How to Calculate Aspect Ratio

Simplify width and height by dividing both numbers by their greatest common divisor.

To find a missing side, multiply the known dimension by the missing side's ratio value, then divide by the known side's ratio value.

Simplify width and height

Write the dimensions as width:height, then divide both sides by the largest whole number that divides evenly into each. A 1920 x 1080 frame divides by 120, so its simplified aspect ratio is 16:9. Simplifying changes the numbers, not the shape.

Calculate a missing width or height

Keep the two sides in the same proportion. For a ratio written as ratio width:ratio height, use these formulas:

height = width x ratio height / ratio widthwidth = height x ratio width / ratio height

For a faster check with your own numbers, open the aspect ratio calculator.

Worked examples

1920 x 1080: divide both values by 120 to get 16:9.
4:5 at 1200 pixels wide: height = 1200 x 5 / 4, so the height is 1500 pixels.
16:9 at 720 pixels high: width = 720 x 16 / 9, so the width is 1280 pixels. See the 16:9 guide for more matching sizes.

Decimal ratio and orientation

You can also divide width by height to get a decimal ratio. For example, 16 divided by 9 is about 1.78. A decimal above 1 is landscape, below 1 is portrait, and 1 is square. The colon form is usually easier to compare and calculate with.

Aspect ratio vs resolution

Aspect ratio describes shape; resolution describes the number of pixels. 1920 x 1080 and 1280 x 720 are both 16:9, but 1920 x 1080 has more pixels and can show more detail. Use the aspect ratio finder when you want to simplify actual dimensions or check whether they are an exact common ratio.

Rounding and near matches

A calculated side can land between pixels. Round to the nearest whole pixel when the destination accepts it, then compare the result with the intended ratio. One rounded pixel is usually harmless, but repeated resizing can change the crop or leave a close ratio that is not exact.