Aspect Ratio & Pixels
Aspect ratio of any pixel size reduced to its simplest form, the nearest named ratio, megapixels, a proportional resize, and print size at a given dpi.
The ratio as 16:9 and 1.7778:1, megapixels and orientation, the missing side when you resize, the print size in inches and cm, and common ratios at your width.
Example: 1920 × 1080 is 16:9 (2.07 MP); resized to 1280 wide it is 720 tall; at 300 dpi it prints 6.4 × 3.6 in.
Divide by the gcd,
keep the ratio.
How a pixel size reduces to a ratio, why 1366 × 768 is “almost” 16:9, how a resize keeps proportions, and how dpi turns pixels into inches.
Reducing the ratio
Width and height are divided by their greatest common divisor: 1920 and 1080 share 120, so 16:9. The decimal form (1.7778 : 1) makes comparison easy. A frame is matched to a named ratio when it is within 0.5 % — 1366 × 768 reduces to 683:384, 0.05 % off 16:9, and is reported as such rather than as an unfamiliar pair.
Resizing
To keep the ratio, the new height is the new width ÷ ratio (or width = height × ratio). If that is not a whole number of pixels the page says so: 1920 × 1080 at 1000 wide wants 562.5 pixels tall, so you round and accept a ratio off by a fraction of a percent, or pick a width that divides evenly. Megapixels are width × height ÷ 10⁶.
Print size
Inches = pixels ÷ dots per inch; centimetres = inches × 2.54. A 1920 × 1080 image prints 6.4 × 3.6 in at 300 dpi and 19.2 × 10.8 in at 100 dpi — the image does not change, only the density. What dpi a print needs depends on viewing distance and the printer, and is your choice; nothing is assumed.
What the number does not settle
Square pixels are assumed (anamorphic and some broadcast formats have a pixel aspect ratio of their own), cropping and bleed are not included, and the named ratios are for orientation only — a display or delivery spec fixes the exact size. Video Bitrate & File Size covers what a frame size costs in storage. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.