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Adding and Subtracting Decimals

Level 1 Decimals · Reviewed by The NumThink editorial team · Last reviewed

Short version: Line up the decimal points, fill gaps with zeros, then add or subtract as with whole numbers: 2.5 + 1.75 = 2.50 + 1.75 = 4.25.

This page is part of the NumThink Decimals 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

Lining up the points lines up the place values — ones under ones, tenths under tenths. You can only add tenths to tenths, just as you add tens to tens.

Writing 2.5 as 2.50 changes nothing (it adds zero hundredths) but makes both numbers the same length, so no digit gets added in the wrong place.

The steps

  1. Give both numbers the same number of decimal places. 2.5 + 1.75 becomes 2.50 + 1.75.
  2. Add or subtract as whole numbers. 250 + 175 = 425.
  3. Put the point back in the same place. Two decimal places: 4.25.

Worked examples

Easy 1.9 + 2.8

Show the thinking

Ones: 1 + 2 = 3

Tenths: 0.9 + 0.8 = 1.7

3 + 1.7 = 4.7

So 1.9 + 2.8 = 4.7.

Answer: 4.7

Medium 2.5 + 1.75

Show the thinking

2.5 = 2.50

2.50 + 1.75 = 4.25

So 2.5 + 1.75 = 4.25.

Answer: 4.25

Hard 5 − 1.25

Show the thinking

Count up from 1.25

1.25 + 0.75 = 2, then 2 + 3 = 5

0.75 + 3 = 3.75

So 5 − 1.25 = 3.75.

Answer: 3.75

When to use it

  • Money: 12.5 + 3.75 = 16.25.
  • Measurements: 1.8 m minus 0.65 m leaves 1.15 m.

When NOT to use it

  • Numbers close to a whole number: 6.9 + 2.8 is quicker as 7 + 2.8 − 0.1 = 9.7 (round and correct).
  • Mixed with fractions — convert first: 1/2 + 0.25 = 0.5 + 0.25 = 0.75.

Common mistakes

Lining up the last digits instead of the points. Writing 2.5 + 1.75 = 2.00 by adding 25 and 175. Fix: Line up the points: 2.50 + 1.75 = 4.25.
Forgetting the zeros in "whole minus decimal". Working out 5 − 1.25 as 4.25. Fix: Write 5 as 5.00 first: 5.00 − 1.25 = 3.75.
Check yourself: Round to check: 2.5 + 1.75 is about 2.5 + 2 = 4.5, and 4.25 is close ✓.