Aspect Ratio Calculator: Resize Without Distortion

The math behind aspect ratios

An aspect ratio is width divided by height, written as two whole numbers. To simplify any size, divide both sides by their greatest common divisor. 1920 and 1080 share a GCD of 120, so 1920 × 1080 reduces to 16:9. 1080 × 1350 reduces to 4:5, and 3440 × 1440 reduces to 43:18, which is marketed as 21:9 because 21:9 reads better on a box.

Resizing uses a simple proportion. New height = new width × original height ÷ original width. Going from 1920 × 1080 to 1280 wide: 1280 × 1080 ÷ 1920 = 720. Change either new dimension in the calculator and the other one updates to keep the proportions locked.

Why some sizes don't simplify cleanly

Plenty of real resolutions are not exactly a standard ratio. 1366 × 768, common on budget laptops, simplifies to 683:384. As a decimal it is 1.7786:1 against 1.7778:1 for 16:9, a difference under 0.05 percent, so everyone calls it 16:9. The calculator shows the exact simplified ratio, the decimal, and the closest standard ratio within 1 percent so you get both the truth and the label.

Rounding causes the same issue in reverse. Scaling 1920 × 1080 down to 1000 wide gives 562.5 pixels tall, which rounds to 563. The result is off by a fraction of a pixel, invisible in practice. If you need a perfect match, choose widths that are multiples of the ratio: for 16:9, any width divisible by 16.

Common ratios and where they're used

16:9 is the standard for HD and 4K video, YouTube, TVs, and most monitors (1280 × 720, 1920 × 1080, 3840 × 2160). 9:16 is the same shape turned vertical for Reels, TikTok, YouTube Shorts, and Stories at 1080 × 1920.

4:3 was the TV and monitor standard before widescreen and survives in iPads and many presentation templates. 3:2 is the shape of 35mm film and most DSLR sensors (6000 × 4000). 1:1 square and 4:5 portrait are Instagram feed formats; 4:5 at 1080 × 1350 gets the most screen space in the feed. Cinema uses 1.85:1 and 2.39:1 widescreen.

Worked example: letterboxing and pillarboxing

When content and screen ratios differ, you get bars. A 2.39:1 movie on a 1920 × 1080 TV fills the width, so its picture is 1920 ÷ 2.39 = 803 pixels tall. That leaves about 277 pixels of black, split into bars of roughly 138 pixels above and below. That is letterboxing.

Put 4:3 footage on the same screen and it fills the height instead: 1080 × 4 ÷ 3 = 1440 pixels wide, leaving 240-pixel bars on each side. That is pillarboxing. The alternative is cropping, which fills the screen but cuts off part of the image. Knowing the numbers helps you decide which edges of a shot are safe for text and faces.

Mistakes that cause stretched images

Typing both a new width and a new height from different ratios is the classic error. A 4:3 photo forced into 1920 × 1080 gets squashed. Lock the ratio, set one side, and crop if the destination has a fixed shape.

On the web, the CSS aspect-ratio property (for example, aspect-ratio: 16 / 9) reserves space for images and embeds before they load, which prevents layout shift and helps your Core Web Vitals. Setting width and height attributes on img tags does the same thing, because browsers derive the ratio from them.

Using this calculator

Enter the original width and height, or tap a preset, then type the new width or height you need. The tool shows the matching dimension, the simplified ratio, its decimal form, and the closest standard. Everything runs in your browser, so it works offline once the page has loaded.

For image exports, decide the ratio before the pixel size. A social graphic that must work as a 1:1 post and a 9:16 story is easiest to design at 1080 × 1920 with the key content inside a central 1080 × 1080 square, so one crop serves both formats without re-layout.