A3, 18×24, 50×70 — and which of them fits a frame.

Arithmetic, not opinion. The numbers on this page are calculated, and the working is shown so you can check it yourself.

There are three families of print size in circulation, and sellers rarely tell you which one they are using:

• The A series (A4, A3, A2, A1) — the international paper standard. Every A size has the same proportions, so an A2 is exactly two A3s.
• Round metric sizes (30×40, 40×50, 50×70 cm) — the European framing standard, and what most off-the-shelf frames in Australia are built to.
• US inch sizes (8×10, 11×14, 18×24, 24×36 in) — what American sellers use, including most of the big print-on-demand shops.

The mismatch that costs people money

Look at two rows in that table. A3 is 29.7 × 42 cm. A 30 × 40 frame is 30 × 40 cm. Three millimetres narrower, twenty millimetres shorter. They look interchangeable on a listing and they are not: an A3 print in a 30×40 frame is two centimetres too long and will not sit down.

The same trap runs the other way. 16 × 20 inches is 40.6 × 50.8 cm, which is almost a 40×50 frame — over by six millimetres on one edge and eight on the other. Enough to stop the backing board closing.

Neither gap is big enough to notice in a photograph of a listing, and both are big enough to make the frame useless.

What actually fits an off-the-shelf frame here

Frames sold in Australian homeware and furniture shops are overwhelmingly built to the round metric sizes — 21×30, 30×40, 40×50, 50×70 — with a smaller run of A-series sizes in art and stationery shops. US inch sizes are the hardest to frame here and generally mean custom framing, which starts at a few hundred dollars and can easily cost more than the print.

So the order of preference, if you would like to spend nothing on framing:

1. A round metric size — widest choice of cheap frames.
2. An A size — findable, slightly fewer options.
3. A US inch size — assume custom framing, and price that in before you buy.

Two things worth checking on the listing

Whether the size includes a border. Some sellers quote the paper size and print the image smaller with a white margin around it. That margin is not a mat — it sits under the frame’s own mat and the image ends up considerably smaller than you expected.

Whether “framed” means framed. Print-on-demand framed options are usually a thin moulding with an acrylic front, which is fine, but they are also the most expensive thing to ship because glazing is heavy and fragile. Buying the print alone and framing it locally is often both cheaper and better.