Aspect ratio calculator for video

What the calculator does

It does four jobs, all with whole-number pixel maths and no video file involved. First, it reduces any width and height to its simplest ratio and says how close that is to a familiar one. Second, it resizes: type a new width and the height follows, or the other way round. Third, it works out the biggest window of a target ratio (9:16 by default) that fits inside your frame, where it sits, and how much of the picture it keeps. Fourth, it checks the shape against the ratio statements platforms publish.

Nothing leaves the page. You type numbers, the page does arithmetic, and no network request is made. The size boxes also accept commas, so 3,840 works.

How to use it

1. Enter the width and height of your video in pixels, or tap a common size. The ratio appears at once, with the decimal value and whether the frame is landscape, portrait or square.
2. To resize, type a new width or a new height in the second card. The other side is filled in and rounded to the nearest whole pixel. If rounding was needed, the card says the ratio is a hair off.
3. To crop, pick a target ratio. The preview shows the window on your frame. Move the slider to slide the window left and right (or up and down for a tall source). The numbers show its size, its start point and how much of the picture it keeps.
4. Read the last card to see which platforms accept that shape, with a link to each platform's own page.

Tick the even-numbers box if you want the window rounded down to an even pixel count. A 1920 x 1080 source gives 607 x 1080 by default and 606 x 1080 with the box ticked. At 606 x 1080 the ratio is a hair off a true 9:16, which is why the box is off until you ask for it.

Worked examples

The share is the same for every 16:9 source because the ratio is the same. A 9:16 window is 9/16 of the frame height wide, and a 16:9 frame is 16/9 of its height wide, so the window keeps 81/256 of the width: 31.6%.

A size that looks like 16:9 may not be. 1366 x 768 reduces to 683:384, about 0.05% away from 16:9. The calculator shows it as close to 16:9 rather than calling it exact. For resizing, 1920 x 1080 to a width of 1000 gives a height of 562.5, which rounds to 563.

The limits it applies, and where they come from

The platform check is about the shape of the frame only. Length, file size and codecs are separate rules. Every row below was read from the platform's own page on 11 October 2026.

Instagram's API documentation accepts a wider range than the Help Center does. The calculator uses the Help Center range because that is the one for app uploads. TikTok is shown as a note, not a pass or fail, because the pages we read give no organic ratio range.

What it will not tell you, and the caption margin

No platform page we read gives pixel margins for captions on organic clips. Meta publishes percentages for Reels ads: keep about 14% of the top, 35% of the bottom and 6% on each side clear of text and logos. TikTok says its ad safe zone depends on the video size and caption length and offers downloadable files instead of numbers. YouTube's help pages gave no safe-zone figures at all.

The dashed box on the 9:16 preview reuses Meta's ad percentages as a cautious starting point. It is KeyMo's conservative recommendation, not a rule from any platform, and it does not apply to ads on TikTok or YouTube. On a 1080 x 1920 clip it leaves a text area of 950 x 979 pixels.

• It does not give a recommended pixel size for each platform. YouTube's resolution table covers 16:9 only and lists no vertical size, and Instagram's Help page gives a 720 pixel minimum without saying which side.
• It does not read your video, so it cannot tell where the speaker is. The window position is yours to set.
• It does not check duration, file size or frame rate.

When a fixed 9:16 window is enough, and when it is not

If you have one speaker who stays in roughly one spot, set the window once and you are done. If two people sit apart, the maths works against you. A 9:16 window from a 1920-wide frame is 607 px across, so two faces 1,000 px apart cannot both be in it. You then need a cut between speakers, a split layout, or a window that moves.

Moving the window by hand through an hour of footage is the slow part. KeyMo takes a long video and returns vertical 9:16 clips at 1080 x 1920, 30 fps, with automatic face-tracking reframe, scored clips and 12 caption styles. It is a paid product with no free plan, while this calculator is free and works on its own. See how pricing works, or read the safe zone glossary entry for the caption-margin background.