Rounding and estimating

Rounding makes numbers simpler while keeping them close to the real value. Look at the digit just after the place you are rounding to: 5 or more rounds up, 4 or less rounds down. Estimating means rounding each number to 1 significant figure, then working it out in your head.

Key facts

  • Find the place you round to, then look one digit to the right.
  • 5, 6, 7, 8 or 9: round up. 0 to 4: keep it.
  • Significant figures start at the first non-zero digit.
  • To estimate, round every number to 1 significant figure first.

Worked examples

Example 1

Round 3.476 to 1 decimal place.

  1. The first decimal place holds 4. 3.4|76
  2. The next digit is 7, which is 5 or more, so round the 4 up. 3.5

Answer: 3.5

Example 2

Round 0.04082 to 2 significant figures.

  1. The first significant figure is 4, the second is 0. 0.040|82
  2. The next digit is 8, so round up. 0.041

Answer: 0.041

Example 3

Estimate 49.6 × 3.1 ÷ 0.52.

  1. Round each number. 50 × 3 ÷ 0.5
  2. Multiply. 50 × 3 = 150
  3. Dividing by 0.5 is the same as doubling. 150 ÷ 0.5 = 300

Answer: ≈ 300