, multiply or divide by a conversion factor. The key is to know the relationship between the units—for example, 1 kilometer equals 1,000 meters, 1 kilogram equals 1,000 grams, and 1 liter equals 1,000 milliliters.

This guide covers the most useful conversions for length, weight, and volume, along with simple methods for metric and imperial units.

Basic Conversion Method

Use this rule:

• To change to a smaller unit, multiply.
• To change to a larger unit, divide.

For example, to convert 3 meters to centimeters:

3 \times 100 = 300 \text{ cm}

To convert 300 centimeters to meters:

300 \div 100 = 3 \text{ m}

A conversion factor tells you how many of one unit equal another. Keep track of whether you are multiplying or dividing, especially when converting between metric and imperial systems.

Length Conversions

Metric length units

Examples:

• 2.5 km = 2,500 m
• 180 cm = 1.8 m
• 45 mm = 4.5 cm

Imperial length units

Examples:

• 2 yards = 6 feet
• 3 feet = 36 inches
• 5,280 feet = 1 mile

Metric to imperial length

Examples:

• 10 inches = 25.4 cm
• 5 km = about 3.11 miles
• 6 feet = about 1.83 m

For everyday estimates, remember that 1 mile is roughly 1.6 km, and 1 meter is slightly longer than 3 feet.

Weight and Mass Conversions

Strictly speaking, kilograms and grams measure mass, while pounds and ounces are commonly used as units of weight. In everyday use, the terms are often used interchangeably.

Metric weight units

Examples:

• 2.4 kg = 2,400 g
• 750 g = 0.75 kg
• 3,000 mg = 3 g

Imperial weight units

Examples:

• 3 pounds = 48 ounces
• 10 stone = 140 pounds

Metric to imperial weight

Examples:

• 5 kg = about 11.02 pounds
• 10 pounds = about 4.54 kg
• 8 ounces = about 226.8 g

For quick estimates, 1 kg is a little more than 2.2 pounds.

Volume Conversions

Volume measures how much space a liquid or substance occupies.

Metric volume units

Examples:

• 2.5 L = 2,500 mL
• 750 mL = 0.75 L
• 1,500 mL = 1.5 L

US volume units

Examples:

• 2 quarts = 4 pints
• 3 cups = 24 fluid ounces
• 1 gallon = 16 cups

Metric to US volume

Examples:

• 2 liters = about 67.6 US fluid ounces
• 1 US gallon = about 3.785 liters
• 500 mL = about 2.11 US cups

Note that a US gallon is different from an imperial gallon. One imperial gallon equals about 4.546 liters, while one US gallon equals about 3.785 liters.

Step-by-Step Conversion Examples

Convert kilometers to miles

Convert 12 km to miles.

12 \times 0.621 = 7.452

So, 12 km is about 7.45 miles.

Convert pounds to kilograms

Convert 150 pounds to kilograms.

150 \times 0.454 = 68.1

So, 150 pounds is about 68.1 kg.

Convert liters to US cups

Convert 3 liters to US cups.

3 \times 4.23 = 12.69

So, 3 liters is about 12.7 US cups.

Convert feet and inches to centimeters

Convert 5 feet 6 inches to centimeters.

First convert the height to inches:

(5 \times 12) + 6 = 66 \text{ inches}

Then convert inches to centimeters:

66 \times 2.54 = 167.64 \text{ cm}

So, 5 feet 6 inches is about 167.6 cm.

Convert Units in a Spreadsheet

Spreadsheets are useful when converting many values.

For example, if kilometers are in cell A2, use this formula to convert them to miles:

=A2*0.621371

To convert kilograms in A2 to pounds:

=A2*2.20462

To convert liters in A2 to US gallons:

=A2*0.264172

Enter the conversion factor once, then copy the formula down the column to convert an entire list.

Common Mistakes to Avoid

Multiplying when you should divide

Going from a larger unit to a smaller unit requires multiplication. Going from a smaller unit to a larger unit requires division.

For example:

• 2 kg to grams: multiply by 1,000, giving 2,000 g
• 2,000 g to kilograms: divide by 1,000, giving 2 kg

Mixing US and imperial units

US and imperial measurements share names but do not always have the same values. A US gallon and an imperial gallon are different, and fluid ounces can also differ depending on the system.

Rounding too early

Keep extra decimal places during the calculation, then round only the final answer. This is especially important for cooking, medicine, engineering, shipping, and scientific work.

Confusing weight and volume

Grams measure mass, while milliliters measure volume. They are not interchangeable unless you know the substance’s density. For example, 100 mL of water weighs about 100 g, but 100 mL of honey weighs more.

Using approximate conversions for precise tasks

Approximate conversions are fine for travel, fitness, and general estimates. For medical dosing, lab work, construction, shipping, or engineering, use the exact conversion factor required by the relevant standard or instruction.

Quick Conversion Reference

Identify the units, choose the correct conversion factor, decide whether to multiply or divide, and round only at the end. For metric-to-imperial conversions, keep a reliable reference table nearby—especially when precision matters.