Aspect Ratio Calculator
Resize while keeping proportions exact.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- New height
- 720 px
- Aspect ratio
- 16:9
- Decimal ratio
- 1.7778
- Scale factor
- 0.6667×
Working
- 1h₂ = 1280 × 1080 ÷ 1920 = 720 px
Quick answer
An aspect ratio is width divided by height, written as two whole numbers such as 16:9. To resize without stretching, scale both sides by the same factor: new height = new width × original height ÷ original width. Shrinking a 1920 × 1080 video to 1280 pixels wide gives 1280 × 1080 ÷ 1920 = 720 pixels high, still 16:9.
What the Aspect Ratio calculator does
This calculator does two things. Give it an original width and height and it reduces them to their simplest ratio (1920 × 1080 becomes 16:9) and a decimal ratio. Give it a new width as well and it returns the matching height, so an image, video, canvas or embed keeps its proportions when resized.
Designers use it to size images for websites and social posts, video editors to set export resolutions, and developers to size responsive embeds. The units do not matter as long as both numbers use the same one — pixels, inches and centimeters all work the same way.
The ratio shown is exact arithmetic, which sometimes produces an unfamiliar result: screens marketed as "21:9" are often 64:27 or 43:18 when reduced exactly. For screen sharpness rather than shape, the screen PPI calculator combines resolution with physical size.
The formula
h₂ = w₂ × h₁ ÷ w₁ Aspect ratio = (w₁ ÷ g) : (h₁ ÷ g), where g = greatest common divisor of w₁ and h₁
| Symbol | Meaning | Unit |
|---|---|---|
| w₁, h₁ | Original width and height | px (or any length unit) |
| w₂ | New width you want | same unit as w₁ |
| h₂ | Height that keeps the same proportions | same unit as h₁ |
| g | Greatest common divisor, used to reduce the ratio to whole numbers | — |
This is ordinary proportional scaling (similar rectangles). To solve for width from a new height instead, swap the roles: w₂ = h₂ × w₁ ÷ h₁. Pixel dimensions must be whole numbers, and many video codecs prefer even numbers, so round the result — rounding by a pixel changes the ratio by an amount no viewer will notice. The general ratio calculator handles proportions that are not width and height.
Worked examples
Resizing a photo for the web
A 4000 × 3000 pixel camera photo needs to be 1200 pixels wide for a blog.
Inputs
- Original width4000 px
- Original height3000 px
- New width1200 px
Result
- New height900 px
- Aspect ratio4:3
- Decimal ratio1.3333
- Scale factor0.3×
1200 × 900 keeps the camera's 4:3 shape at 30% of the original size. Resizing before upload saves bandwidth and speeds up the page.
Vertical video for Reels, TikTok and Shorts
A 1080 × 1920 vertical clip needs a lighter 720-pixel-wide version.
Inputs
- Original width1080 px
- Original height1920 px
- New width720 px
Result
- New height1280 px
- Aspect ratio9:16
- Decimal ratio0.5625
- Scale factor0.6667×
720 × 1280 stays at 9:16, the vertical format used by short-form video apps. Keep the original 1080 × 1920 for upload where possible, since platforms re-compress smaller files harder.
Ultrawide monitor resolution
Checking the true ratio of a 2560 × 1080 "21:9" monitor and the half-size resolution for a window.
Inputs
- Original width2560 px
- Original height1080 px
- New width1280 px
Result
- New height540 px
- Aspect ratio64:27
- Decimal ratio2.3704
- Scale factor0.5×
The exact ratio is 64:27, about 2.37:1. "21:9" (2.33:1) is a marketing label covering several ultrawide resolutions, so use the exact pixel numbers when sizing content to fill the screen.
Common aspect ratios and where they are used
The decimal ratio is width ÷ height. Ratios above 1 are landscape, below 1 portrait.
| Ratio | Decimal | Typical sizes | Used for |
|---|---|---|---|
| 16:9 | 1.78 | 1280 × 720, 1920 × 1080, 3840 × 2160 | HD and 4K TV, YouTube, most monitors |
| 9:16 | 0.56 | 1080 × 1920 | Stories, Reels, TikTok, YouTube Shorts |
| 4:3 | 1.33 | 1024 × 768, 4000 × 3000 | Older TVs and monitors, many phone and compact cameras |
| 3:2 | 1.5 | 6000 × 4000 | 35 mm film, most DSLR and mirrorless cameras |
| 1:1 | 1 | 1080 × 1080 | Square social posts, profile images |
| 4:5 | 0.8 | 1080 × 1350 | Portrait social feed posts |
| 16:10 | 1.6 | 1920 × 1200, 2560 × 1600 | Many laptops and productivity monitors |
| 2.39:1 | 2.39 | Widescreen cinema | Anamorphic "scope" feature films |
Fitting one ratio into another
When content and screen ratios differ, something has to give:
- Letterboxing: a wider image on a narrower screen gets black bars above and below. A 2.39:1 film on a 16:9 TV fills only about 74% of the screen height (1.78 ÷ 2.39).
- Pillarboxing: a narrower image on a wider screen gets bars on the sides, as when 4:3 footage plays on a 16:9 display.
- Cropping: filling the frame means cutting off part of the image. Cropping 16:9 to 9:16 keeps only about a third of the original width, so frame vertical versions on purpose rather than cropping later.
- Stretching: changing one dimension alone distorts the picture. This calculator exists to avoid it.
Getting the aspect ratio of a TV or screen size
Screen sizes are measured diagonally, so the same diagonal gives different widths at different ratios. For a 16:9 screen, width ≈ diagonal × 0.872 and height ≈ diagonal × 0.490; a 65-inch TV is therefore about 56.7 inches wide and 31.9 inches high. For any ratio W:H, width = diagonal × W ÷ √(W² + H²). Video resolution and its bitrate decide file size, which the video file size calculator estimates.
How to use the Aspect Ratio Calculator
- 1Enter the original width and height in pixels (or any single unit).
- 2Enter the new width you need; the matching height appears.
- 3To fix height instead, enter the dimensions rotated (height as width), then swap the result back.
- 4Round the new size to whole pixels — to an even number for video exports.
Frequently asked questions
How do I calculate aspect ratio?
Divide width and height by their greatest common divisor. For 1920 × 1080 the divisor is 120, giving 16:9. Width ÷ height gives the decimal form, 1.78.
What is the aspect ratio of 1920 × 1080?
16:9. Other common 16:9 resolutions include 1280 × 720, 2560 × 1440 and 3840 × 2160.
What height should a 1080-pixel-wide 16:9 image be?
1080 × 9 ÷ 16 = 607.5 pixels, so round to 608, which is also an even number for video. Odd results like this are why widths that are multiples of 16 are popular for video.
Is 21:9 the same as 64:27?
Not exactly. 21:9 is about 2.33:1 and 64:27 (2560 × 1080) is about 2.37:1. Manufacturers use "21:9" as a family name for ultrawide screens.
What aspect ratio is best for Instagram and TikTok?
Use 9:16 (1080 × 1920) for Stories, Reels and TikTok, 4:5 (1080 × 1350) for portrait feed posts and 1:1 (1080 × 1080) for square posts.
References
- Recommendation ITU-R BT.709, Parameter values for the HDTV standards — International Telecommunication Union
- Recommendation ITU-R BT.2020, Parameter values for ultra-high definition television systems — International Telecommunication Union
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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