How do I find the aspect ratio of an image from its width and height?

Open the Dimensions to Ratio tab and enter Width (px) and Height (px). It shows the Simplified ratio, such as 1920 by 1080 as 16:9, plus the Decimal ratio, Named ratio and Megapixels.

How do I get the height for a given width at 16:9?

Use the Ratio to Dimensions tab, enter the Aspect ratio and a Known dimension, such as a width of 1920. The other side is calculated, giving 1080 for 16:9.

How do I resize an image while keeping its proportions?

Open the Resize tab, enter the Original size, then type a new width or height. The other side follows the ratio and a Scale percentage is shown. Results are rounded to whole pixels.

Why does the tool say Nearest exact for my dimensions?

The calculated side was not a whole number, so it was rounded. Nearest exact shows the closest whole-pixel size that keeps the ratio perfectly, so you can choose between them.

Does the calculator have presets for social media sizes?

Yes. Common dimensions include 4K UHD, 1080p, 720p, Instagram Post, Instagram Story, Twitter Header, YouTube Thumb, Facebook Cover, LinkedIn Banner, A4 and US Letter.

Yes. Every tool is free to use, with no account and no credit card. The work runs on your device rather than on our servers, which is what keeps it free to run.

Does it work on mobile?

Yes. It runs in current mobile browsers on iOS and Android, with nothing to install.

📐

Free Aspect Ratio Calculator

Calculate the aspect ratio of any dimensions, or find the missing dimension from a known side and ratio. Includes 12 common presets.

Runs in your browserNo sign-up, no pop-upsNo login requiredFree forever

Works offline once loaded

Width (px)

Height (px)

Simplified ratio16:9

Decimal ratio1.7778:1

Named ratio16:9

Megapixels2.07 MP

16:9

Exact resolutions for 16:9

480 × 270

640 × 360

960 × 540

1024 × 576

1280 × 720

1600 × 900

Common dimensions

Common uses

Calculate Ratios for Responsive DesignFind the correct height for responsive images and containers given a fixed width and target ratio.
Calculate Aspect Ratios for VideoFind the correct dimensions for 16:9, 4:3, 9:16 and other common video aspect ratios.
Calculate Dimensions for Social MediaGet exact pixel dimensions for Instagram, Twitter, LinkedIn, and Facebook image formats.
Calculate Image Dimensions for WebEnsure images fit their containers perfectly — find the right width and height for any layout.
Calculate Print DimensionsConvert print sizes between different aspect ratios — plan crops before sending to a print shop.

How It Works

All three calculation modes — simplify a ratio, derive missing dimensions, and lock-ratio resize — run as pure JavaScript arithmetic in your browser using a hand-written Euclidean GCD algorithm. No library, no server, no file upload involved.

1
GCD reduces the ratio
When you enter width and height, the tool runs Euclid's algorithm (gcd(a, b) = gcd(b, a % b)) repeatedly until the remainder is zero, then divides both sides by that divisor to produce the simplified integer ratio, e.g. 1920 × 1080 becomes 16:9.
2
Named-ratio lookup
The decimal ratio (width / height) is compared against a hard-coded table of 12 common ratios — 1:1, 4:3, 16:9, 21:9, 2.39:1, and so on. If the difference is within 0.05, the closest name is displayed alongside the reduced fraction.
3
Derived dimension or resize calculation
In Ratio-to-Dimensions mode, the missing side is calculated as knownValue / ratio (or knownValue * ratio), rounded to the nearest integer, with a fallback that finds the nearest exact-integer multiple of the simplified ratio. In Resize mode, the locked side is computed as (newDimension * originalOther) / originalThis — a single cross-multiplication.
4
Matching resolutions enumerated
After simplification, the tool walks a fixed list of 14 standard widths (480 px through 7680 px) and tests tw % simplifiedWidth === 0 to find up to six resolutions that share the exact same ratio, listing them for one-click copy.

Related Tools

✂️

Video Crop

Crop a video to 9:16, 1:1, 4:5 or 16:9, or fit the whole picture in with a blurred, coloured or black background, with nothing cut off.

Try itFiles stay local

⚖️

Ratio Calculator

Simplify a ratio, solve a proportion (a:b = c:x), or split a total into ratio parts.

Try it

📸

Video Frame Grabber

Capture a still image from any frame of a video at full resolution. Scrub, step frame-by-frame, and save as PNG or JPG. Instant and entirely in your browser.