Rounding to the nearest 10, 100 and 1,000
Maths · Key Stage 2 · The School
Why we round
A rounded number is easier to picture and easier to say. A stadium holds 34,872 people, but you would say "about 35,000" in conversation. Rounding trades a little accuracy for a number that is much easier to hold in your head. News reports round for exactly this reason. A precise figure would only get in the way of the story.
Which way does it round
Look at the digit just after the one you are rounding to. If that digit is 5 or more, round up. If it is 4 or less, round down. Rounding 148 to the nearest ten: look at the 8, which is 5 or more, so round up to 150. Rounding 143 the same way rounds down to 140 instead, because the 3 is less than 5.
The exact halfway case
What about 250, rounded to the nearest hundred? It sits exactly halfway between 200 and 300. The rule still applies: the digit after the hundreds is 5, so round up. 250 rounds to 300. The halfway case always rounds up, by this rule, with no exceptions. It can feel arbitrary the first time you meet it. It is simply the rule everyone has agreed to follow, so answers stay consistent.
Using rounding to check an answer
You work out 3,842 add 5,193 and get 90,035. Round each number first: 4,000 and 5,000. Their rough total is 9,000. Your actual answer, 90,035, is ten times too big. Rounding first catches the mistake instantly, long before you check every single digit by hand. A rough check like this takes seconds. It is one of the fastest ways to catch a slipped decimal point.