Aspect Ratio Calculator

About Aspect Ratio Calculator

An aspect ratio is width divided by height, written as the smallest pair of whole numbers: 16:9, 4:3, 3:2. Enter a width and height in pixels and this calculator reduces them by their greatest common divisor, shows the decimal ratio (1.7778 for 16:9) and names the nearest common ratio. Give it a new width or a new height and it works out the other side without distorting the image.

The arithmetic is simple. The confusing parts are questions like “is my 21:9 monitor actually 21:9?”, “how much of my video survives a vertical crop?” and “why does 1366×768 reduce to something ugly?” Every number below came out of the tool.

What GCD reduction does to real resolutions

1920×1080 divides by its GCD of 120 to give 16:9. 1920×1200 divides by 240 to give 8:5, which is the same ratio people call 16:10, so the tool labels the nearest common ratio as 16:10. The two are identical; 16:10 is just written to be compared with 16:9.

The once-ubiquitous laptop resolution 1366×768 reduces to 683:384, because 683 is prime and nothing more cancels. At 1.7786 it's within 0.0008 of true 16:9 (1.7778), so it's sold and treated as 16:9, and the tool shows 16:9 as the nearest match. A true 16:9 image scaled to fill it is stretched by a fraction of a percent.

The practical lesson is to trust the decimal more than the fraction. Two resolutions with ugly reduced ratios can be visually identical, while two that are both marketed under the same name can differ enough to matter.

21:9 monitors aren't 21:9

The common ultrawide resolutions are 2560×1080, which reduces to 64:27 (2.370), and 3440×1440, which reduces to 43:18 (2.389). Neither is 21:9 (2.333), and they aren't equal to each other either. “21:9” is a marketing label meaning “roughly cinema-wide.” The tool normally names a common ratio only when the difference is within 0.01, but it knows these two — and 3840×1600 (12:5) — are sold as 21:9, so it labels their nearest ratio “~21:9”.

Cinema itself uses different numbers. Theatrical widescreen, “scope,” is 2.39:1, and standard “flat” is 1.85:1. In the DCI digital cinema specification, the 2K containers are 2048×858 for scope and 1998×1080 for flat. Enter 1.85 by 1 and the tool returns 37:20; 2.39 by 1 returns 239:100. When a ratio doesn't come from whole pixel counts, the exact fraction is unwieldy, which is why film people say “two-three-nine” rather than quoting a fraction.

Letterboxing, pillarboxing and anamorphic squeeze

When the picture and the screen disagree, something has to give: crop the picture or pad it with black. A 2.39:1 film fitted to the width of a 1920×1080 screen is 1920 ÷ 2.39 ≈ 803 pixels tall, leaving bars of about 138 pixels top and bottom (letterboxing). A 4:3 picture fitted to the height of the same screen is 1440 pixels wide, leaving 240-pixel bars on each side (pillarboxing). A 16:9 video on a 1920×1200 display gets 60-pixel bars above and below.

Anamorphic lenses solve a related problem at capture time. A 2× anamorphic lens squeezes the scene horizontally onto a taller, narrower area of film or sensor, and projection or post-production stretches it back out; a roughly 1.2:1 capture area unsqueezed by 2 gives about 2.39:1. Files from such workflows store one pixel shape and display another, so computing the aspect ratio from raw pixel counts gives the wrong answer. Check the container's display aspect ratio or pixel aspect ratio before resizing.

3:2 photos and 9:16 vertical video

Most DSLRs and mirrorless cameras shoot 3:2, inherited from 35 mm film; phones and Micro Four Thirds cameras default to 4:3. A 6000×4000 frame prints on 4×6 paper with no cropping, but an 8×10 print is 5:4, so keeping the full 4000-pixel height means the width must be 5000, and 1000 pixels, a sixth of the frame, are cut off. That's why 8×10 enlargements so often chop off someone at the edge of a group photo.

Going from horizontal to vertical is far more drastic. Keep the full 1080-pixel height of a 1920×1080 clip and a 9:16 frame is only 607.5 pixels wide: about 32% of the original width. If footage is meant to be recut for Shorts, Reels or TikTok, keep the subject inside the middle third while shooting.

Worked examples

How to use it

1. Enter the original width and height to get the reduced and decimal ratio.
2. Type a new width or a new height; the other dimension is calculated to keep the ratio.
3. Use the presets for 16:9, 4:3, 9:16 and others, or swap width and height for portrait.

Frequently asked questions