An aspect ratio is the shape of a picture: its width compared with its height. It is why a square photo looks lost on YouTube and why a landscape photo gets cropped into a strip on Reels. This guide shows how to work a ratio out, how to turn a square photo into 16:9 or 9:16 without cutting anything off, and which ratio each platform wants. The free aspect ratio calculator and converter does both jobs in your browser.
How to calculate aspect ratio
Take the width and height in pixels and divide both by the largest number that divides them evenly, the greatest common divisor (GCD). For 1920 × 1080 the GCD is 120: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9. The same works for 1080 × 1350 (GCD 270) = 4:5 and 1080 × 1920 = 9:16.
Some sizes don't simplify neatly. 1366 × 768, a common laptop screen, reduces only to 683:384. Its decimal ratio is 1.779, and 16:9 is 1.778, so in practice it is 16:9. The calculator shows that “practically 16:9” match for you.
To find a missing side, cross-multiply: height = width × h ÷ w and width = height × w ÷ h. A 16:9 image 1920 px wide is 1920 × 9 ÷ 16 = 1080 px tall; a 9:16 image 1920 px tall is 1920 × 9 ÷ 16 = 1080 px wide.
How to convert a square image to 16:9 without cropping
A square photo has the wrong shape for a 16:9 frame, so something has to fill the gap. There are two honest options:
- Pad it. Keep the whole photo and fill the empty sides. A 1000 × 1000 photo in a 1920 × 1080 frame is scaled to 1080 × 1080 and centred, leaving a 420 px band on each side. Filled with a blurred, slightly darkened copy of the photo, the bands look intentional, which is why video editors use this for vertical clips on landscape timelines.
- Crop it. Cut a 16:9 strip out of the square. The photo fills the frame, but a 1:1 picture loses 1 − 9/16 = 43.75% of its height. Fine for a landscape, bad for a portrait whose head and feet both matter.
- Open the Image Aspect Ratio Converter and drop in your photo (or up to 20 at once).
- Choose 16:9, or a platform size such as YouTube thumbnail 1280 × 720.
- Pick a fill: Blur background (adjust blur and darkness), Solid colour (the auto option picks the photo's dominant colour), Gradient (blends the colours of the photo's two edges), or Crop and drag to choose what is kept.
- Download as JPG, PNG or WebP. Batches download as a ZIP. Nothing is uploaded.
Aspect ratio chart: common ratios and pixel sizes
| Ratio | Used for | 720 | 1080 | 2160 (4K) |
|---|---|---|---|---|
| 16:9 | YouTube, TV, laptop screens | 1280 × 720 | 1920 × 1080 | 3840 × 2160 |
| 9:16 | Reels, Shorts, TikTok, Stories | 720 × 1280 | 1080 × 1920 | 2160 × 3840 |
| 4:5 | Instagram portrait post | 720 × 900 | 1080 × 1350 | 2160 × 2700 |
| 1:1 | Square posts, profile photos | 720 × 720 | 1080 × 1080 | 2160 × 2160 |
| 4:3 | iPad, older TV, most phone photos | 960 × 720 | 1440 × 1080 | 2880 × 2160 |
| 3:2 | DSLR and mirrorless photos | 1080 × 720 | 1620 × 1080 | 3240 × 2160 |
| 21:9 | Ultrawide monitors, cinematic video | 1680 × 720 | 2520 × 1080 | 5040 × 2160 |
The column heading is the short side; 21:9 at 1080 works out to 2520 × 1080 by the formula, though ultrawide monitors sell as 2560 × 1080, which is really 64:27.
Best aspect ratios for YouTube, Instagram, TikTok and more
| Where | Pixels | Ratio |
|---|---|---|
| YouTube thumbnail | 1280 × 720 | 16:9 |
| Shorts / Reels / TikTok | 1080 × 1920 | 9:16 |
| Instagram square | 1080 × 1080 | 1:1 |
| Instagram portrait | 1080 × 1350 | 4:5 |
| Instagram 3:4 | 1080 × 1440 | 3:4 |
| X / Twitter post | 1600 × 900 | 16:9 |
| LinkedIn post | 1200 × 627 | 1.91:1 |
| Facebook cover | 851 × 315 | 2.70:1 |
Sizes checked in November 2026 against current platform guides. Platforms change these from time to time; Facebook's cover in particular is shown cropped differently on phones and desktops, so keep text away from the edges. For more detail on YouTube and Instagram limits, see resizing images for Instagram and YouTube.
16:9 vs 9:16 vs 4:5: which should you use?
- 16:9 for anything watched on a laptop or TV: YouTube videos and thumbnails, presentations, website headers.
- 9:16 for full-screen phone content: Reels, Shorts, TikTok and Stories. Keep faces and text in the middle, because the app's buttons cover the bottom and right edges.
- 4:5 for Instagram feed posts. It fills more of a phone screen than a square or landscape post. Instagram's profile grid now shows posts as 3:4, so 1080 × 1440 is worth considering too.
Why video sizes should be even numbers
Most video files store colour at half the resolution of brightness (4:2:0 chroma subsampling), in 2 × 2 pixel blocks. An odd width or height leaves half a block, and many encoders, including H.264 in browsers and phones, refuse the file or quietly crop a pixel. Choose Even (video) rounding in the calculator when sizing video. The video resolution guide covers this and bitrate in more depth. For still images, odd sizes are fine.
