How do I convert between metric and imperial units?

In one line

The rule

To change from one unit system to another, use a conversion factor: multiply or divide your value by this number.

For example:

• 1 inch = 2.54 centimetres (cm)
• 1 kilogram (kg) = 2.2046 pounds (lb)
• 1 mile = 1.609 kilometres (km)

Let's understand it simply

Imagine you are buying cloth in a market. The shopkeeper measures in inches, but your mother wants to know the length in centimetres. You need a way to change the number from one system to another. This is like changing money from rupees to dollars — you use a rate. Here, we use a conversion factor (the number you multiply or divide by).

How it works

1. Find the conversion factor for the units you want (for example, inches to centimetres is 2.54).
2. Multiply or divide: If you are going from a smaller unit to a bigger one, usually multiply. If going from bigger to smaller, usually divide.
3. Write the answer in the new unit.
4. Check if your answer makes sense (for example, 10 inches should be more than 10 cm).

The story behind it

The metric system was made in France in the late 1700s to make measurements simple and standard. The imperial system comes from old British ways of measuring. Different countries used different units, so conversion factors were agreed upon to help people trade and work together.

Where you see it in real life

• Cricket bat sizes: Some bats are measured in inches (imperial), but you may want to know the length in centimetres (metric).
• Buying vegetables: A vendor might say "2 pounds of tomatoes"; you want to know how many kilograms that is.
• Measuring height: Your height may be in feet and inches, but a doctor may write it in centimetres.
• Road distances: In India, we use kilometres, but in the UK or US, you see miles.
• Cooking recipes: Some recipes use grams, others use ounces or pounds.
• International sports: Track events may use metres, but some records are kept in yards.

Why it matters

• Travel: You can understand distances and weights in any country.
• Science and exams: Many questions ask you to convert units.
• Business: Traders and shopkeepers need to convert weights and lengths for international customers.
• Everyday life: From buying cloth to cooking, you need to switch between units.

Worked example

Let's convert 10 inches to centimetres.

Given: 10 inches
Step 1: Write the rule        1 inch = 2.54 cm
Step 2: Multiply              10 × 2.54 = 25.4
Step 3: Answer                10 inches = 25.4 cm

Common mistakes

• Forgetting to multiply or divide: Always check if you should multiply or divide by the conversion factor.
• Using the wrong factor: Make sure you use the correct number for the units you are changing.
• Not writing the new unit: Always write the unit after converting, so you know what the number means.

Remember it

Think: "To convert, multiply or divide by the conversion factor — like changing rupees to dollars!"