Aspect ratio calculator

What the calculator is actually doing

An aspect ratio is a shape, not a size. It is the width of an image divided by its height, reduced to the smallest whole numbers that describe the same proportion. A 6000 × 4000 pixel camera file, a 6 × 4 inch print and a 30 × 20 inch poster are all 3:2. They are wildly different sizes and identical shapes, and any of them will fill any of the others perfectly.

That is the whole reason this tool exists. Frame shopping goes wrong when people match on size and ignore shape. You buy an 8 × 10 frame for a photo straight out of a camera, and the lab quietly trims a full inch off each end of the long edge to make a 3:2 file behave like a 4:5 one. Usually that inch contains somebody's elbow. Occasionally it contains their head.

The four numbers the tool gives you

The simplified ratio

Enter any two dimensions and the calculator reduces them to the simplest whole-number pair that describes the same shape. 1920 × 1080 becomes 16:9. 8.5 × 11 becomes 17:22, which is a useful thing to know: US Letter is not a clean ratio, and that is exactly why letter-size documents look awkward in frames designed around 8 × 10.

The decimal ratio

Some shapes never reduce neatly. A-series paper is √2:1 — approximately 1.414:1 — and no pair of small whole numbers describes it. When the simplified form would be unreadable, the calculator falls back to a decimal, which is still the number you want when you are comparing two shapes to see how close they are.

The metric conversion

One inch is exactly 2.54 cm. That constant hides a genuine trap: an 8 × 10 inch frame is 20.32 × 25.4 cm, and the 20 × 25 cm frames sold across Europe are very slightly smaller. Close enough to look identical in a listing, not close enough for the print to sit flat.

Pixels needed at 300 DPI

300 dots per inch is the standard for photographic printing, so a 16 × 20 print wants 4800 × 6000 pixels — 28.8 megapixels. Before you enlarge anything, check that figure against what your file actually contains. Large posters viewed from across a room hold up fine at 150 DPI; a print someone will stand two feet from does not.

Frame size families worth memorising

Standard frames cluster into a handful of ratio families. Once you know which family your image belongs to, choosing a frame stops being guesswork.

When the shapes do not match

Sooner or later you will have a 3:2 photo and a 4:5 frame, and you have three honest options. Crop the image and accept the loss — decide yourself where the trim falls rather than letting a print lab decide. Add a mat with an opening cut to the image's own ratio, which is the framer's answer and almost always looks better than a full-bleed print anyway. Or order a custom frame, which costs three to five times a standard size and is worth it perhaps once a year.

There is a fourth option people try and regret: printing with white borders and hiding them under the frame's rabbet. The rabbet only covers about an eighth of an inch, the borders are never perfectly even, and the result sits crooked in a way you cannot un-see.