Key facts

• The first significant figure is the first digit that is not zero.
• Zeros between other digits count (305 has 3 s.f.). Zeros at the start do not (0.0042 has 2 s.f.).
• Zeros at the end of a whole number may or may not count (3400 has 2, 3 or 4 s.f.).
• To round, look at the next digit: 5 or more rounds up.
• Decimal places count digits after the decimal point.
• In a calculation, give the answer to the same number of significant figures as the least precise data.

How to do it

1. Find the first significant figure.
2. Count along to the number of figures you need.
3. Look at the next digit to decide whether to round up.
4. Keep the place value: fill with zeros if needed (4380 to 2 s.f. is 4400).