Image Ratio Calculator
This calculator works out the exact aspect ratio of an image, screen, video or print size from its width and height, then shows you how to resize it without stretching or squashing it. Enter your original width and height in any unit, or pick a preset such as Full HD, 4K, a square Instagram post or A4 at 300 dpi, and the tool reduces the two numbers to their simplest whole-number ratio using the greatest common divisor, plus the equivalent decimal ratio. Choose whether to lock the width or the height, enter your target size, and it calculates the missing dimension so the proportions match the original exactly. The results show your simplest aspect ratio, the decimal ratio, and the scaled width and height, backed by a breakdown of the GCD calculation and the scale factor used. A comparison table then checks your ratio against ten common standards, including 16:9, 4:3, 1:1, 3:2 and 9:16, flagging an exact match and naming its typical use, from widescreen video and DSLR photos to Instagram Stories and ultrawide monitors. Use it before resizing photos for social media, planning a video project, laying out a print job, or building a responsive web layout, so you know whether your new size will distort the image and what the correctly proportioned dimensions should be instead.
1. Original Image Size
2. Scale to New Size
Ratio Breakdown
Scaled Dimensions
Common Aspect Ratios
| Ratio | Decimal | Common use | Example size | Match? |
|---|
How the Image Ratio Calculator Works
The aspect ratio of an image is the proportional relationship between its width and its height. It is expressed as two numbers separated by a colon, such as 16:9. The numbers represent how many units wide the image is for every unit of height. To find the simplest form, both numbers are divided by their greatest common divisor (GCD).
The Formula
Given a width W and height H:
- Find the greatest common divisor: GCD(W, H) using the Euclidean algorithm
- Ratio width component = W / GCD(W, H)
- Ratio height component = H / GCD(W, H)
- The aspect ratio is written as (W/GCD):(H/GCD)
To scale while keeping the same ratio, fix one dimension (for example, a new width W2) and calculate the other: H2 = W2 x (H / W). Alternatively, fix H2 and calculate W2 = H2 x (W / H).
Worked Example
Original image: 1920 x 1080 pixels. Target width: 1280 pixels.
- GCD(1920, 1080): 1920 = 1 x 1080 + 840; 1080 = 1 x 840 + 240; 840 = 3 x 240 + 120; 240 = 2 x 120 + 0. GCD = 120.
- Ratio = (1920/120):(1080/120) = 16:9
- Decimal = 1920/1080 = 1.778
- Scaled height = 1280 x (1080 / 1920) = 1280 x 0.5625 = 720 px
- Scaled size: 1280 x 720 pixels
These match the default inputs in the calculator above.
Common Aspect Ratios Explained
| Ratio | Typical use | Example pixel sizes |
|---|---|---|
| 16:9 | Widescreen video, modern monitors, YouTube, TV | 1920x1080, 1280x720, 3840x2160 |
| 4:3 | Older monitors, standard definition TV, iPad | 1024x768, 800x600, 1400x1050 |
| 1:1 | Square, Instagram posts, profile pictures | 1080x1080, 600x600 |
| 3:2 | 35mm film, DSLR cameras, standard photo prints | 2736x1824, 6000x4000 |
| 4:5 | Portrait Instagram posts | 1080x1350 |
| 9:16 | Vertical video, Reels, TikTok, Stories | 1080x1920 |
| 21:9 | Ultrawide monitors, cinema | 2560x1080, 3440x1440 |
Why Aspect Ratio Matters
If you resize an image without maintaining the original aspect ratio, the image will appear stretched or squashed. This is especially important when uploading photos to social media platforms, which often crop or resize images automatically. Knowing the correct ratio lets you crop to the required shape without distortion.
For web designers and developers, understanding aspect ratios helps maintain proportional layouts across different screen sizes. For photographers, the ratio determines how an image will look when printed at standard sizes such as 4x6 inches (3:2) or 5x7 inches (5:7).
Related Calculators
- Maths and Stats Calculators: all maths tools in one place.
- Aspect Ratio Calculator: general aspect ratio tool for screens and displays.
- Area of a Circle Calculator: calculate area from radius or diameter.
- Percentage Calculator: find percentages, changes, and portions.
Method: Aspect ratio reduced to simplest form using the Euclidean algorithm for the greatest common divisor (GCD). Scaled dimensions calculated by fixing one side and multiplying the other by the ratio of original dimensions. Standard ratios sourced from common industry conventions for video, photography, and web design.