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Aspect Ratio Calculator

Reduce any width and height to its simplest aspect ratio, or find the missing width or height that keeps a ratio such as 16:9.

Level: Grades 6–7

  • Common Core standard 6.RP.A.3.aMake tables of equivalent ratios relating quantities with whole-number measurements, find missing values in the tables, and plot the pairs of values on the coordinate plane. Use tables to compare ratios.
Printable worksheet

Aspect Ratio Calculator

Inputs
Calculation

Pixels, inches, centimeters: any unit, as long as both sides use the same one.

Result

Aspect ratio (simplest form)

16:9

1,920 × 1,080 has an aspect ratio of 16:9 (1.7778 : 1).

Decimal ratio (width ÷ height)
1.7778 : 1
Orientation
Landscape (wider than tall)
Closest common ratio
16:9 (exact)Useful when the exact ratio has large terms, such as 683:384 for 1366 × 768.
CSS property
aspect-ratio: 16 / 9;
Padding-top for the older CSS ratio box
56.25%Height ÷ width × 100: the percentage that keeps a box at this ratio without the aspect-ratio property.

See the step-by-step solution ↓

Step-by-step solution

  1. Find the greatest common divisor (GCD), the largest whole number that divides both sides

    GCD(1,920,1,080)=120

  2. Divide both sides by 120 to get the simplest ratio

    1,920120:1,080120=16:9

  3. Divide width by height for the decimal form, which makes different ratios easy to compare

    1,9201,080=1.777778

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the shape of any screen, image or video, or work out a missing width or height without stretching it. An aspect ratio describes that shape: how wide it is compared with how tall. A 1920 × 1080 video frame and a 1280 × 720 one are different sizes but the same shape, and both reduce to 16:9. The ratio says nothing about physical size or resolution, only proportion.

This calculator works in both directions. Give it a width and a height, and it returns the ratio in lowest terms, the decimal form, the closest common ratio and ready-made CSS. Or pick a ratio and one side, and it returns the other side. That lets you resize an image, a video or a layout box at the same shape.

Where it's used

Why this math matters outside the classroom:

  • Video exports. Video editors check that 1920 × 1080 reduces to 16:9 before exporting for a widescreen display.
  • Resizing images. Web designers scaling a 16:9 image to 800 px wide set its height to 800 × 9 ÷ 16 = 450 px.
  • Photo prints. Photographers see that a 3:2 camera image must be cropped to fit an 8 × 10 print, which is 4:5.
  • Social media posts. Social media managers crop images to square 1:1 or vertical 9:16 so posts aren't cut off.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.a. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose whether you want to find the ratio of a size you have, or find a missing width or height.
  2. To find a ratio, enter the width and height in any unit, as long as both use the same unit.
  3. To find a missing side, choose a ratio from the list (or Custom), say whether you know the width or the height, and enter it.
  4. Read the answer and the working. For pixel sizes, use the rounded whole-pixel value.

What the inputs mean

Width
The horizontal size. Pixels, inches and centimeters all work, because the units cancel out in a ratio.
Height
The vertical size, in the same unit as the width.
Aspect ratio
Common ratios. 21:9 is a marketing name: real ultrawide panels are usually 64:27 (2560 × 1080) or 43:18 (3440 × 1440), both close to 2.37 : 1.
Known side
The side you already have. The calculator returns the other side at the chosen ratio.

Formula and method

ratio = (width ÷ GCD) : (height ÷ GCD)
decimal ratio = width ÷ height
missing height = known width × H ÷ W
missing width = known height × W ÷ H

The greatest common divisor (GCD) is the largest whole number that divides both sides exactly. Dividing both sides by it gives the smallest whole numbers with the same proportion. If either side has decimals, both are first multiplied by a power of ten so they become whole numbers.

W and H are the two numbers of the ratio, such as 16 and 9 in 16:9. To keep that shape, the width divided by the height must equal W divided by H. Rearranging that one equation gives the missing side. The decimal form (for example 1.7778 : 1) is the same ratio written with a height of 1, which makes very different ratios easy to compare.

Worked example

A video is 1920 pixels wide and 1080 pixels tall. What is its aspect ratio, and how tall should a 1280-pixel-wide copy be?

  1. Find the GCD, the biggest number that divides both sides: for 1920 and 1080 it is 120.
  2. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9.
  3. The decimal form is 1920 ÷ 1080 = 1.7778 : 1.
  4. For the smaller copy, choose 16:9 and enter a known width of 1280. To keep the same shape, the height is 1280 × 9 ÷ 16 = 720.

Result: The video is 16:9, and a 1280-pixel-wide version at the same shape is 1280 × 720.

Understanding the result

A ratio above 1 : 1 (decimal greater than 1) is landscape, below 1 is portrait, and exactly 1 is square. Swapping the two numbers turns a landscape ratio into its portrait counterpart, so 9:16 is 16:9 stood on its end.

Some real resolutions reduce to awkward numbers. 1366 × 768 becomes 683:384, which is within a tenth of a percent of 16:9, so the calculator also names the closest common ratio and how far off it is.

The CSS results give the modern aspect-ratio property, plus the padding-top percentage (height ÷ width × 100) that older layouts use to hold a box at a fixed shape.

Assumptions and limitations

  • Pixels are assumed to be square, which is true for computer screens, phones and almost all modern video.
  • Both sides are measured in the same unit, and decimals are reduced exactly up to eight decimal places.
  • A missing side is often not a whole number. Rounding to whole pixels changes the ratio very slightly. That is normal, and you won't see it at ordinary sizes.
  • The calculator does not crop or letterbox. If you force a 4:3 picture into a 16:9 frame you must either crop it, add bars, or stretch it.
  • Video formats with non-square pixels (some older broadcast standards) need a pixel aspect ratio correction that is not modeled here.

Frequently asked questions

What resolutions are 16:9?

Any size whose width is exactly 16/9 of its height: 1280 × 720, 1600 × 900, 1920 × 1080, 2560 × 1440 and 3840 × 2160 are the common ones. Each reduces to 16:9 because its GCD divides out cleanly.

Why is my monitor's ratio shown as 8:5 instead of 16:10?

They are the same shape. 16:10 is not in lowest terms because both numbers divide by 2. The industry keeps the 16:10 name so it is easy to compare with 16:9, and the calculator lists it as the closest common ratio.

Does the aspect ratio tell me the resolution?

No. A ratio only fixes the shape. A 16:9 image can be 640 × 360 or 7680 × 4320. Use the missing-side mode to find the other dimension once you choose one.

How do I keep a box at a fixed ratio in CSS?

In current browsers, set the aspect-ratio property shown in the results, for example aspect-ratio: 16 / 9. In older layouts, give the box a height of zero. Then set its padding-top to height ÷ width × 100 percent, which is 56.25% for 16:9.

How this calculator is checked

Method: Reduction by greatest common divisor (Euclid's algorithm); proportional scaling of a rectangle. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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