Image Aspect Ratio Calculator
Use this image aspect ratio calculator to find the ratio of a photo, design or graphic from its pixel dimensions. Enter the width and height, or use RatioForge’s local image reader, then calculate the simplified ratio and matching resize dimensions without distorting the image.
Image ratio calculator
Common image ratios
| Size / ratio | Label | Use-case |
|---|---|---|
| 1080×1080 | 1:1 | Square social post or avatar |
| 1080×1350 | 4:5 | Instagram portrait image |
| 1200×630 | 1.91:1 | Link preview image |
| 1920×1080 | 16:9 | Web hero or video still |
| 4000×3000 | 4:3 | Common camera/photo shape |
Image ratio formula
Simplify image ratio: divide width and height by their greatest common divisorResize by width: new height = new width × original height ÷ original widthResize by height: new width = new height × original width ÷ original height.
Worked example. A phone or DSLR photo comes off the sensor at 4000×3000 px. Their greatest common divisor is 1000, so 4000÷1000 : 3000÷1000 = 4:3 — the source shape. To shrink it for the web at 1600 px wide while keeping every pixel of the frame, new height = 1600 × 3000 ÷ 4000 = 1200 px. The result, 1600×1200, is still 4:3, so nothing is stretched or cropped — only scaled down.
What ratio does your camera or phone shoot?
Before you resize, it helps to know the shape a file started as, because that determines whether a target crop loses pixels. Digital cameras and phones capture at a handful of fixed sensor ratios, and the number in your file is almost always one of these:
| Sensor ratio | Typical example size | Where it comes from |
|---|---|---|
| 3:2 | 6000×4000 | Most DSLR and mirrorless cameras (full-frame, APS-C) |
| 4:3 | 4000×3000 | Micro Four Thirds and most phone main cameras |
| 16:9 | 1920×1080 | Video mode and widescreen still crops |
| 1:1 | 4000×4000 | Square crop mode and some medium-format backs |
The reader above tells you which of these you actually have: drop a file in and RatioForge measures its pixels in the browser. That is worth stressing — the image is never uploaded to a server, so private photos, client work and screenshots stay on your device while you read their shape.
Crop vs. stretch vs. letterbox
When a source ratio and a target ratio disagree, you have three honest choices, and only two of them keep the picture looking right.
- Crop — trim pixels off the edges until the frame matches. A 4000×3000 (4:3) photo cropped to a 1:1 square keeps the full 3000 px height but discards 4000 − 3000 = 1000 px of width (500 off each side). Content is lost, but nothing is distorted.
- Stretch — force the old pixels into the new shape. This is the one to avoid: squeezing that same 4:3 photo into a 1:1 box makes faces look wide and circles look like ovals.
- Letterbox / pad — keep the whole image and add bars to fill the rest. A 1920×1080 (16:9) still placed into a 4:5 portrait box keeps all 1920 px of width but needs a 2400 px-tall canvas, leaving 2400 − 1080 = 1320 px of padding split top and bottom.
Sensor shapes at a glance
Drawn to the same height, 3:2 is the widest of the everyday sensor shapes, 4:3 is a little boxier, and 1:1 is a true square. Moving a photo from a wider shape to a narrower one always costs width; moving to a wider one costs height. Knowing the two ratios in advance tells you exactly how much.
Source ratio to target size
Once you know the source shape, this maps common sources to platform-ready sizes. Where the target ratio differs from the source, the last column notes what gets trimmed:
| Source (ratio) | Target | Resize / crop result |
|---|---|---|
| 4000×3000 (4:3) | Web hero 16:9 | Crop to 4000×2250, then scale to 1600×900 |
| 4000×3000 (4:3) | Instagram 4:5 | Crop to 2400×3000, then scale to 1080×1350 |
| 6000×4000 (3:2) | Square 1:1 | Crop to 4000×4000, then scale to 1080×1080 |
| 1920×1080 (16:9) | Portrait 4:5 | Pad to 1920×2400 or crop to 864×1080 |
Switch the calculator to centimetres or inches when the destination is print. Image sharpness in print depends on how many pixels land in each inch: a 1800×1200 px file prints a clean 6×4 inch photo at 300 DPI, because 1800 ÷ 300 = 6 and 1200 ÷ 300 = 4. Halve the DPI to 150 and the same file would cover 12×8 inches, but noticeably softer.
How to use image ratios before resizing
Images often need to move between platforms: a camera photo, a website hero, an Instagram crop and a link preview can all require different shapes. Calculating the ratio first tells you whether a resize will preserve the whole image or whether a crop is unavoidable.
RatioForge reads a dropped image locally: it measures the pixel dimensions in your browser and does not upload the file. That makes the tool more trustworthy than generic upload-based resizers.
Start with diagnosis before resizing. You often know you have a file but not whether it is 4:3, 3:2, 16:9 or something unusual. Enter the pixel width and height to identify the shape before choosing a crop — useful for photographers, bloggers, ecommerce sellers and designers preparing assets for multiple placements.
The privacy angle matters: because the dimension reader runs in your browser, private files stay on your device. Once you know the source shape, turn it into a platform-ready crop with the 4:5 or 1:1 calculator, or read the math step by step in how to calculate an aspect ratio.
Frequently asked questions
Related calculators and guides
- Aspect ratio calculator — the main calculator with custom ratios
- 4:5 aspect ratio calculator — portrait crops for the feed
- 1:1 aspect ratio calculator — square crops for avatars and posts
- How to calculate an aspect ratio — the math, step by step
- Aspect ratios explained — what the common ratios mean