Enter the width and height of an image or screen in pixels. The tool simplifies them to the lowest whole-number ratio and finds the closest standard ratio.
Enter two positive whole numbers for width and height.
Pick a standard ratio, choose whether you already have the width or the height, then enter that value. The tool calculates the matching second dimension.
Enter a positive number for the known dimension.
Why standard aspect ratios exist
An aspect ratio describes the relationship between an image's width and its height, written as two numbers such as 16:9. Screens, cameras, printers and video platforms are built around a small set of standard ratios so that content displays correctly across devices without unexpected cropping or black bars. When a photo, video or design element uses a ratio that matches its intended output, it fills the space edge to edge instead of leaving gaps or forcing a distorted resize.
Common ratios and a UK-specific mix-up worth knowing
16:9 is the standard for widescreen video, HD and 4K displays, and most modern monitors and TVs. 4:3 was the classic television and early digital photo standard before widescreen took over. 1:1 (square) is used across most social feeds. 21:9 is an ultrawide ratio associated with cinema and some gaming monitors. 9:16 is the vertical format used for stories and reels. 3:2 traces back to 35mm film photography and remains the native ratio of many camera sensors. 5:4 matches a classic photo print size. Printed material in the UK almost always follows the ISO A-series (A4, A3 and so on), which has a ratio close to 1:1.41, not the same as any of these screen standards. That matters in practice: a UK designer working with US clients or templates will occasionally receive a Letter-sized (8.5x11 inch) layout instead of A4, and the two shapes are close enough in proportion to be confused at a glance but different enough that text and images end up misaligned once printed.
Limits of a simplified ratio
The ratio this tool returns is only exact when your width and height divide down cleanly. Many real photos and exports are a pixel or two off a perfect standard because of rounding during resizing or compression, so the simplified fraction may look unfamiliar even when the image is visually close to 16:9 or 4:3. Also remember that forcing an image into a different ratio by cropping changes its composition: cropping a 4:3 photo down to 16:9 removes content from the top and bottom, it does not just relabel the same picture.
How the calculation works and when to use it
The tool finds the greatest common divisor (GCD) of the width and height using the Euclidean algorithm, then divides both numbers by it to get the simplest whole-number ratio, the same logic used to reduce a fraction. This is useful before setting up a video export at a specific ratio, sizing a thumbnail or banner to match a platform's requirements, or checking whether a supplied image already matches the ratio you need before you crop it. For proportions based on the golden ratio instead of a screen or print standard, see the Golden Ratio Calculator.