Ratio Calculator

Enter the two sides of a ratio, A and B. The calculator divides both by their greatest common factor to give the simplified ratio, then shows A ÷ B as a decimal and as a percentage. The default 16 : 9 is already in lowest terms, so you see 16 : 9, 1.78 and 177.78%.

The percentage is A compared with B, not A's share of the total. A 3 : 1 ratio shows 300%, because A is three times B. To find A's share of the whole, divide A by A + B instead. For 16 : 9 that's 16 ÷ 25 = 64%.

1366 × 768 doesn't reduce to 16 : 9, but its decimal is so close that for most purposes it behaves like 16 : 9. When a simplified ratio comes out awkward, compare decimals instead.

To resize an image while keeping its shape, use new height = new width × B ÷ A. A 16 : 9 image scaled to 1,280 pixels wide should be 720 pixels high.

The calculator rounds both values to whole numbers before it does anything else, and it doesn't warn you. That can give a wrong answer:

• 1.5 : 2.5 is treated as 2 : 3. The correct simplified ratio is 3 : 5.
• 0.75 : 2 is treated as 1 : 2, with a decimal of 0.5. The true decimal is 0.375.

The fix is to scale both sides to whole numbers first, multiplying by 10, 100 or whatever clears the decimals. Enter 15 and 25 to get 3 : 5, or 75 and 200 to get 3 : 8 and a decimal of 0.38.

A ratio describes parts of a whole, and the whole is A + B parts. To split a total, use A's share = total × A ÷ (A + B).

• $25 split 2 : 3 is $10 and $15, since there are 5 parts of $5 each.
• 2 liters of drink mixed 1 : 4, concentrate to water, is 0.4 liters of concentrate and 1.6 liters of water.

A frequent slip is reading 2 : 3 as two thirds. The first part is 2 out of 5, which is 40% of the total, and the second is 3 out of 5, which is 60%.

If B is 0, the decimal and percentage show a dash, because nothing can be divided by zero, and the simplified ratio reduces to 1 : 0. If both sides are 0, the ratio shows 0 : 0, which has no meaning, so check your inputs.