Rule: Line up the decimal points, pad with zeros, then add column by column
The golden rule for adding decimals is to align the decimal points in a column. Then pad shorter numbers with zeros so every row has the same number of decimal places. Finally add from right to left, carrying as with whole numbers.
Example 1 — Calculate \(3.47 + 12.6\)
Step 1
Write in columns with decimal points aligned. Pad 12.6 as 12.60.
Common mistake: Not aligning decimal points. Always write the decimal point in the same column for every number, then pad shorter numbers with zeros.
✓ Quick Check 1 — Adding Decimals
Question 1 of 10
Subtracting Decimals — Column Method with Borrowing
Subtracting decimals uses the same column approach. Align the decimal points, pad with zeros, then subtract from right to left — borrowing (exchanging) when a digit on top is smaller than the one below.
Example 3 — Calculate \(10.00 - 3.47\)
Step 1
Set up in columns: 10.00 on top, 03.47 below. Decimal points aligned.
Step 2
Hundredths: \(0 - 7\) — can't do it. Borrow from tenths. Tenths is 0, so borrow from units. Units is 0, so borrow from tens. Chain borrow: tens becomes 0, units becomes 9, tenths becomes 9, hundredths becomes 10.
Real-Life Applications — Shopping, Change and Bank Statements
Adding and subtracting decimals is essential for everyday money and measurement calculations. The method is always the same: align the decimal point and work in columns.
Example 5 — Shopping total and change
Step 1
Items cost £1.20, £0.89, £1.45, £2.30. Align decimal points and add in columns.
Work through these without a calculator. Type your answer, then click Check.
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Question
Answer
Result
1
A plank is 2.45 m long. You cut off 0.8 m. How much is left (m)?
2
You buy items costing £4.99, £1.35 and £2.60. What is the total (£)?
3
Balance £56.40. You spend £12.75 then receive £30.00. New balance (£)?
4
£15.60 + £3.45 = ?
5
8.7 − 3.25 = ?
6
You buy 3 items: £2.15, £0.99 and £4.50. What is the total (£)?
7
You have £100. You spend £34.75. How much change (£)?
8
12.6 + 3.47 = ?
9
10.00 − 3.47 = ?
10
A recipe needs 1.25 kg flour. You have 0.6 kg. How much more do you need (kg)?
⚠ Things to Watch Out For
Not aligning the decimal points: Always write numbers with decimal points directly below each other. This is the single most common error — misalignment means wrong column values.
Not padding with zeros: 12.6 + 3.47 — write 12.60 + 3.47 so both have the same number of decimal places. This prevents misreading the hundredths column.
Moving the decimal point: The decimal point stays in the same column throughout the calculation. It does not move.
Subtraction from zero: 10.00 − 3.47: you must borrow through the zeros. Work carefully column by column — don't subtract 0 from 4 and write 4.
Dropping the decimal in the answer: Always include the decimal point in the answer, aligned exactly with the decimal points in the calculation.