Image Aspect Ratio Calculator
Enter your image width and height to see the simplified aspect ratio, its decimal equivalent, total pixels, and orientation. You can also lock a known ratio and solve for a missing dimension, making it easy to resize images proportionally or check whether a file matches a platform requirement. Common presets cover every major social media and video format.
What is an aspect ratio and how is it calculated?
An aspect ratio is the relationship between the width and height of an image or video frame, expressed as two numbers separated by a colon (e.g. 16:9) or as a single decimal (1.778). To calculate it, divide the width by the height to get the decimal form, then find the greatest common divisor (GCD) of both integers and divide each by it to get the simplified whole-number ratio. For example, a 1920 x 1080 pixel image gives a decimal of 1920 / 1080 = 1.778 and a GCD of 120, which simplifies to 16:9. Aspect ratios do not carry units - they work equally in pixels, centimetres, or any other consistent unit, so a 16:9 shape always looks the same whether it is a thumbnail or a cinema screen.
How to resize an image while keeping the ratio
Maintaining the aspect ratio when resizing prevents stretching or squashing. The rule is simple: new height = new width / (original width / original height). Equivalently, new width = new height x (original width / original height). If you start with a 1920 x 1080 image (ratio 16:9) and want it 1280 pixels wide, the new height is 1280 / (1920 / 1080) = 1280 / 1.778 = 720 px - giving the standard 1280 x 720 HD resolution. This calculator does both directions: enter your original dimensions and a new width or height, and it returns the missing dimension rounded to the nearest pixel.
Common aspect ratios and when to use them
16:9 is the default for HD television, YouTube, and most laptop and desktop displays. 4:3 was the classic broadcast and computer-monitor standard, still used in some print workflows. 1:1 (square) is the default Instagram feed format and suits product photos. 9:16 is the vertical flip of 16:9 and is required by Instagram Stories, TikTok, and YouTube Shorts. 3:2 matches the 35 mm film frame and is the native sensor ratio of most DSLR and mirrorless cameras. 4:5 (slightly taller than a square) is Instagram's maximum portrait ratio for feed posts and is considered the format that gets the most screen real estate on mobile. 21:9 and 32:9 are ultrawide formats used by gaming monitors and cinematic video. When creating content for a specific platform, matching its native ratio avoids black bars (letterboxing or pillarboxing) or unexpected cropping.
Letterboxing, pillarboxing, and cropping
When a source ratio and a display ratio do not match, the player or renderer must handle the mismatch in one of two ways. Letterboxing adds horizontal black bars above and below the image when the source is wider than the display (e.g. a 21:9 film on a 16:9 screen). Pillarboxing adds vertical bars on the sides when the source is narrower (e.g. a 4:3 recording on a 16:9 screen). The alternative is cropping, which fills the frame but loses part of the image. Some platforms crop automatically - Instagram will centre-crop a non-compliant image, sometimes removing important detail. Knowing the target ratio before you shoot or export lets you frame for the crop zone and avoid surprises.
Platform aspect ratio requirements
| Platform | Use case | Ratio | Recommended resolution |
|---|---|---|---|
| YouTube | Video upload | 16:9 | 1280 x 720 min |
| Feed landscape | 1.91:1 (16:9 acceptable) | 1080 x 566 | |
| Feed square | 1:1 | 1080 x 1080 | |
| Feed portrait | 4:5 | 1080 x 1350 | |
| Stories / Reels | 9:16 | 1080 x 1920 | |
| TikTok | Videos | 9:16 | 1080 x 1920 |
| Shared image | 1.91:1 | 1200 x 630 | |
| Twitter / X | In-stream photo | 16:9 | 1600 x 900 |
| Shared image | 1.91:1 | 1200 x 627 | |
| Standard pin | 2:3 | 1000 x 1500 | |
| Print / A4 | Portrait | 1:1.414 (ISO 216) | 2480 x 3508 at 300 dpi |
| DSLR photo | Default | 3:2 | Varies by sensor |
Common requirements as of mid-2025. Always verify with the platform directly before publishing, as guidelines change.
Frequently asked questions
What does 16:9 mean?
It means the image is 16 units wide for every 9 units tall. The actual pixel count can be anything that simplifies to that ratio - 1280 x 720, 1920 x 1080, and 3840 x 2160 (4K) are all 16:9. The ratio describes shape, not size.
How do I find the aspect ratio of an image I already have?
Right-click the file on most operating systems and choose "Properties" (Windows) or "Get Info" (macOS) to see the pixel dimensions. Then enter the width and height in this calculator and it will compute the ratio instantly. On a Mac, Quick Look (spacebar) also shows dimensions in the preview window.
What aspect ratio should I use for social media?
It depends on the platform and placement. Instagram feed posts support 1.91:1 (landscape), 1:1 (square), and 4:5 (portrait) - 4:5 gets the most screen space on mobile. Instagram and TikTok Stories/Reels use 9:16. Facebook shared images target 1.91:1. YouTube videos are 16:9. Pinterest pins are 2:3. Twitter in-stream photos are 16:9. Check the platform table on this page for recommended pixel dimensions.
Why does my image look different after uploading?
Most platforms crop or resize automatically when your image does not match their required ratio. Instagram crops 16:9 landscape images to 1.91:1 (cutting the sides slightly), and TikTok may crop non-9:16 videos with black bars or a blur fill. Always export or crop to the exact target ratio before uploading to avoid unexpected results.
What is the difference between aspect ratio and resolution?
Aspect ratio is the proportional relationship between width and height (a shape). Resolution is the actual pixel count (a size). Two images can share the same ratio but have very different resolutions - a 320 x 180 thumbnail and a 3840 x 2160 4K frame are both 16:9. Resolution affects sharpness and file size; aspect ratio affects how the image looks on a given display.
How do I calculate a proportional height for a new width?
Divide the original height by the original width to get a scale factor, then multiply by the new width. For a 1920 x 1080 image scaled to 800 px wide: scale factor = 1080 / 1920 = 0.5625, and new height = 800 x 0.5625 = 450 px. This calculator handles both directions in the "Scale by new width" and "Scale by new height" modes.