Adding & Subtracting Decimals — KS3 Maths

Adding and subtracting decimals is ordinary column arithmetic — once the decimal points are lined up. Stack the points one above the other, fill any gaps with zeros so every column is complete, work column by column as usual, and bring the point straight down into the answer.

Why do you line up the decimal points?

Because column arithmetic only works when digits of the same value sit above each other — tenths above tenths, hundredths above hundredths. The decimal point is the marker that guarantees it, and filling gaps with zeros keeps every column complete.

Writing $12.4$ as $12.40$ doesn't change its value at all: it has nothing in the hundredths column, and a zero says exactly that.

  1 2 . 4 0
+    3 . 7 5
-----------
  1 6 . 1 5

  9 . 2 0
- 4 . 6 5
---------
  4 . 5 5

  1 3 . 6 0
+    8 . 7 4
-----------
  2 2 . 3 4

Where you'll use this next

• Money problems are decimal addition and subtraction with two places fixed — change from a note, a bill split, a running total.
• Perimeter and measures add decimal lengths, often mixing 1 and 2 decimal places, which is exactly what the zero-filling handles.
• Multiplying and dividing decimals completes the four operations.

Practice questions

Set each one out in columns, line up the points, and fill the gaps with zeros before you add or subtract. Then click to check.