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Aspect Ratio Calculator
This aspect ratio calculator finds the ratio of any width and height, work out the missing dimension for 16:9, 9:16, 4:5 or your own ratio, and see how much to crop — or how big the bars are — when you change shape.
How aspect ratio is calculated
- Ratio in lowest termswidth ÷ GCD : height ÷ GCD (GCD = greatest common divisor)
- Decimal ratiowidth ÷ height, written as x:1
- Missing dimensionheight = width ÷ ratio · width = height × ratio
- Fill (center crop)scale = max(frame W ÷ W, frame H ÷ H); crop the overflow, half each side
- Fit (bars)scale = min(frame W ÷ W, frame H ÷ H); bars fill the gap, half each side
The greatest common divisor is found with Euclid's algorithm: replace the larger number by the remainder of dividing it by the smaller, until the remainder is zero. The nearest common ratio is the one with the smallest percentage difference. When a crop or bar total is odd, the extra pixel goes to the right or bottom.
Worked examples
- 1920 × 1080: GCD 120, so 16:9, or 1.778:1. That's 2.07 megapixels.
- 1366 × 768: GCD 2, so 683:384 = 1.7786:1. It is sold as 16:9 but is 0.049% wider, because exact 16:9 at 768 high would be 1,365.333 pixels wide.
- 2.39:1 at 1920 wide: 1920 ÷ 2.39 = 803.347, rounded to an even 804 — 1,920 × 804, -0.081% off the target ratio.
- 1920 × 1080 into a 1080 × 1350 feed post: scale by 125% to 2,400 × 1,350, then crop 660 pixels from each side. 45% of the picture is kept.
Common sizes and their ratios
Platform sizes are the recommended upload sizes, not hard limits — most platforms accept other sizes and scale or crop them. Ratios are computed from the sizes.
| Use | Size (px) | Ratio |
|---|---|---|
| YouTube video 1080p | 1920 × 1080 | 16:9 |
| YouTube video 4K | 3840 × 2160 | 16:9 |
| YouTube Shorts | 1080 × 1920 | 9:16 |
| Instagram / Facebook Reels | 1080 × 1920 | 9:16 |
| TikTok video | 1080 × 1920 | 9:16 |
| DCI 2K full container | 2048 × 1080 | 1.896:1 |
| DCI 2K flat | 1998 × 1080 | 37:20 |
| DCI 2K scope | 2048 × 858 | 2.387:1 |
| DCI 4K full container | 4096 × 2160 | 1.896:1 |
| Instagram post — square | 1080 × 1080 | 1:1 |
| Instagram post — portrait 4:5 | 1080 × 1350 | 4:5 |
| Instagram post — landscape 1.91:1 | 1080 × 566 | 1.908:1 |
| Instagram story / reel cover 9:16 | 1080 × 1920 | 9:16 |
| Facebook Page cover | 851 × 315 | 2.702:1 |
| X header | 1500 × 500 | 3:1 |
| X profile photo | 400 × 400 | 1:1 |
| X post image — square | 1200 × 1200 | 1:1 |
| X post image — landscape 1.91:1 | 1200 × 628 | 1.911:1 |
| LinkedIn profile background | 1584 × 396 | 4:1 |
| YouTube thumbnail — HD | 1280 × 720 | 16:9 |
| YouTube thumbnail — 4K | 3840 × 2160 | 16:9 |
| YouTube channel banner | 2560 × 1440 | 16:9 |
Named ratios
- 16:9 (1.778) — HD and 4K video, YouTube, TVs
- 9:16 (0.563) — Shorts, Reels, TikTok, stories
- 4:3 (1.333) — older TVs, iPads, many compact cameras
- 3:4 (0.75) — portrait phone photos
- 3:2 (1.5) — 35 mm film and most DSLR / mirrorless photos
- 2:3 (0.667) — portrait photos, Pinterest pins
- 1:1 (1) — square posts and profile photos
- 4:5 (0.8) — portrait Instagram / Facebook feed posts
- 5:4 (1.25) — 8 × 10 prints, old 1280 × 1024 monitors
- 21:9 (2.333) — ultrawide monitors (marketing name)
- 1.85:1 (1.85) — "flat" widescreen cinema
- 2.39:1 (2.39) — anamorphic "scope" cinema
Sources: video platform sizes are the widely published recommendations for YouTube, Shorts, TikTok and Instagram — they change over time, so check the platform's current guidance before a big upload; social image sizes from each platform's help pages as listed in the Northlane Image Resizer, checked October 7, 2026; DCI container sizes from the DCI Digital Cinema System Specification; 4K UHD from ITU-R BT.2020. Checked 14 October 2026.
Frequently asked questions
How do I calculate an aspect ratio?+
Divide the width and the height by their greatest common divisor. For 1920 × 1080 the greatest common divisor is 120, so the ratio is 16:9. As a decimal it is width ÷ height = 1.778:1.
Is 1366 × 768 16:9?+
Not exactly. Its greatest common divisor is only 2, so it reduces to 683:384, or 1.7786:1. True 16:9 at 768 high would be 1,365.333 pixels wide, which isn't a whole number, so panel makers rounded up to 1366. It is 0.049% wider than 16:9, which you can't see.
How do I find the height for a 16:9 width?+
Divide the width by 16 and multiply by 9 — the same as dividing by 1.7778. 1280 wide gives 720 high. 1000 wide gives 562.5, which isn't a whole pixel; rounded to an even number it is 562, a 0.089% ratio error.
Why should video sizes be even numbers?+
Most H.264 and H.265 video uses 4:2:0 chroma subsampling, which stores color at half the width and half the height of the picture. An odd width or height can't be halved evenly, so many encoders refuse it or add a pixel. Multiples of 8 or 16 match the block size some older encoders and hardware prefer.
What is the difference between 4K UHD and DCI 4K?+
4K UHD, used by TVs, streaming and YouTube, is 3840 × 2160 — exactly 16:9. DCI 4K, the cinema container, is 4096 × 2160, which reduces to 256:135 or about 1.896:1. Films shot for cinema are usually framed inside it at 1.85:1 (flat) or 2.39:1 (scope).
Is 21:9 a real aspect ratio?+
It's a marketing name. A 2560 × 1080 ultrawide monitor is 64:27 (2.37:1) and a 3440 × 1440 one is 43:18 (about 2.389:1). Exact 21:9 would be 2.333:1. All three are close to the 2.39:1 used for widescreen cinema.
How do I turn a 16:9 video into 9:16 for Shorts, Reels or TikTok?+
Either crop it or add bars. Center-cropping 1920 × 1080 to 9:16 keeps a 608 × 1,080 strip and cuts 656 pixels from each side — only 31.7% of the picture. Fitting it instead keeps everything but leaves large bars above and below. Most editors let you move the crop to follow the subject.
What is the difference between letterbox and pillarbox?+
Letterboxing adds bars above and below a picture that is wider than the frame, like a 16:9 video in a 4:3 frame (180 pixels top and bottom at 1920 wide). Pillarboxing adds bars at the left and right of a picture that is narrower than the frame, like a 4:3 video on a 16:9 screen.
What aspect ratio is a 4 × 6 or 8 × 10 photo?+
A 4 × 6 print is 3:2, the same shape as most camera photos, so it prints without cropping. An 8 × 10 is 5:4, which is squarer: filling it from a 6000 × 4000 photo keeps a 5,000 × 4,000 area, 83.3% of the picture, and crops the rest from the sides.
What aspect ratio is 1080 × 1350?+
It's 4:5, a portrait shape 1:1.25 — the tall size long used for Instagram and Facebook feed posts. Turned sideways, 1350 × 1080 is 5:4.
How do I resize an image without stretching it?+
Change the width and height by the same factor, so the ratio stays the same. To shrink a 4000 × 3000 photo to 1,200 pixels wide, divide the height by the same amount: 3000 × 1,200 ÷ 4000 = 900. Enter the ratio and the width you know, and the calculator works out the height.
Is anything I enter saved or sent?+
No. Everything is calculated in your browser and nothing is stored or uploaded.
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