ratio

Measure of the relative size of two quantities or of two measurements (in similar units), expressed as a proportion. For example, the ratio of vowels to consonants in the alphabet is 5:21. As a fraction 5/26 of the letters are vowels. The ratio of 500 m to 2 km is 500:2,000, or in its simplest integer form 1:4 (dividing both sides of the ratio by 500).

Ratios are normally expressed as whole numbers, so 2:3.5 would become 4:7 (the ratio remains the same provided both parts of the ratio are multiplied or divided by the same number).

For example, to make up 1 litre of mauve paint, blue, red, and white paints have to be mixed in the ratio 5:2:3. The amount of each colour needed can be worked out as follows:

1 litre is the same as 1,000 ml. The ratio 5:2:3 means 5 parts blue, 2 parts red, and 3 parts white – making 10 parts altogether. 1,000 ÷ 10 = 100, so 1 part = 100 ml. The amounts needed, therefore, are

5 × 100 = 500 ml of blue paint; 2 × 100 = 200 ml of red paint; 3 × 100 = 300 ml of white paint

Alternatively, by dividing the given quantity into the ratio, the quantity for each paint can be found. For instance, the amount of blue paint = 5/10 × 1,000 = 500 ml.