Give any three values of A:B = C:D to find the fourth, or enlarge and shrink a ratio by a set number of times.
Fill in three of the four values in A : B = C : D and leave one empty.
Make a ratio bigger or smaller by a number of times, for example to resize an image and keep its shape.
A ratio compares two quantities and shows how many times one contains the other. A ratio of 3 to 4, written 3 : 4, means that for every 3 of the first thing there are 4 of the second. Ratios appear in recipes, maps, screen sizes, mixing paint and sharing money. The same ratio can be written many ways, and 3 : 4, 6 : 8 and 600 : 800 all describe the same relationship.
A proportion says two ratios are equal, A : B = C : D, or as fractions A ÷ B = C ÷ D. If you know any three of the values, multiply across to find the fourth. D equals B × C ÷ A, and C equals A × D ÷ B. A equals B × C ÷ D, and B equals A × D ÷ C. For 3 : 4 = 600 : D, D is 4 × 600 ÷ 3, which is 800. None of A, B, C or D can be zero in the calculation.
To simplify a ratio of whole numbers, divide both numbers by their greatest common factor. The calculator also writes the ratio as 1 : x and x : 1, which is useful for comparing, and shows what share of the total each part makes up. The scaling calculator multiplies or divides both sides of a ratio by the same number, so enlarging 250 : 280 by 2.5 times gives 625 : 700.
Write it as a proportion, A : B = C : D, and cross-multiply. The missing value is the product of the two numbers on the diagonal that are known, divided by the number opposite the unknown.
Divide both numbers by their greatest common factor. For example, 600 : 800 divided by 200 is 3 : 4.
It means the second quantity is 1.33 times the first. It is the same relationship as 3 : 4, written with 1 on the left.
Divide the part by the total of both parts. For 3 : 4 the first part is 3 ÷ 7, which is about 42.9%.
Use the scaling calculator. Enter the width and height, choose enlarge or shrink, and enter how many times, and it gives the new width and height in the same ratio.
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