Then you look at what is actually in the menu. Printful’s centimetre framed poster offers nine sizes, and its own product data shows they come from two different traditions. Four are genuinely metric. Four are standard American frame sizes with the decimals filed off. And one carries a label that is three millimetres away from the thing it names: the product copy says “21 × 30cm posters are size A4”, while ISO A4 is 21 × 29.7 cm.
None of this is a defect, and nothing here is hidden. It is a rounding convention, published openly in the vendor’s own catalogue. It only costs you something if you build a file to a size name instead of to the numbers, which is exactly what a size name invites you to do.
What the catalogue actually lists
Printful publishes its catalogue as a public API, so you can read the size list yourself rather than trust a dropdown. Product 304 is Enhanced Matte Paper Framed Poster (cm), and its raw catalogue entry returns 24 variants across nine sizes.
| Size | Frame colours offered |
|---|---|
| 21 × 30 cm | Black, White, Oak |
| 30 × 40 cm | Black, White, Oak |
| 50 × 70 cm | Black, White, Oak |
| 61 × 91 cm | Black, White, Oak |
| A2 (42 × 59.4 cm) | Black, White, Oak |
| A1 (59.4 × 84.1 cm) | Black, White, Oak |
| 30 × 45 cm | Black, White |
| 40 × 50 cm | Black, White |
| 45 × 61 cm | Black, White |
The frames themselves are the same across the range: black and white in semi-hardwood alder, oak in solid oak, on 189 g/m² paper measuring 0.26 mm thick. Those are the vendor’s own figures.
Notice the pattern in the right-hand column. Six sizes come in three colours. Three come in two. That split is the first clue that the menu is not one ladder.
Which of the nine are actually metric?
Take the four sizes that only come in black and white, and convert the nearest standard American frame at exactly 2.54 cm to the inch.
| Listed size | Nearest US frame | Exact conversion | Rounded to |
|---|---|---|---|
| 30 × 45 cm | 12 × 18 in | 30.48 × 45.72 cm | 30 × 45 cm |
| 40 × 50 cm | 16 × 20 in | 40.64 × 50.8 cm | 40 × 50 cm |
| 45 × 61 cm | 18 × 24 in | 45.72 × 60.96 cm | 45 × 61 cm |
| 61 × 91 cm | 24 × 36 in | 60.96 × 91.44 cm | 61 × 91 cm |
All four are American frame sizes wearing metric clothes. Printful sells the same ladder natively in inches as product 2, where 12 × 18, 16 × 20, 18 × 24 and 24 × 36 all appear as themselves.
That leaves the genuinely metric half of the menu: 30 × 40 cm and 50 × 70 cm, which are standard European poster sizes and convert to nothing tidy in inches, plus true ISO A1 at 59.4 × 84.1 cm and A2 at 42 × 59.4 cm.
And then there is 21 × 30 cm, which is neither. It is not an inch size, and it is not A4.
The 3 mm, and what it does to your file
ISO A4 is 210 × 297 mm. Printful’s smallest framed poster is 210 × 300 mm. The width matches exactly. The height is 3 mm greater.
Three millimetres sounds like nothing, and on the wall it is nothing. It matters at one moment only: when you export the file.
The two shapes have different proportions. A4 is 1 : 1.414. A 21 × 30 cm sheet is 1 : 1.4286. So a file built to true A4 proportions cannot fill a 21 × 30 cm opening without one of two things happening.
| If you scale your A4-shaped file to | You get | Result |
|---|---|---|
| 210 mm wide (full width) | 297 mm tall in a 300 mm opening | A 3 mm empty strip along one edge |
| 300 mm tall (full height) | 212.12 mm wide in a 210 mm opening | 2.12 mm of artwork cropped away, about 1 mm each side |
Neither outcome is a disaster. A 3 mm strip is a hairline. A 1 mm crop eats a margin, not a subject. But if your artwork runs to the very edge, a border that was meant to be even will not be, and that is the kind of detail people notice in a photograph of their own wall without being able to say why.
The fix costs nothing if you do it first. Build the file at 210 × 300 mm and the question never arises.
Does the mat version change anything?
Yes, and in a useful direction. Printful’s matted framed poster is product 366, and it is offered in only four sizes: 21 × 30, 30 × 40, 50 × 70 and 61 × 91 cm. Eight variants in total, against the unmatted version’s 24.
So if you want a mat, the rounded-inch sizes and both true ISO sizes are simply not on the menu. The choice gets made for you. Its product copy carries the same line, written slightly differently: “21×30cm posters are size A4”.
Worth noting for the largest size: Printful’s own hanging instruction for 61 × 91 cm frames is to place each mounting hook 2.5 cm from the frame corners when hanging horizontally.
What to do before you order
- Read the size in numbers, not in names. Order the 21 × 30 cm frame and design at 210 × 300 mm. If you genuinely need ISO A4 proportions, order A2 or A1, which are the only true ISO sizes in this range, and scale down.
- If a mat matters to you, decide that first. It removes five of the nine sizes.
- If your artwork bleeds to the edge, add a 3 mm safety margin on every side and keep nothing important inside it.
- Check this yourself rather than trusting this guide. Open the catalogue entry for whatever product you are about to order and read the size list. It is public, it needs no account, and it is the same data the vendor prints from.
What this guide does not claim
We checked one vendor. Printify’s and Gelato’s catalogues were not examined for this guide, and nothing here should be read as a claim about how they structure their size menus. The point generalises; the specifics do not.
We are not calling the A4 label an error in any sense that matters commercially. Rounding 29.7 to 30 is ordinary, the real dimension is published alongside it, and the frame you receive will be the frame you ordered.
We have no data on how often this actually causes a print problem. Nobody publishes that, this guide does not measure it, and the honest answer is that most artwork has enough margin to absorb 3 mm without anyone noticing.
Every figure above was read directly from Printful’s public catalogue on 4 September 2026 and converted at exactly 2.54 cm to the inch. Catalogues change. If you are reading this months from now, spend the ten seconds and check the current list.
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