Aspect ratio calculator
Solve any missing width or height for a ratio, simplify W:H with the GCD, read the megapixels and preview the shape. Presets for 16:9, 4:5 and 9:16.
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height = width × (ratio height ÷ ratio width)
= 1,280 × (9 ÷ 16)
= 1,280 × 0.562500
= 720 px1280 : 720 ÷ 80 both sides = 16:9
16:9 · 1,280 × 720 px
16:9 — HD and 4K video, most monitors and TVs
1,280 × 720 (16:9)
The whole calculation, once
An aspect ratio is a single number: width divided by height. Everything a ratio calculator does is one of two rearrangements of that.
ratio = width ÷ height
height = width ÷ ratio
width = height × ratioType into the width box and the height is solved; type into the height box and the width is solved. The page prints the substituted arithmetic with your own numbers, so you can check it by hand rather than trusting the box.
Simplifying a ratio
To reduce 1920 × 1080 to 16:9, divide both sides by their greatest common divisor. Euclid's algorithm finds it in a few steps: gcd(1920, 1080) → gcd(1080, 840) → gcd(840, 240) → gcd(240, 120) → gcd(120, 0) = 120. Then 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9. Dimensions that do not reduce neatly — 1366 × 768, for instance, which is 683:384 — are reported as a decimal ratio instead, because a huge integer pair tells you nothing useful.
Ratios worth knowing
| Ratio | Decimal | Where it is used |
|---|---|---|
| 16:9 | 1.778 | HD and 4K video, most monitors and televisions, YouTube |
| 9:16 | 0.563 | Stories, Reels, Shorts, TikTok |
| 4:5 | 0.800 | Instagram portrait — the tallest allowed in the feed |
| 1:1 | 1.000 | Square posts, avatars, album art |
| 4:3 | 1.333 | Classic television, iPad, Micro Four Thirds, most projectors |
| 3:2 | 1.500 | 35 mm film and almost every DSLR and mirrorless sensor |
| 64:27 (“21:9”) | 2.370 | Ultrawide monitors — 2560 × 1080 and 3440 × 1440 |
| 2.39:1 | 2.390 | Anamorphic cinema scope |
| √2:1 | 1.414 | ISO paper: A0 through A6, which is why halving A4 gives A5 |
Why “21:9” is not 21:9
A literal 21:9 is 2.333. No ultrawide panel is that shape: 2560 × 1080 and 3440 × 1440 both reduce to 64:27, which is 2.370. The name stuck because it sits neatly next to “16:9” on a spec sheet. The preset button on this page uses the real 64:27, so the dimensions it produces actually match hardware. The same caution applies to “18:9” phones (really 2:1) and “19.5:9” (2.167).
Common resolutions at a glance
| Resolution | Ratio | Megapixels |
|---|---|---|
| 1280 × 720 | 16:9 | 0.92 |
| 1920 × 1080 | 16:9 | 2.07 |
| 2560 × 1440 | 16:9 | 3.69 |
| 3840 × 2160 | 16:9 | 8.29 |
| 1080 × 1350 | 4:5 | 1.46 |
| 1080 × 1920 | 9:16 | 2.07 |
| 1024 × 768 | 4:3 | 0.79 |
| 3440 × 1440 | 64:27 | 4.95 |
Letterboxing, pillarboxing and cropping
When source and destination ratios differ you have three choices and no fourth. Fit keeps everything and adds bars — horizontal bars (letterboxing) when the source is wider, vertical bars (pillarboxing) when it is taller. Fill covers the frame and crops the overflow. Stretch distorts, and is essentially never right. Work out how much you would lose before deciding: fitting 16:9 video into a 9:16 frame means bars over 68% of the height, which is why vertical video is usually reshot rather than converted.
Megapixels
Megapixels are simply width × height ÷ 1,000,000, and they scale with the square of the linear size. Doubling both dimensions quadruples the pixel count — and roughly quadruples the memory a browser needs to hold the image, which is worth remembering before building a collage from twenty 24-megapixel photos.
Related tools
Screen Size Calculator turns a diagonal into real width and height, Resize Image applies the dimensions you calculated here, Crop Video has the social ratios as presets, and Combine Images is useful once you know the output shape you want.
Questions
How do I keep an image proportional when resizing?
Enter the original width and height as the ratio, then type the new width — the height is solved as you type from newHeight = newWidth × ratioHeight ÷ ratioWidth. The page shows that arithmetic with your own numbers substituted.
What ratio is 1920 × 1080?
16:9. The greatest common divisor of 1920 and 1080 is 120, and dividing both by it gives 16:9. Type any pair of dimensions into the width and height boxes and the simplified ratio appears immediately.
Why is 21:9 shown as 64:27?
Because “21:9” is marketing shorthand. Real ultrawide panels are 2560 × 1080 or 3440 × 1440, both of which reduce to 64:27 — that is 2.370:1, not the 2.333:1 that a literal 21:9 would give.
What are the common social media ratios?
1:1 for a square feed post, 4:5 for the tallest allowed in-feed portrait, 9:16 for Stories, Reels, Shorts and TikTok, and 16:9 for YouTube and most landscape video. All of them are preset buttons on this page.
Does a ratio have to use whole numbers?
No. Cinema scope is 2.39:1 and ISO paper is √2:1 (about 1.414:1). Type decimals into the ratio boxes and the calculator will report a decimal ratio rather than forcing a misleading integer pair.