How to teach rounding

Rounding replaces a number with a nearby friendly number, usually the nearest ten or hundred. So 47 rounds to 50 and 432 rounds to 400. It powers estimation, letting you make quick mental checks and sense whether an exact answer is reasonable.

Rounding is usually taught from Grade 2 to Grade 5. Students start by rounding to the nearest ten on a number line, then extend to hundreds and thousands as place value grows, and use rounding to estimate totals and check calculations.

Look at the digit just to the right of the place you are rounding to. If it is 5 or more, round up; if it is 4 or less, keep the digit the same. So rounding 47 to the nearest ten, the 7 rounds it up to 50. Only the digits to the right change.

By convention, 45 rounds up to 50. Whenever the deciding digit is exactly 5, the standard rule is to round up. Some children assume a 5 could round either way, so it is worth stating clearly that 5 always rounds up.

Rounding is the basis of estimation. Rounding numbers first makes mental checks quick, so you can tell whether an exact answer is roughly right. For example 298 plus 51 is about 300 plus 50, which is 350, a fast way to sense-check the real total.

A common error is looking at the tens digit when rounding to the nearest hundred, or changing digits to the left as well as the right. So 473 to the nearest ten is 470, not 500. Teach children to find the rounding place first, then check only the single digit to its right.

Look at the tens digit, the one just to the right of the hundreds place. If it is 5 or more, round the hundreds up; if it is 4 or less, keep the hundreds the same, and set the tens and ones to zero. So 432 rounds to 400 and 470 rounds to 500.