Aspect Ratio Explained: 16:9, 4:3, 1:1 and More

By HSP · Published October 16, 2026 · 8 min read · Images & Photos

Every photo, video, screen and printed page has a shape. Aspect ratio is simply the name for that shape: how wide it is compared with how tall it is. Two pictures with the same ratio can be scaled into each other perfectly. Two pictures with different ratios cannot, so something has to give: you crop, you add bars, or you stretch (which almost always looks wrong). This guide explains the numbers in plain language and shows the maths with real pixel sizes.

What does aspect ratio mean?

An aspect ratio is written as two numbers separated by a colon, width first. 16:9 means that for every 16 units across there are 9 units down. The units do not matter: pixels, inches or centimetres all give the same ratio, because a ratio is a comparison, not a size. A 1280 × 720 thumbnail and a 3840 × 2160 4K frame are both 16:9 even though one has nine times as many pixels.

You will also see ratios written as one decimal number, such as 1.78:1 or 2.39:1. That is just width divided by height. 16 ÷ 9 = 1.778, so 16:9 and 1.78:1 describe the same shape. Film-makers tend to use the decimal style; screen and phone makers usually use whole numbers.

How to find the aspect ratio of any size

Take the width and height and divide both by their greatest common divisor (the biggest whole number that divides both evenly). That gives the simplest whole-number ratio.

1. Write down width and height, for example 1080 × 1350.
2. Find the greatest common divisor. Here it is 270.
3. Divide both: 1080 ÷ 270 = 4 and 1350 ÷ 270 = 5, so the ratio is 4:5.

Some sizes do not reduce neatly. A 1366 × 768 laptop screen reduces to 683:384, which looks odd but is 1.779 as a decimal, almost exactly 16:9 (1.778). Our aspect ratio calculator shows both the exact ratio and the nearest standard one, so you do not have to work out divisors by hand.

The common ratios and where they are used

16:9 is the standard shape for HD and Ultra HD television, most computer monitors and online video. The international HDTV standard (ITU-R BT.709) defines a 1920 × 1080 picture, and the UHDTV standard (BT.2020) uses 3840 × 2160 and 7680 × 4320, all 16:9. YouTube’s help centre also gives 16:9 as the standard player ratio on computers.

4:3 was the shape of older televisions and computer monitors (640 × 480, 1024 × 768). Many phone cameras still shoot 4:3 photos by default, and tablets often have screens close to it. It is a comfortable shape for slides and documents.

3:2 comes from 35 mm film and is still used by many camera sensors. It matches a 4 × 6 inch print exactly, which is why camera photos print on that size without cropping.

1:1 square is popular for profile pictures, product photos and social feeds because it looks the same on phones and computers. 4:5 is a slightly taller portrait shape that takes up more screen space in a phone feed. 9:16 is simply 16:9 turned on its side, the full-screen shape of a phone held upright, used for stories, reels and short videos.

1.91:1 is the usual shape for link previews when a page is shared. Meta’s sharing guide for webmasters recommends images of at least 1200 × 630 pixels, which is about 1.91:1. 2.39:1 and 1.85:1 are cinema shapes; ultrawide monitors marketed as 21:9 are close to 2.39:1.

Worked example: resize without stretching

You have a 1920 × 1080 image and need a version 1280 pixels wide. Keep the ratio by scaling height by the same factor as width:

New height = new width × original height ÷ original width
1280 × 1080 ÷ 1920 = 720. So the new size is 1280 × 720, still 16:9.

Going the other way, if you know the new height, use new width = new height × original width ÷ original height. For a 6000 × 4000 camera photo shrunk to 800 pixels tall: 800 × 6000 ÷ 4000 = 1200 wide. Type the original size and one new number into the calculator and it fills in the other. When you actually shrink the file, the image resizer and compressor keeps proportions for you, and our guide to reducing image size without losing quality covers the file-size side.

Worked example: change the shape by cropping

Resizing cannot change the shape. To turn a 4:3 photo into a 16:9 banner you have to crop. Keep the full width and work out the height that matches the new ratio:

You lose a quarter of the picture height, so leave the subject in the middle band before you crop. In the free image cropper choose “Wide 16:9” and the crop box locks to that shape; drag it to keep the important part, then download. The same idea gives a square: crop a 4000 × 3000 photo to 3000 × 3000 by removing 500 pixels from each side.

Bars instead of cropping: letterbox and pillarbox

When a picture must be shown whole inside a frame of another shape, players add plain bars. A 4:3 video on a 1920 × 1080 screen is scaled to 1080 tall, which makes it 1440 wide (1080 × 4 ÷ 3). That leaves 480 spare pixels, shown as two 240-pixel bars on the left and right, called pillarboxing. A 2.39:1 film on the same screen is about 803 pixels tall (1920 ÷ 2.39), leaving bars of roughly 138 pixels above and below, called letterboxing. Bars keep everything visible; cropping fills the screen but cuts edges. Stretching does neither well, because people and circles visibly change shape.

Printing photos: why some sizes crop

Print sizes have ratios too. A 4 × 6 inch print is 2:3 (the same shape as 3:2 turned upright), so a 3:2 camera photo fits perfectly. An 8 × 10 print is 4:5, so the same photo loses part of its long side. A 5 × 7 print is 5:7, which matches neither. Before you order prints, check the ratio and crop yourself so the lab does not cut off heads or feet. For ID photos with strict sizes, see our guide on making a passport size photo online.

Screens: ratio and diagonal size

Screens are sold by diagonal, but the ratio decides the real width and height. For a 16:9 screen, width = diagonal × 16 ÷ √(16² + 9²) and height = diagonal × 9 ÷ √(16² + 9²). √337 is about 18.36, so a 27-inch 16:9 monitor is roughly 23.5 inches wide and 13.2 inches tall. A 4:3 screen with the same diagonal is taller and narrower, which is why a 16:9 and a 4:3 monitor of the same “size” do not have the same area.

Quick tips

• Pick the ratio first, then the pixel size. Changing shape later always costs part of the picture.
• Shoot a little wider than you need so you can crop to several shapes from one photo.
• For web images, keep a few standard sizes (for example 1200 × 630 for sharing) and save them in an efficient format; see PNG vs JPG vs WebP.
• Round pixel results to whole numbers; a fraction of a pixel cannot be displayed.
• If a site gives a file-size limit as well as a shape, crop first and then compress, as in compressing an image to 20 KB.

Platform size recommendations change from time to time, so check the current help page of the service you are uploading to. The maths of ratios, though, never changes.