When you resize a photo for a website, a print lab or a social network, the width and height must change by the same factor or the picture looks stretched. That factor can come from a target width (a blog column 1,200 px wide), a target height (a slider 800 px tall) or a percentage (50% for a quick preview). Each case is one line of proportion arithmetic, but it is easy to slip up when the numbers are not round.
This calculator keeps the proportion H1 : W1 = H2 : W2 for you. It accepts exact pixel sizes or common shapes – 4:3, 3:2, 16:9, 16:10 and 1:1 – and reports the simplified ratio of the original, the decimal ratio, the linear scale and how much the pixel count (and therefore roughly the file size) changes.
Because images can only have whole pixels, it also gives the rounded size and warns you when rounding shifts the ratio by more than 0.2%, which is the point where a careful eye or a strict upload checker may notice.
How to use the Image Ratio Calculator
- Original image
- Original width W1 (px)
- Original height H1 (px)
- I know
- New width W2 (px)
- New height H2 (px)
- Percentage of the original size A
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
Choose a preset or “Custom size” and enter the original dimensions, then say whether you know the new width, the new height or the scale percentage. The missing side follows from the cross-multiplication of the two ratios; the scale is W2 ÷ W1, and the pixel count scales with its square.
Worked example
A 6,000 × 4,000 camera photo (3:2) must be 1,200 px wide for a blog. H2 = 1,200 × 4,000 ÷ 6,000 = 800 px, so the new size is 1,200 × 800. The scale is 1,200 ÷ 6,000 = 20%, and the pixel count drops to 20%² = 4% of the original (0.96 MP instead of 24 MP). Scaling the same photo to 25% instead gives 1,500 × 1,000 px.
Common ratios and sizes
| Ratio | Decimal | Typical sizes | Used for |
|---|---|---|---|
| 1:1 | 1.000 | 1080 × 1080 | Profile pictures, square posts |
| 4:3 | 1.333 | 1024 × 768, 2048 × 1536 | Compact cameras, iPad, old TVs |
| 3:2 | 1.500 | 6000 × 4000 | DSLR and mirrorless photos, 4 × 6 in prints |
| 16:10 | 1.600 | 1920 × 1200, 2560 × 1600 | Laptop and office monitors |
| 16:9 | 1.778 | 1920 × 1080, 3840 × 2160 | Video, TVs, YouTube thumbnails |
| 4:5 | 0.800 | 1080 × 1350 | Portrait social posts |
Formula
H1 ÷ W1 = H2 ÷ W2 · W2 = H2 × W1 ÷ H1 · H2 = W2 × H1 ÷ W1 · W2 = W1 × A% , H2 = H1 × A% · pixel count changes by (A%)²
Worked example
Inputs — Original image: Custom size; Original width W1 (px): 6,000; Original height H1 (px): 4,000; I know: the new width W2; New width W2 (px): 1,200.
Results — New size W2 × H2 (px) = 1,200 × 800; New width W2 = 1,200 px; New height H2 = 800 px; Rounded to whole pixels = 1,200 × 800 px; Original width W1 = 6,000 px; Original height H1 = 4,000 px.
Key terms
- Aspect ratio
- Width divided by height, written W:H (16:9) or as a decimal (1.778).
- Scale factor
- The number both sides are multiplied by when resizing.
- Cross-multiplication
- Solving H1/W1 = H2/W2 for the unknown side.
- Resampling
- Calculating new pixel values when an image is resized.
Frequently asked questions
How do I resize an image and keep the aspect ratio?
Multiply both sides by the same factor. For a new width, H2 = W2 × H1 ÷ W1; for a new height, W2 = H2 × W1 ÷ H1.
What is 1920 × 1080 scaled to a width of 1280?
1280 × 720 – a factor of 2/3.
What ratio is 6000 × 4000?
3:2, the shape of most DSLR and mirrorless sensors.
How much smaller is an image at 50%?
Each side halves, so the pixel count – and roughly the uncompressed file size – drops to 25%.
What is the ratio of 1080 × 1350 (Instagram portrait)?
4:5 (0.8).
Why do I get a fraction of a pixel?
Some ratios do not divide evenly. Round to the nearest pixel and accept a tiny distortion, or crop a pixel to keep the ratio exact.