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How to Round Numbers

Level 1 Rounding and estimation · Reviewed by The NumThink editorial team · Last reviewed

Short version: Look at the digit just after the place you round to: 5 or more rounds up, 4 or less stays. 3,478 to the nearest 100: the next digit is 7, so 3,500.

This page is part of the NumThink Rounding and estimation course. Read why the method works, watch it run on worked examples, then drill it in the practice box below until it feels automatic.

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Why it works

Rounding finds the nearest round number. 3,478 lies between 3,400 and 3,500: it is 78 above 3,400 but only 22 below 3,500, so 3,500 is nearer.

The digit after the rounding place tells you which half you are in. A 5 there means exactly halfway (or past it); the usual rule is to round up, so 45 rounds to 50.

The steps

  1. Find the rounding place. Nearest 100 in 3,478: the hundreds digit is 4.
  2. Look at the next digit to the right. It is 7.
  3. 5 or more: round up. 4 or less: keep the digit. 7 is 5 or more, so 3,478 → 3,500. Every digit after the rounding place becomes 0 (after the decimal point, just drop them).

Worked examples

Easy Round 85 to the nearest 10

Show the thinking

The ones digit is 5

5 or more rounds up

So 85 → 90.

Answer: 90

Medium Round 3478 to the nearest 100

Show the thinking

The digit after the hundreds is 7

7 is 5 or more, so round up

So 3,478 → 3,500.

Answer: 3,500

Hard Round 3.46 to the nearest 0.1

Show the thinking

The digit after the tenths is 6

6 is 5 or more, so round up

So 3.46 → 3.5.

Answer: 3.5

When to use it

  • Reporting sizes sensibly: a crowd of 23,817 is "about 24,000".
  • Before estimating a calculation — see the next lesson.

When NOT to use it

  • Money you actually pay, or change you give — that must be exact.
  • Rounding in steps: round the ORIGINAL number once. 3,449 to the nearest 1,000 is 3,000; rounding it to 3,450, then 3,500, then 4,000 is wrong.

Common mistakes

Looking at the wrong digit. Rounding 3,478 to the nearest 100 by looking at the 8. Fix: Look only at the digit right after the rounding place — here the 7.
Changing a digit instead of replacing with zeros. Writing 3,478 → 3,578. Fix: Round the hundreds up (4 → 5) and turn everything after it into zeros: 3,500.
Check yourself: The rounded number is never more than half a step away: 3,500 is 22 away from 3,478, and 22 is less than 50 ✓.