Aspect Ratio Calculator & Converter (Simplify & Resize)
Instantly calculate and convert aspect ratios from an image or video's width and height — free. Simplify a ratio (e.g. 1920x1080 to 16:9), convert to a target ratio, or compute the missing dimension when resizing while keeping the ratio.
Common Aspect Ratios
| Ratio | Typical Use |
|---|---|
| 1:1 | Instagram square posts, social media profile pictures |
| 4:3 | Older TVs and monitors, standard photo prints |
| 16:9 | Widescreen video, HD/4K displays, the YouTube standard |
| 21:9 | Ultrawide monitors, cinematic-style footage |
| 3:2 | 35mm film cameras, the standard ratio for most digital SLR cameras |
| 9:16 | Vertical video for Instagram/TikTok Stories and Reels |
What an aspect ratio is
An aspect ratio is the proportion between the width and height of an image or a video, written as a ratio of whole numbers such as 16:9 or 4:3. Told that something is 1920×1080 you learn only how big it is; put as a ratio, the shape itself — wide, square or tall — is conveyed in a word. Two things with the same ratio look the same shape whether you enlarge or shrink them.
This tool has two modes. “Simplify the ratio” divides the width and height by their greatest common divisor, turning 1920×1080 into the familiar 16:9. “Resize while keeping the ratio” takes the original width and height plus **only one** of a new width or height, and works out the other in proportion. Results are rounded to whole pixels, so they may differ by about one pixel from what an image editor shows.
How to use the aspect ratio calculator
- Choose a mode Pick “simplify the ratio” if you want the ratio in its usual notation, or “resize while keeping the ratio” if you want the dimensions after resizing.
- Enter the original width and height Give the pixel counts as whole numbers. Values of zero or less, or anything that is not a number, raise an error.
- In resize mode, fill in only one side Enter either the new width or the new height. The other field clears itself, and the calculation always runs from whichever you filled in.
- Read the result You get either the simplified ratio or the width and height after resizing.
- Check the uses in the table A table below sets out the main uses of 1:1, 4:3, 16:9, 21:9, 3:2 and 9:16 — a guide when you are unsure which to work in.
Tips for getting more out of it
- In "Simplify Ratio" mode, enter a video resolution (e.g. 3840x2160) to reduce it by its greatest common divisor into a familiar ratio like 16:9.
- "Resize Keeping Ratio" mode is handy when creating a thumbnail and you need the resulting dimensions without distorting the original image.
- You don't need to fill in both the new width and new height — typing into one automatically clears the other, so the result is always calculated from a single target value.
- Results are rounded to the nearest whole pixel, so they may differ very slightly from the values shown in image editing software.
Where the aspect ratio calculator helps
Naming the ratio behind a video resolution
Enter a resolution such as 3840×2160 and it reduces to 16:9 — useful for confirming that footage is in the ratio you expected.
Settling the size of a thumbnail
When you want every thumbnail 320 pixels wide, this gives the height that keeps the original proportions, so the heights do not come out uneven when you line them up.
Choosing a shape for a social post
Decide between square, wide and tall, then get the dimensions that match. For a vertical video, 9:16 is the usual choice.
Checking whether an image has been distorted
Enter the width and height of an exported image and read off the ratio to see whether it drifted from what you intended — that is, whether it was stretched.
When you want to convert the image itself
Once the dimensions are settled, use image resizing to carry the change out, or image cropping if you would rather cut it down.
Terms used with aspect ratios
- Aspect ratio
- The proportion between width and height, written as a ratio of whole numbers such as 16:9. It describes the shape of an image, a video or a display.
- Greatest common divisor
- The largest number that divides two integers exactly. Dividing width and height by it reduces the ratio to its simplest whole-number form.
- Resolution
- The number of pixels making up an image or a screen, written as width by height, as in 1920×1080. Two different resolutions can perfectly well share the same ratio.
- Pixel
- The smallest unit an image is built from. The results of these calculations are rounded to whole numbers of this unit.
- Letterboxing
- Fitting a picture into a screen of a different ratio by adding black bars above and below, so the original proportions survive. Bars at the sides instead are called pillarboxing.
- 9:16 (vertical)
- The ratio 16:9 with width and height exchanged. It is the standard for video meant to be watched with a phone held upright.
Frequently Asked Questions
Side Note — How aspect ratio standards came to be
The history of film aspect ratios goes back to the early 1900s. Early motion picture film was shot at a ratio close to 4:3 (precisely 1.33:1), but in the 1950s, in competition with the rise of television, cinemas began adopting wider "CinemaScope"-style formats (around 2.35:1) to offer a more dramatic viewing experience.
Television went through a similar transition. CRT televisions used 4:3 for decades, but as digital broadcasting and flat-panel TVs spread in the 2000s, 16:9 (1.78:1) became the standard for high-definition broadcasts. It's often noted that 16:9 was chosen through a mathematical compromise (a geometric mean) between the many competing widescreen film formats of the era and the older 4:3 television screen.
More recently, the rise of smartphones has given new prominence to the "tall" 9:16 ratio. Vertical video formats popularized by TikTok and Instagram Stories are optimized for how people naturally hold and watch content on a phone with one hand — a shift that challenges over a century of assumptions that video content should always be wider than it is tall.