Estimate Sums by Rounding

Estimating a Sum by Rounding

When you need a quick estimate of a sum, round each addend first, then add the rounded numbers. This gives an answer that is close to the exact total and is often easier to compute mentally.

Steps

1. Look at each addend. Decide which place value you will round to, such as the nearest ten or nearest hundred.
2. Round each number separately. Use the digit to the right to decide whether to round up or keep the number the same.
3. Add the rounded numbers. This total is your estimate.
4. Check your estimate. If needed, compare it to the exact sum to see whether the estimate is reasonable.

Example method

If the numbers are 48 and 26, round to the nearest ten: 48 → 50 and 26 → 30. Then add: 50 + 30 = 80.

Good checking habit

Your estimate should be close to the exact answer, but not necessarily equal. If the exact sum is much larger or smaller, recheck how you rounded each addend.

Tip

Be consistent: round all addends to the same place value unless the directions say otherwise.