SizeKit reference

Aspect Ratio Calculator

Simplify any width and height to its aspect ratio, or scale a known ratio to a new size without distorting.

1920 x 1080 = 16:9. 1080 x 1920 = 9:16. 1080 x 1080 = 1:1. Enter your width and height to see the simplified ratio, or pick a common ratio and scale.

SizeKit

Common results first

These cover frequent one-off checks. Use the full calculator below only for custom values.

ISO A A4 210 x 297 mm 2480 x 3508 px at 300 DPI Open North America US Letter 215.9 x 279.4 mm 2550 x 3300 px at 300 DPI Open Print 2480 x 3508 px 8.27 x 11.69 in at 300 DPI Customize Ratio 1920 x 1080 16:9 common video / screen ratio Customize

SizeKit

Three calculation modes

Pick the mode that matches what you already know.

1 Find ratio from width and height Enter width and height in pixels. Result is the simplified ratio (e.g. 1920 x 1080 = 16:9). 2 Scale by width Keep the same ratio. Enter the original width and height, then a new target width. Result is the matching height. 3 Scale by height Keep the same ratio. Enter the original width and height, then a new target height. Result is the matching width.

SizeKit

Aspect ratio calculator

Fill the inputs needed for the mode you want. Other inputs are ignored.

Width

Height

Target width

Target height

SizeKit

Common aspect ratios

Copyable presets for the ratios used in video, photo, and social media.

Ratio	Common sizes	Where it is used
16:9	1280 x 720, 1920 x 1080, 2560 x 1440, 3840 x 2160	HD/FHD/QHD/4K video, YouTube, monitors
9:16	720 x 1280, 1080 x 1920	TikTok, Instagram Reel, Story, YouTube Shorts
1:1	1080 x 1080, 1200 x 1200, 2048 x 2048	Instagram square post, profile picture, album cover
4:3	1024 x 768, 1280 x 960, 1600 x 1200	Older monitors, iPad, presentations
3:4	1080 x 1440, 1200 x 1600	Xiaohongshu cover, vertical product photo
4:5	1080 x 1350, 1200 x 1500	Instagram portrait post
21:9	2560 x 1080, 3440 x 1440, 5120 x 2160	Ultrawide monitors, cinematic video
3:2	1080 x 720, 6000 x 4000	DSLR / mirrorless photo
2:3	720 x 1080, 4000 x 6000	Portrait DSLR photo, photo print 4x6

SizeKit

Formula

Aspect ratio is width divided by height, simplified.

Mode 1: ratio = width : height, divided by gcd(width, height). Example: 1920 / gcd(1920, 1080) = 16; 1080 / gcd = 9. Mode 2: target_height = round(target_width x height / width). Mode 3: target_width = round(target_height x width / height).

SizeKit

Use this tool for

When the aspect ratio calculator is the right answer.

Resizing video or image without cropping or stretching.
Checking whether a source matches a target ratio (e.g. fit a 4:3 photo into 16:9 video frame).
Picking the right export size for Instagram, TikTok, YouTube, or other platform.
Finding the simplified ratio of any width x height for spec sheets, design tokens, or layout rules.

SizeKit

Source and calculation method

Math and conventions used by SizeKit.

Source type

Calculated, exact

Ratios are calculated via greatest common divisor. Common ratio names (16:9, 4:3) are industry conventions, not standards.

Calculation method

Browser-side, rounded

Math runs in the browser. Scaled dimensions are rounded to the nearest whole pixel.

Last reviewed

2026-05-29

Common platform export ratios re-verified twice a year. Mathematical ratios do not change.

SizeKit

Related paths

Use a reference page if you know the size name; use a calculator only when your value is custom.

A4 size page

Get mm, inches, and common pixel presets without configuring anything.

PAPER

All paper sizes

Browse by name, country, and use case.

SOC

Social and web sizes

Find recommended export dimensions.

SizeKit

FAQ

Practical calculator questions.

What aspect ratio is 1920 x 1080?

1920 x 1080 simplifies to 16:9. This is the standard ratio for HD video, YouTube, and most modern monitors.

What size is 16:9?

16:9 is a ratio, not a size. Common 16:9 sizes are 1920 x 1080 (Full HD), 1280 x 720 (HD), 3840 x 2160 (4K UHD), and 2560 x 1440 (QHD).

What size is 9:16?

9:16 is vertical video. Common sizes: 1080 x 1920 (Instagram Reel, TikTok, YouTube Shorts), 720 x 1280 (lower bitrate).

How do I simplify an aspect ratio?

Divide width and height by their greatest common divisor (GCD). Example: 1920 / 120 = 16, 1080 / 120 = 9. So 1920 x 1080 = 16:9.