How to Calculate Aspect Ratio (Formula, Common Ratios, and Resizing Without Distortion)
An aspect ratio is the width and height of an image or video reduced to their simplest whole-number ratio, found by dividing both dimensions by their greatest common divisor. A 1920 by 1080 video reduces to 16:9 because 1920 and 1080 share a greatest common divisor of 120, and dividing both by that number leaves 16 and 9, the smallest pair that describes the same shape at any size.
The formula
Ratio = Width : Height, both divided by GCD(Width, Height)Take the 1920×1080 example step by step. The greatest common divisor of 1920 and 1080 is 120. Divide each side by 120: 1920 / 120 = 16, and 1080 / 120 = 9. The result, 16:9, holds for every resolution that shares the same proportions, from a 1280×720 laptop screen to a 3840×2160 4K panel.
You can also express the same relationship as a decimal by dividing width by height: 1920 / 1080 = 1.78. That single number is handy for comparing ratios at a glance without reducing them to whole numbers first.
Common aspect ratios (cheat sheet)
| Ratio | Decimal | Typical resolution | Where you’ll see it |
|---|---|---|---|
| 16:9 | 1.78:1 | 1920×1080, 3840×2160 | YouTube, TVs, most laptop screens |
| 4:3 | 1.33:1 | 1024×768 | Older TVs, classic slide decks |
| 1:1 | 1:1 | 1080×1080 | Instagram feed posts, profile photos |
| 4:5 | 0.8:1 | 1080×1350 | Instagram portrait posts |
| 9:16 | 0.56:1 | 1080×1920 | Stories, Reels, TikTok, Shorts |
| 3:2 | 1.5:1 | 6000×4000 | DSLR and mirrorless camera photos |
| 21:9 | 2.33:1 | 3440×1440 | Ultrawide monitors |
| 2.39:1 | 2.39:1 | 2048×857 (DCI scaled) | Widescreen cinema |
Notice that several very different pixel counts, like 1280×720 and 3840×2160, land on the exact same 16:9 ratio. Resolution and aspect ratio answer different questions: resolution is how many pixels you have, ratio is the shape those pixels form.
Worked example: resizing without stretching
Say you have a 3000×2000 photo (a 3:2 ratio, typical of a DSLR) and you need a thumbnail exactly 600 pixels wide. Stretching the height to whatever fits the layout would squash the photo, so instead you solve for the matching height using cross multiplication:
New height = New width × (Original height / Original width)
New height = 600 × (2000 / 3000) = 400| Step | Width | Height |
|---|---|---|
| Original | 3000 px | 2000 px |
| Target thumbnail | 600 px | 400 px (solved) |
A second example: fitting a 1920×1080 screenshot into a 1280-pixel-wide blog column. The same cross multiplication gives 1280 × (1080 / 1920) = 720, so the screenshot becomes 1280×720, still exactly 16:9.
Calculate it with your own numbers
Enter any two dimensions to get the simplified ratio, or lock a ratio and fill in one side to solve for the other. The tool below does both.
Common mistakes
Dragging a corner handle without holding the proportion lock. Most image editors stretch width and height independently unless you hold Shift (or an equivalent lock), which distorts faces, logos and straight lines the moment the drag isn’t perfectly diagonal.
Confusing resolution with aspect ratio. 1280×720, 1920×1080 and 3840×2160 are all 16:9. Changing the resolution alone keeps the shape intact; changing the ratio is what causes cropping or letterboxing.
Rounding a computed dimension to an odd number. Video encoders like H.264 and H.265 generally require both width and height to be divisible by 2. If your cross-multiplication math lands on an odd pixel count, round to the nearest even number rather than truncating.
Assuming a platform shows the full image edge to edge. Instagram, for example, displays feed posts at up to 4:5 but crops wider or taller originals to fit, so text or faces placed near the edges of a differently-shaped source image can get cut off.
Frequently asked questions
What is an aspect ratio? It’s the proportional relationship between width and height, written as two numbers such as 16:9 or 4:3. A 1920×1080 image has a 16:9 ratio because both dimensions divide down evenly to 16 and 9.
How do I calculate aspect ratio from pixel dimensions? Find the greatest common divisor of the width and height, then divide both by it. For 1920×1080, the greatest common divisor is 120, giving 16:9.
What’s the difference between 16:9 and 4:3? 16:9 is wider relative to its height (the modern standard for video and screens), while 4:3 is closer to a square (the older TV and photo standard). The same diagonal screen size shows noticeably more width in 16:9 than in 4:3.
What aspect ratio should I use for social media? 1:1 or 4:5 for Instagram feed posts, 9:16 for Stories, Reels, TikTok and YouTube Shorts, and 16:9 for standard YouTube uploads. Check the specific platform, since crop rules change occasionally.
Can I resize an image without losing quality? Resizing down (making it smaller) rarely causes visible quality loss. Resizing up (making it larger) stretches existing pixels and can look soft, since no new detail is created. Keeping the aspect ratio locked avoids distortion either way, but it doesn’t add resolution that isn’t already there.