Home › Math › Aspect Ratio Calculator
Aspect Ratio Calculator
Every screen, photo and video has an aspect ratio — the relationship between its width and height, written like 16:9. Enter any width and height to get the ratio in its simplest form, or enter a ratio and one side to find the matching other side, so a resize never stretches or squashes the picture.
Resize keeping the ratio
A ratio only describes shape, not size: 1920×1080, 1280×720 and 640×360 are all 16:9. Resizing to the same ratio keeps every part of the picture in proportion; changing the ratio stretches it. Note that marketed 21:9 screens are often exactly 64:27 (2560×1080) or 43:18 (3440×1440) — close to 21:9, but not identical.
How it works
An aspect ratio is the width divided by the height, reduced to its simplest whole-number form. First find the greatest common divisor (GCD) of the two sides — the largest number that divides both evenly — then divide both sides by it. Worked example: 1920×1080 has GCD 120, so 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, giving 16:9; as a decimal that is 1920 ÷ 1080 = 1.778:1. To resize without distortion, keep the same ratio: a new width W at 16:9 needs height W × 9 ÷ 16, so 1280 px wide → 720 px high. Worked example 2: 1024×768 has GCD 256, giving 4:3 — the classic TV ratio.
Frequently asked questions
What is an aspect ratio?
The ratio of an image, video or screen width to its height, written width:height — 16:9 means the width is 16 units for every 9 units of height. It describes shape only: 1920x1080, 1280x720 and 640x360 are all 16:9, just at different sizes.
How do I find the aspect ratio of an image?
Divide both dimensions by their greatest common divisor. For 1920x1080 the GCD is 120, so the ratio is 16:9. This calculator does that step for you — just enter the pixel width and height.
What aspect ratio should I use for YouTube, Instagram or TikTok?
YouTube and most TV use 16:9 (1920x1080). Instagram feed posts are often 1:1 square or 4:5 portrait; Stories, Reels and TikTok use 9:16 vertical (1080x1920). Pick the ratio the platform displays in, or the video will be cropped or letterboxed.
Why does resizing to a different ratio stretch my picture?
Changing the ratio changes the shape: squeezing 16:9 footage into a 4:3 frame makes everything narrower. To keep proportions, scale both sides by the same factor — or crop, which removes picture instead of distorting it.
Is 21:9 really 21:9?
Not exactly. Screens marketed as 21:9 are usually 2560x1080, which simplifies to 64:27 (2.370:1), or 3440x1440, which is 43:18 (2.389:1). True 21:9 would be 2.333:1. The difference is small, but this calculator shows the exact simplified ratio, not the marketing label.