Aspect Ratio Calculator
Reduce a resolution to its ratio, or find the missing width or height.
About
Aspect Ratio Calculator
An aspect ratio is the proportion between width and height, and it stays constant however the image is scaled. Get it wrong when resizing and the result is stretched, squashed or cropped in a way that is immediately visible.
Finding the ratio
Divide both numbers by their greatest common divisor. For 1920 by 1080, the greatest common divisor is 120:
- 1920 ÷ 120 = 16
- 1080 ÷ 120 = 9
- Ratio: 16:9
The decimal form, 1.778, is often more useful for comparison, since it can be checked against another figure directly without reducing anything.
Finding a missing dimension
Hold the ratio and solve for the side you need:
height = width × ratio height ÷ ratio width
width = height × ratio width ÷ ratio height
A 16:9 image 2560 pixels wide is 2560 × 9 ÷ 16 = 1440 tall. That is the entire calculation behind every "scale proportionally" checkbox.
Ratios worth knowing
| Ratio | Decimal | Where it is used |
|---|---|---|
| 16:9 | 1.778 | HD and 4K video, most monitors, phones in landscape |
| 4:3 | 1.333 | Older televisions, many tablets, compact cameras |
| 3:2 | 1.500 | 35mm film and most DSLR sensors |
| 1:1 | 1.000 | Square social posts and profile images |
| 9:16 | 0.563 | Vertical phone video and short-form platforms |
| 64:27 | 2.370 | Ultrawide monitors, sold as 21:9 |
| 2.39:1 | 2.390 | Anamorphic widescreen cinema |
| √2:1 | 1.414 | A-series paper |
The 21:9 that is not 21:9
Ultrawide monitors are marketed as 21:9, and 2560 × 1080 actually reduces to 64:27, a decimal ratio of 2.370. A true 21:9 would be 2.333.
The marketing name was chosen because 21:9 reads as an obvious step up from 16:9, and because 64:27 means nothing to a shopper. It is worth knowing when the exact figure matters, since designing to 21:9 and displaying on a 64:27 panel leaves a visible gap.
Why A-series paper is the shape it is
A4 is 210 by 297 mm, a ratio of 1.414, which is the square root of two. That specific number has a property no other ratio has: cutting the sheet in half across its longer side produces two sheets of exactly the same proportion.
A0 is one square metre, A1 is half of it, A2 half again, and each keeps the same shape. Scaling A4 to A3 is a clean factor of √2 with no cropping, which is why photocopiers offer the enlargement as a preset. US Letter, at 8.5 by 11 inches, has no such property, and halving it changes the shape.
Fitting content into the wrong ratio
Three options when the source and target ratios differ, each with a cost.
Letterbox or pillarbox. Add bars to fill the gap. Nothing is lost and screen space is wasted, which is why a 4:3 programme on a 16:9 television has black bars at the sides.
Crop. Fill the frame and cut the overflow. Nothing is distorted and something leaves the picture, which is why faces get clipped in automatically cropped thumbnails.
Stretch. Force the image to fit. Nothing is lost or cropped and everything is distorted. This is the option to avoid, and it is the one that happens by default when someone drags a corner handle without holding shift.
Ratio against resolution
These get conflated and are separate things. The ratio is a shape; the resolution is a pixel count. 1280 × 720 and 3840 × 2160 share the 16:9 shape and differ ninefold in pixels.
An image can be the right shape and too low a resolution, which shows as softness rather than distortion. It can also be a high resolution and the wrong shape, which shows as bars or cropping. Diagnosing which problem you have saves time, since the fixes are unrelated.
Photo print sizes
Standard print sizes do not share one ratio, which is why prints come back cropped in ways people do not expect.
| Ratio | Decimal | |
|---|---|---|
| 6 × 4 in | 3:2 | 1.500 |
| 7 × 5 in | 7:5 | 1.400 |
| 10 × 8 in | 5:4 | 1.250 |
| 12 × 8 in | 3:2 | 1.500 |
Most cameras shoot 3:2, which matches 6 × 4 and 12 × 8 exactly. Printing the same file at 10 × 8 means scaling the short side to 8 inches, which makes the long side 12, and then cutting 2 inches away. That is 16.7% of the image gone, taken off one or both ends.
Phones commonly shoot 4:3, which fits none of the standard prints cleanly. Leaving some empty space around the subject when composing gives the crop somewhere to take from, and it is the only reliable defence against a print that clips someone's head.
Cropping deliberately
When a crop is unavoidable, choosing it beats letting software choose. Automatic cropping usually takes from the centre outward, which is exactly wrong for a photograph composed with the subject off-centre.
The rule worth following is to crop from the side with least information. A landscape with a large empty sky can lose the top; a portrait cannot lose the top of the head. Where the software offers a crop preview before printing, it is worth the extra click.
Pixel aspect ratio
A complication that appears in video: pixels are not always square. Standard definition digital video used rectangular pixels, so a 720 × 576 frame displayed as 4:3 rather than the 5:4 the numbers suggest.
This is why old footage sometimes appears stretched or squashed when opened in software that assumes square pixels. Modern formats use square pixels throughout, and the problem is confined to archive material and to anamorphic cinema, where the image is optically squeezed onto the film and unsqueezed by the projector lens.
Ratios in web layout
CSS has an aspect-ratio property that holds a box to a fixed shape as the page resizes, which removes the padding trick that used to be required. Setting aspect-ratio to 16 / 9 on a container keeps embedded video the right shape at any width.
It matters for page stability as much as for appearance. A image or embed without reserved space causes the content below it to jump when the asset loads, which counts against a page in Core Web Vitals and is irritating to read. Giving every image its width and height attributes, or an explicit ratio, lets the browser reserve the space before anything arrives.
Video and the shape of the frame
Film has changed shape repeatedly. Early cinema used 1.375:1, close to the 4:3 that television inherited. Widescreen formats arrived in the 1950s partly to distinguish cinema from television, and 2.39:1 anamorphic remains the widest in general use.
16:9 was chosen as a compromise when digital television was standardised: wide enough to feel cinematic, narrow enough that older 4:3 material did not waste too much of the screen. It sits close to the geometric mean of the formats it had to accommodate, which is why it fits everything moderately well and nothing perfectly.
Common mistakes
Rounding one dimension. Rounding 1440 to 1400 breaks the ratio and produces a slight stretch that is hard to name and easy to see.
Assuming 21:9 is exact. It is a marketing name for 64:27.
Confusing ratio with orientation. 16:9 and 9:16 are the same shape rotated, and swapping them is what produces sideways video.
Dragging without holding shift. Most editors constrain the ratio only while shift is held.
Common questions
Frequently asked questions
The proportion between width and height, written as two numbers with a colon between them. It describes the shape rather than the size, so 1280 by 720 and 3840 by 2160 are both 16:9 despite one holding nine times the pixels.
Divide the width and height by their greatest common divisor. For 1920 by 1080 that divisor is 120, giving 16:9. The decimal form, width divided by height, is often more practical since it can be compared directly without reducing.
Any resolution where width divided by height equals 1.778. Common ones are 1280 by 720, 1920 by 1080, 2560 by 1440 and 3840 by 2160. All four are the same shape at different pixel counts.
No. Ultrawide monitors sold as 21:9 are usually 2560 by 1080 or 3440 by 1440, both of which reduce to 64:27, a decimal ratio of 2.370. A true 21:9 would be 2.333, and the marketing name was chosen because it reads as a step up from 16:9.
Change one dimension and calculate the other from the ratio. A 16:9 image resized to 2560 wide must be 1440 tall. Most editors do this automatically while shift is held, and stretch the image when it is not.
The ratio is the shape and the resolution is the pixel count. Getting the ratio wrong produces stretching or unexpected cropping; getting the resolution wrong produces a soft or oversized image. They are separate problems with separate fixes.
Because the square root of two is the only ratio where halving the sheet across its long side produces two sheets of the same shape. That is what lets A4 scale to A3 or A5 without cropping, and why photocopiers offer those steps as presets.
It depends on the placement rather than the platform. Square at 1:1 and vertical at 4:5 suit feed posts, 9:16 suits full-screen stories and short video, and 16:9 suits landscape video. Checking the current specification is worth doing, since platforms change them.