Aspect Ratio Calculator: Simplify Any Ratio and Find the Missing Side

This aspect ratio calculator turns any width and height into a clean ratio, like 1920 × 1080 into 16:9. The aspect ratio is simply the shape of an image, screen or video, written as two numbers separated by a colon, and it’s the first thing every platform checks before it crops your upload.

How to calculate aspect ratio with the calculator

1. Enter width and height

   Done

   In pixels, straight from the file’s properties. Or tap a common ratio or pick a social media format.

2. Read the ratio

   Done

   You get the simplified ratio, the decimal, the orientation and the closest standard ratio, instantly.

3. Find the missing side

   Done

   With the ratio locked, type a new width to get the height, or the other way around, rounded to the pixel.

4. Copy it

   Done

   Copy “16:9” for your brief, or the CSS aspect-ratio rule for your site.

It works as an image aspect ratio calculator and for video frames, screens or print sizes alike.

The formula: greatest common divisor

To calculate aspect ratio, divide the width and the height by their greatest common divisor (GCD), the largest number that goes evenly into both. Here is the math.

ratio = (width ÷ GCD) : (height ÷ GCD)
1920 ÷ 120 = 16   and   1080 ÷ 120 = 9   →   16:9
new height = new width × height ÷ width = 1280 × 1080 ÷ 1920 = 720

Sometimes the answer is ugly. A 1366 × 768 laptop screen comes out as 683:384, because its GCD is just 2. That’s why the calculator also shows the decimal form (1.78:1) and flags that it’s within 0.05% of 16:9. Another fun one: monitors sold as “21:9” are usually 2560 × 1080, which is really 64:27.

16:9 aspect ratio, 4:3 and social media sizes

Format	Ratio	Sample size
YouTube video	16:9	1920 × 1080
Reels, Shorts and TikTok	9:16	1080 × 1920
Instagram portrait post	3:4	1080 × 1440
Instagram feed ad	4:5	1440 × 1800
LinkedIn post image	1.91:1	1200 × 627
Pinterest standard Pin	2:3	1000 × 1500
Older TVs and monitors	4:3	1600 × 1200

The 16:9 aspect ratio is YouTube’s standard on desktop, and 9:16 rules everything vertical. The 4:3 aspect ratio gives you more height relative to width, which is why it still shows up in photography and older displays. If you’re hunting for the right Instagram aspect ratio, note that Instagram’s help center now keeps photos between 1.91:1 and 3:4 uncropped, while 4:5 comes from Meta’s ads guides. We checked every format in October 2026.

Aspect ratio for YouTube: 16:9, vertical and what the player adds

The standard aspect ratio for YouTube on a computer is 16:9, according to YouTube’s help page. Anything else still plays: the player resizes to fit and fills the gap with padding, white by default and dark gray in dark theme. YouTube also advises against adding black bars or padding of your own, because it stops the player from resizing.

Here is what that means in numbers. Play a vertical 1080 × 1920 clip in a 1920 × 1080 desktop player. It scales to the height, so its width is 1080 × 9 ÷ 16 = 607.5 px, about 32% of the player. A square 1080 × 1080 video takes 56%. Pick the shape by where most of your viewers watch.

Where they watch	Export size	Ratio
Desktop, TV and laptops	1920 × 1080	16:9
Phones and Shorts	1080 × 1920	9:16
Mixed audience, one file	1080 × 1080	1:1

Repurposing a Reel for YouTube desktop? Expect the padding.

Cinema aspect ratio: 2.39:1, 1.85:1 and their pixel sizes

Cinema runs on two projection formats: 1.85:1 (flat) and 2.39:1 (scope). Older guides write the second as 2.35:1 or 2.40:1. With the ratio locked, type a width in the calculator and it returns the height.

Format	Ratio	At 1920 wide	At 3840 wide
Flat	1.85:1	1920 × 1038	3840 × 2076
Scope	2.39:1	1920 × 804	3840 × 1608

The exact results are 803.3 and 1,606.7. Most video encoders need even dimensions, so round up to the next even number.

Inside a 1920 × 1080 frame, a 2.39:1 picture leaves about 138 px of black bar above and below, the look people expect from a film on a TV.

A marketing warning. On a vertical 1080 × 1920 phone screen, a 2.39:1 video is only 452 px tall, about 23.5% of the screen. Cinematic looks great in a TV spot and wastes space in a feed.

Frequently asked questions about aspect ratios

01

Is 3:4 or 9:16 better for photos?

It depends on where the photo will live. 9:16 fills a phone screen, so it’s the pick for Stories, Reels, Shorts and TikTok. 3:4 is the tallest photo Instagram’s feed shows without cropping, according to its help center, and it keeps more of the frame than 9:16 would. Shooting for both? Frame loosely in 3:4 and crop to 9:16 later.

02

What are the common 16:9 aspect ratios?

The usual 16:9 resolutions are 1280 × 720 (HD), 1920 × 1080 (Full HD), 2560 × 1440 (QHD) and 3840 × 2160 (4K UHD). They all share one shape, so you can scale between them without black bars or cropping. Smaller ones work too, like 854 × 480, which rounds to 16:9 because 480 doesn’t divide evenly. Type any of them above and the calculator confirms the ratio.

03

Can I change the aspect ratio of a photo?

Yes, but only by cropping or by adding space, never by stretching. Cropping cuts the edges to reach the new shape; padding adds bars or a background around it. Stretching changes the ratio too, but it distorts faces, logos and text. Use the calculator to get the exact crop size first, then crop in your editor and check the result at full size.

04

What is aspect ratio?

Aspect ratio describes an image’s shape, not its size. An 800 × 600 photo and a 4000 × 3000 photo share the same aspect ratio, 4:3, even though the second has 25 times as many pixels. That’s why it’s different from resolution: resolution tells you how much detail fits, while aspect ratio tells you whether the image is wide, tall or square.

05

What aspect ratio is best for Instagram?

For feed photos, 3:4 (1080 × 1440) takes up the most screen space: Instagram’s help center says it keeps photos between 1.91:1 and 3:4 without cropping. For Reels and Stories, go vertical 9:16. And if you’re running feed ads, Meta’s ads guide recommends 4:5. Square 1:1 still works everywhere, but it gives up vertical space on a phone.

06

What aspect ratio is 1920x1080?

It is 16:9. Divide both numbers by their greatest common divisor, 120, and you get 16 and 9. That is the Full HD standard for TVs, monitors and YouTube. The same shape holds for 1280 × 720, 2560 × 1440 and 3840 × 2160, so you can scale between them with no bars and no cropping. Type any other width and height into the calculator above to check its ratio.