Aspect Ratio Calculator
Three tools in one: reduce any pixel dimensions to their simplest ratio (1920x1080 becomes 16:9), lock a ratio and solve for the missing dimension when resizing, or enter a screen diagonal to find its physical width, height, and pixel density. Common presets let you jump to 16:9, 4:3, 1:1, and more in one click.
Formula
Worked example
1920x1080: gcd(1920, 1080) = 120, ratio = 16:9, total = 2,073,600 px. To keep 16:9 at width 1280, height = 1280 x 9 / 16 = 720 px. A 27-inch 16:9 screen: width = 27 x 16 / sqrt(337) = 23.54 in, height = 13.24 in. At 2560 px wide, PPI = 2560 / 23.54 = 108.8.
What an aspect ratio actually means
An aspect ratio describes the proportional relationship between a frame's width and its height, written as two numbers separated by a colon, such as 16:9. It says nothing about absolute size: a 16:9 frame can be a thumbnail or a cinema screen. To find it from pixel dimensions you divide both the width and the height by their greatest common divisor, which strips out the shared factor and leaves the smallest whole-number pair describing the same shape. That is why 1920x1080, 1280x720, and 3840x2160 all reduce to the same 16:9 ratio.
Resizing without distortion
When you scale an image or video, keeping the aspect ratio constant prevents stretching. If you fix one dimension, the other follows from the proportion: width / height = ratio width / ratio height. Solving gives height = width x ratio height / ratio width, or width = height x ratio width / ratio height. Because the result is rarely a whole number, rounding to the nearest pixel introduces a sub-pixel ratio error too small to see. Most video codecs (H.264, H.265, AV1) require even pixel dimensions, so the codec-safe rounding option rounds up to the next multiple of 2, keeping the error negligible while ensuring compatibility.
Screen diagonals and pixel density
Display manufacturers advertise screen sizes as a diagonal in inches. To find the physical width and height, treat the ratio as the legs of a right triangle: the hypotenuse is sqrt(ratio_w^2 + ratio_h^2), so physical width = diagonal x ratio_w / hypotenuse, and physical height = diagonal x ratio_h / hypotenuse. Pixel density (PPI, pixels per inch) is then the pixel width divided by the physical width. A 27-inch 16:9 monitor at 2560x1440 resolution has a physical width of about 23.5 inches and a pixel density of roughly 109 PPI, which is a standard non-retina desktop density.
Common ratios and where they appear
A handful of ratios dominate everyday media. 16:9 is the standard for HD and 4K video and most monitors, while 4:3 still appears in classic broadcast and many tablets. Photographers shoot 3:2 on full-frame cameras and DSLRs. Social platforms favour 1:1 squares and 4:5 portraits for feeds, and 9:16 vertical for Stories, Reels, and TikTok. Cinema leans on 2.39:1 scope. Ultrawide monitors use 21:9, and super-ultrawide gaming displays stretch to 32:9. Knowing the target ratio before cropping or exporting prevents black bars, letterboxing, and awkward re-cropping later.
Common aspect ratios and platform presets
| Ratio | Decimal | Example resolution | Typical use |
|---|---|---|---|
| 1:1 | 1.000 | 1080 x 1080 | Square social posts (Instagram, Facebook) |
| 4:5 | 0.800 | 1080 x 1350 | Instagram portrait feed posts |
| 9:16 | 0.563 | 1080 x 1920 | TikTok, Instagram Reels, YouTube Shorts |
| 4:3 | 1.333 | 1024 x 768 | Classic TV, iPads, some tablets |
| 3:2 | 1.500 | 3000 x 2000 | Full-frame DSLR photography |
| 16:10 | 1.600 | 1920 x 1200 | Laptop and MacBook displays |
| 16:9 | 1.778 | 1920 x 1080 | HD / 4K video, YouTube, most monitors |
| 2:1 | 2.000 | 2048 x 1024 | Univisium cinema, some mobile banners |
| 21:9 | 2.333 | 2560 x 1080 | Ultrawide monitors, cinematic video |
| 2.39:1 | 2.390 | 2048 x 858 | Cinema scope (anamorphic widescreen) |
| 32:9 | 3.556 | 5120 x 1440 | Super-ultrawide gaming monitors |
Standard ratios, example resolutions, and where each is used.
Frequently asked questions
How do I find the aspect ratio of an image?
Select "Reduce" mode, enter the pixel width and height, and the calculator divides both by their greatest common divisor to give the lowest-terms ratio, the decimal value, and the total pixel count. For example, 1600x900 shares a GCD of 100, so it reduces to 16:9.
How do I resize an image while keeping its aspect ratio?
Switch to "Find a missing dimension" mode. Pick a preset ratio or enter custom numbers, choose whether you know the width or height, and enter that value. The calculator uses height = width x (ratio height / ratio width) to find the other side and rounds to a whole pixel.
What does codec-safe rounding do?
H.264, H.265, and AV1 video codecs require that width and height are each divisible by 2 (some profiles require multiples of 4 or 16). If the exact solved dimension is, say, 719.4 px, rounding to 719 would fail encoding. Codec-safe rounding bumps it up to 720 px, keeping the ratio error sub-pixel.
How do I calculate physical screen dimensions from a diagonal?
Use "Screen size" mode. Enter the diagonal (the advertised inch measurement), the ratio, and optionally the pixel width. The calculator uses Pythagoras to split the diagonal into physical width and height, then divides pixel width by physical width to give pixels per inch (PPI).
What is the difference between aspect ratio and resolution?
Resolution is the exact pixel count, such as 1920x1080, while aspect ratio is the simplified shape, 16:9. Many different resolutions share one aspect ratio. Two images can both be 16:9 yet have wildly different sharpness and file sizes.
What aspect ratio should I use for YouTube, Instagram, and TikTok?
YouTube standard video uses 16:9 (1920x1080 or 3840x2160). YouTube Shorts, Instagram Reels, and TikTok all use vertical 9:16 (1080x1920). Instagram feed square posts use 1:1, feed portrait posts use 4:5 (1080x1350). See the reference table above for a full list.