Aspect Ratio Calculator
Reduce width by height to its simplest ratio, or find the missing dimension for a target ratio. Free, instant, runs in your browser.
An aspect ratio only means something once it is reduced, so 1,920 by 1,080 becomes 16 by 9 rather than a pair of large numbers. This calculator does that reduction, and inverts it too: give it a target ratio and one known dimension, and it returns the other side without changing the proportions.
How it works
How the ratio is found
Divide the width and the height by their greatest common divisor. The two resulting numbers are the ratio in its simplest form, and they describe the shape of the image without saying anything about how many pixels it has. That separation is the point: 1,920 by 1,080 and 640 by 360 are both 16 by 9.
ratio = width : height, reduced by their greatest common divisor
Common image and video ratios
These are the ratios that show up constantly, so recognising them saves a calculation. Landscape shapes run wide, portrait shapes are the same numbers reversed, and square is 1:1 regardless of the pixel count.
| Ratio | Decimal | Shape | Typical use |
|---|---|---|---|
| 1:1 | 1.00 | Square | Product photos, profile pictures, app icons |
| 4:3 | 1.33 | Landscape | Older photos, compact cameras, some scans |
| 3:2 | 1.50 | Landscape | DSLR photos, many phone cameras |
| 16:10 | 1.60 | Widescreen | Laptop screens, some tablets |
| 16:9 | 1.78 | Widescreen | Video, streaming, YouTube thumbnails |
| 21:9 | 2.33 | Ultrawide | Cinema framing, phone ultrawide modes |
| 9:16 | 0.56 | Portrait | Phone video, Instagram stories and reels |
| 4:5 | 0.80 | Portrait | Instagram feed posts, portrait photography |
Changing a ratio means cropping or padding
Reducing a 4:3 image to 16:9 cannot be done without a decision. Cropping scales the image until the width is full, then removes the top and bottom, so you lose content. Padding scales the image to fit entirely inside the new frame and fills the remainder with empty space, so you keep everything but add bars. Neither changes the proportions of what is inside the frame.
- Crop when the subject can be trimmed without losing what matters, and the result will be used edge to edge.
- Pad when losing any part of the image is unacceptable, such as screenshots, diagrams, or documents.
- Fill and fit are the two names most resizers give these behaviours. The print size calculator helps you pick target dimensions once you have decided.
NoteCalculating a ratio does not crop anything. This page does the arithmetic, and the resize presets do the cropping.
Finding the missing dimension
When you know the target ratio and one side, the other side follows from a single multiplication or division. This is the calculation a designer does when a layout fixes the width and the ratio decides the height, or when a print size fixes one edge. Enter the ratio and the known side, and the calculator returns the other.
- Width known: height = width × (ratio height ÷ ratio width).
- Height known: width = height × (ratio width ÷ ratio height).
- Round to whole pixels, because a fraction of a pixel does not exist. A 100 pixel width at 16:9 gives 56.25, which becomes 56.
Frequently asked questions
What does an aspect ratio tell me?
It expresses width to height as a reduced pair, such as 16 by 9, independent of the pixel count. A 1,920 by 1,080 image and a 1,280 by 720 image share the same ratio, which is why both are 16 by 9.
How do I convert an image to a different aspect ratio?
Working out the numbers is only half of it. The other half is a decision: crop the image to the new ratio, or pad it with empty space. Reducing a 4:3 photo to 16:9 means losing the top and bottom, while padding keeps everything and adds bars.
What is the aspect ratio of 1920 by 1080?
Dividing both numbers by their greatest common divisor of 120 gives 16 by 9, the standard for full HD. 1,080 by 1,080 is 1 by 1, and 1,080 by 1,920 is 9 by 16, which is why portrait and landscape versions of the same frame share a number.
Is a 4:3 image the same ratio as 4 by 3 inches?
Yes, the ratio is independent of the unit. A 4:3 photo and a 4 by 3 inch print have the same shape. The unit only changes the physical size, not the proportion.
Does changing aspect ratio reduce resolution?
It depends on the method. Cropping discards pixels, so the result is smaller. Padding keeps every pixel and adds empty space around them. If you want the same pixel count in a new shape, pad rather than crop, then check the square resize presets.