Aspect Ratio Calculator

An aspect ratio calculator divides the width and height by their greatest common divisor. For 1920 × 1080 the GCD is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9, or 1.78:1 as a decimal. To keep that shape at a new width, multiply by 9/16: 1280 wide gives 1280 × 9 ÷ 16 = 720 high.

How to use the aspect ratio calculator

1. Simplify: enter a width and height (pixels, inches or any unit) to get the simplified ratio, the decimal form, the nearest standard ratio and a preview drawn to scale.
2. Missing side: pick a preset such as 16:9 or 4:5, or type your own ratio, then enter the width or the height to get the other side.
3. Resize: enter the original size and a new width, a new height, a percentage, or a box to fit into. The box option shows both the “fit” size (no cropping) and the “fill” size (crop the overflow).
4. Decimal: turn a value such as 2.39 or 1.85:1 into an exact whole-number ratio, simpler approximations and the closest standard ratios.

What an aspect ratio means

An aspect ratio compares width to height, written W:H. It describes shape, not size: a 1280 × 720 video and a 3840 × 2160 video are both 16:9, because 3840 ÷ 1280 = 3 and 2160 ÷ 720 = 3. A ratio greater than 1:1 is landscape, less than 1:1 is portrait, and 1:1 is square. The same idea works for any unit, as long as width and height use the same one.

How to simplify an aspect ratio

Simplifying a ratio works like simplifying a fraction. Find the greatest common divisor (GCD) of the width and height, then divide both by it. The Euclidean algorithm finds the GCD quickly: divide the larger number by the smaller, keep the remainder, and repeat until the remainder is 0.

For 1920 × 1080: 1920 = 1 × 1080 + 840; 1080 = 1 × 840 + 240; 840 = 3 × 240 + 120; 240 = 2 × 120 + 0. The last nonzero remainder, 120, is the GCD, and 1920:1080 = 16:9.

If a side has decimals, multiply both sides by 10, 100 or 1,000 first. For 2.35:1, multiply by 100 to get 235:100, then divide by the GCD of 5 to get 47:20.

Aspect ratio formula for a missing side

For a ratio W:H, the sides of any matching rectangle satisfy width ÷ height = W ÷ H. So:

• height = width × H ÷ W
• width = height × W ÷ H

When the answer is not a whole number, round to the nearest pixel. The calculator shows how far the rounded size is from the exact ratio, which is usually a tiny fraction of a percent.

Common aspect ratios

“21:9” is a marketing name. The common 2560 × 1080 panel simplifies to 64:27 (2.370:1) and 3440 × 1440 to 43:18 (2.389:1), both a little wider than a true 21:9 (2.333:1). Some sizes do not simplify neatly at all: 1366 × 768 is 683:384, about 1.779:1, and is sold as 16:9.

Worked examples

A phone photo

A photo is 3024 × 4032 pixels. The GCD is 1008, so the ratio is 3024 ÷ 1008 : 4032 ÷ 1008 = 3:4, a portrait version of 4:3.

Vertical video at 1080 wide

For 9:16 at a width of 1080, height = 1080 × 16 ÷ 9 = 1920, so the frame is 1080 × 1920.

Resizing and fitting

A 4000 × 3000 image resized to 1200 wide uses a scale factor of 1200 ÷ 4000 = 0.3, so the height becomes 3000 × 0.3 = 900. To fit the same image inside a 1080 × 1350 frame, compare 1080 ÷ 4000 = 0.27 and 1350 ÷ 3000 = 0.45. The smaller factor, 0.27, fits it with nothing cut off: 1080 × 810, with empty bars above and below.

Screen size from the diagonal

A TV’s size is its diagonal. For a 55-inch 16:9 screen, the diagonal of a 16 × 9 rectangle is √(16² + 9²) = √337 by the Pythagorean theorem. Width = 55 × 16 ÷ √337 ≈ 47.9 in and height = 55 × 9 ÷ √337 ≈ 27.0 in.

Common mistakes

• Stretching instead of scaling: changing only the width (or only the height) distorts the image. Multiply both sides by the same factor.
• Writing H:W instead of W:H. 9:16 and 16:9 are different shapes.
• Dividing by a common factor that is not the greatest. 1920:1080 ÷ 40 gives 48:27, which can still be divided by 3.
• Assuming every “16:9” or “21:9” size is exact. Check the pixels with the Simplify tab.

Aspect Ratio practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How do I calculate an aspect ratio?

Divide the width and the height by their greatest common divisor. 1920 × 1080 ÷ 120 = 16:9. For the decimal form, divide width by height: 1920 ÷ 1080 ≈ 1.78.

What aspect ratio is 1920 × 1080?

It is 16:9 (1.78:1), the same as 1280 × 720 and 3840 × 2160.

How do I resize an image without changing its aspect ratio?

Multiply both sides by the same scale factor. Find the factor by dividing the new width by the old width (or the new height by the old height).

Is 21:9 the same as 64:27?

Not exactly. 21:9 is 2.333:1, while 64:27 is 2.370:1. Monitors sold as 21:9 at 2560 × 1080 are 64:27.

What is the difference between fit and fill?

Fit scales the image until one side touches the frame, so the whole image shows with bars on two sides. Fill scales until both sides cover the frame and crops whatever sticks out.

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