Aspect Ratio Calculator
Simplify a width and height to a clean ratio, or solve for a missing side at a target ratio like 16:9 or 4:3.
From pixels to a ratio, and back
An aspect ratio strips a pixel size down to the relationship that actually matters for layout: how wide the frame is relative to how tall. Reducing by the greatest common divisor gives the smallest whole numbers that describe it, so 1920×1080 and 1280×720 both read as 16:9 — the same shape at different resolutions.
Once you know the ratio you want, the reverse mode fills in the exact pixels. To then resize an image to those numbers, use the Image Resize Calculator; to crop an off-ratio photo to fit, try the Crop & Fit calculator.
Frequently Asked Questions
How is an aspect ratio worked out from pixel dimensions?
Divide the width and height by their greatest common divisor (GCD) — the largest number that divides both evenly. For 1920×1080 the GCD is 120, so 1920÷120 : 1080÷120 gives 16:9. That reduction is exactly what this calculator does, turning any pixel size into its simplest whole-number ratio.
What are the most common photo and video aspect ratios?
3:2 is the classic 35mm and DSLR frame; 4:3 is typical of phones, Micro Four Thirds, and older monitors; 1:1 is the square used on many social feeds; 16:9 is widescreen video and most displays; and 21:9 is ultrawide cinema. Portrait versions simply swap the numbers, such as 4:5 and 9:16.
How do I find a missing width or height for a ratio?
Keep the ratio fixed and solve for the unknown side. For a 16:9 frame with a known width, height = width × 9 ÷ 16; with a known height, width = height × 16 ÷ 9. Switch to the 'find a missing dimension' mode above, enter the ratio and the side you know, and it fills in the other.
Why doesn't 1366×768 reduce to exactly 16:9?
Because it isn't exactly 16:9. Its true reduced ratio is 683:384, very close to 16:9 but not identical — which is why some content shows tiny bars on those panels. This calculator always gives the exact mathematical reduction rather than rounding to the nearest familiar ratio.
Educational tool. Ratios are exact integer reductions; solved dimensions are rounded to whole pixels.