Two sizes that look nearly identical in a dropdown: A3 and 11x14 inches.
A3 is 297 x 420mm. 11x14 is 279 x 356mm. Within a couple of centimetres on the short edge, so a seller adds both to the same listing, uploads one print file, and assumes the printer will handle it.
It will not. A3 has an aspect ratio of 1:1.414. 11x14 is 1:1.27. Those are different shapes, and the gap between them is about 11% of the long edge. Something has to give: either the printer crops a strip off your artwork, or it centres your file and leaves an uneven white margin down two sides.
This is the single most common technical mistake in poster print-on-demand, and it does not announce itself. The listing looks fine. The mockup looks fine. The buyer receives a print with their title cropped in half.
Two systems that will never line up
The world uses two poster standards, and they were built on incompatible ideas.
ISO 216 (the A-series) is used almost everywhere except North America. Its whole design is one property: every size has a ratio of 1:√2, about 1:1.414. Cut an A2 in half across its long edge and you get two A3s with the same proportions. That is the entire point of the system, and it means one artwork ratio covers every A size you will ever sell.
| Size | Millimetres | Inches |
|---|---|---|
| A5 | 148 x 210 | 5.8 x 8.3 |
| A4 | 210 x 297 | 8.3 x 11.7 |
| A3 | 297 x 420 | 11.7 x 16.5 |
| A2 | 420 x 594 | 16.5 x 23.4 |
| A1 | 594 x 841 | 23.4 x 33.1 |
| A0 | 841 x 1189 | 33.1 x 46.8 |
US sizes grew out of paper mill conventions and picture framing, not mathematics. They have no consistent ratio at all:
| Size | Ratio | Also fits |
|---|---|---|
| 5 x 7 | 1:1.40 | Close to A-series |
| 8 x 10 | 1:1.25 | 16x20, 24x30 |
| 11 x 14 | 1:1.27 | Nothing else neatly |
| 12 x 16 | 1:1.33 | 18x24, 300x400mm |
| 18 x 24 | 1:1.33 | 12x16 |
| 24 x 36 | 1:1.50 | 8x12, 12x18, 20x30 |
Read that column again. Within the US sizes alone there are four different shapes. 8x10 and 11x14 sit next to each other in every frame shop in America and are not the same proportion.
So there is no clever ratio that covers everything. Anyone who tells you to "just design at 2:3 and it works everywhere" is telling you that cropping is fine.
What actually happens to a mismatched file
When your file's ratio does not match the ordered size, the printer does one of three things — and which one depends on the service, not on you.
It fills and crops. Your artwork is scaled until it covers the whole area, and the overflow is trimmed. On a 1:1.414 file printed at 1:1.25, that removes about 11% of the height, split top and bottom. A centred illustration survives. A design with text near the top edge does not.
It fits and borders. Your artwork is scaled until it fits entirely inside, leaving white on two sides. Now a poster that was meant to be full-bleed has a 20mm band down the top and bottom and nothing at the sides. It looks like a printing error, because it looks exactly like one.
It stretches. Rare, and the worst. Circles become ovals and nobody can say why the print looks wrong, only that it does.
The buyer does not know which of these happened. They know the print does not match the photo in your listing, and that is the review you get.
The three ways sellers handle it
1. Pick one ratio family and stay in it
The simplest answer, and the right one for most new shops. Choose the ratio your main market buys in and only offer sizes within it.
- Selling mainly to the UK, EU or Australia: design at 1:1.414 and offer A5 through A0. One file, six sizes, zero cropping.
- Selling mainly to the US: pick 1:1.25 (8x10, 16x20, 24x30) or 1:1.5 (8x12, 12x18, 24x36) and stay there.
You lose some sizes. You also lose every cropping complaint, and a listing with four sizes that are all correct outsells one with nine sizes where three arrive wrong.
2. Produce a real crop per ratio family
The professional answer. Design once at the largest ratio, then produce a deliberate variant for each other family, deciding yourself what gets cut.
That last part is the value. When you crop 11% off your own design you move the composition first, so the title stays clear of the trim and the subject stays centred. When the printer crops it, nobody made a decision at all.
Three master files covers essentially the whole market:
- 1:1.414 — every A size
- 1:1.25 — 8x10, 16x20, 24x30
- 1:1.5 — 4x6, 8x12, 12x18, 24x36
Add 1:1.33 if you sell 18x24 in volume, and 1:1 if you sell squares.
The reason people avoid this is the arithmetic: three ratios across forty designs is 120 exports, and doing that by hand in Photoshop is a week of work. It is also exactly the kind of job a batch mockup and export workflow exists for — the decision is creative and happens once per design, the export is mechanical and happens 120 times.
3. Design with a safe area
If you must run one file across ratios, build the tolerance in. Keep all text and anything that must not be cut inside a margin of roughly 12% from every edge, and let the background run to the edges. Then a crop takes background rather than content.
This limits what you can design. Full-bleed typography and edge-to-edge borders are out, because a border that gets 11% trimmed on two sides is worse than no border at all. But for photographic and illustrative work it holds up, and it is the only honest way to sell one file across several shapes.
Resolution, and when 300 DPI is overkill
The standard advice is 300 DPI at final print size. For anything up to about A2 that is right, and it is what print resolution actually means in practice.
At A1 and A0 it starts to be worth thinking about. A0 at 300 DPI is 9,933 x 14,043 pixels — a 140-megapixel image, and a PSD that will make your machine work for it.
The reason you can often drop to 150 DPI on very large pieces is that viewing distance scales with size. Nobody puts their nose against an A0. A print viewed from two metres at 150 DPI resolves about the same as an A4 held at arm's length at 300.
Two cautions. First, your printer may have a minimum spec regardless of physics — check before you halve anything. Second, this only applies to genuinely large formats. Do not use it to justify 150 DPI on an A3, which people absolutely will hold.
| Size | 300 DPI | 150 DPI |
|---|---|---|
| A4 | 2480 x 3508 | 1240 x 1754 |
| A3 | 3508 x 4961 | 1754 x 2480 |
| A2 | 4961 x 7016 | 2480 x 3508 |
| A1 | 7016 x 9933 | 3508 x 4961 |
| A0 | 9933 x 14043 | 4961 x 7016 |
Note the pattern: each A-series step up at 150 DPI equals the step below at 300. That is the 1:√2 ratio doing its work again.
Listing sizes so buyers understand them
Two rules, both about the buyer rather than the file.
Lead with the unit they think in. A US buyer searching for wall art types "18x24 poster". They do not search for A2, and A2 does not read as a size to them — it reads as a product code. The reverse is just as true: a UK buyer sees "16.5 x 23.4 inches" and assumes something has gone wrong, because no shop in Britain sells a 16.5-inch anything.
Give the frame answer before they ask. The single most common pre-purchase question on a poster listing is whether it fits a standard frame. If you sell A-series, say "fits standard A2 frames". If you sell 11x14, say "fits standard 11x14 frames, no mat required". If a size is non-standard, say so plainly — a buyer who learns at the framer's that their print needs a custom mount leaves a one-star review about it, and they are not wrong to.
The short version
- A-series is one shape. US sizes are four shapes. There is no ratio that covers both.
- A mismatched file gets cropped, bordered or stretched, and you do not get to choose which.
- Pick one ratio family, or produce a real crop for each — but decide the crop yourself rather than leaving it to the printer.
- 300 DPI up to A2. Above that, check your printer's spec before dropping to 150.
- List the size in the unit your buyer already uses, and tell them what frame it fits.
The whole problem comes from one assumption: that a size dropdown is a list of sizes. It is a list of shapes, and two of them being close in area does not make them the same.
Frequently asked questions
Is A3 the same as 11x14 inches?
No, and they are not even the same shape. A3 is 297 x 420mm, an aspect ratio of 1:1.414. 11x14 inches is 279 x 356mm, a ratio of 1:1.27. Printing an A3 design at 11x14 crops roughly 11% off the long edge or leaves an uneven border. The sizes are close enough in area to look interchangeable in a size dropdown and are not interchangeable in practice.
What aspect ratio should I design posters at?
There is no single ratio that covers both markets. 1:1.414 covers the entire A-series in one file. 1:1.25 covers 8x10, 16x20 and 24x30. 1:1.5 covers 4x6, 8x12, 12x18 and 24x36. Pick the ratio family your main market buys in, design to that, and produce a separate crop for the other rather than stretching one file across both.
What resolution do I need for a large poster?
300 DPI at final print size is the safe target for anything a buyer will hold. For very large pieces viewed from a distance, 150 DPI is usually acceptable because the viewing distance scales with the print. A0 at 300 DPI is 9933 x 14043 pixels, which is a genuinely large file — 150 DPI halves each dimension and quarters the pixel count.
Should I sell in inches or centimetres?
Sell in the units your buyer thinks in, and list both. US buyers shop for 18x24. UK, EU and Australian buyers shop for A2. Listing an A2 as 16.5 x 23.4 inches is technically accurate and reads as a mistake. Most poster sellers list the native unit first and the conversion in brackets.
Do I need bleed on a poster print file?
It depends on the printer. Print-on-demand services that trim to size usually ask for 2-3mm of bleed, or they apply their own. Services that print borderless to a standard sheet often want the exact trim size with no bleed at all. Check the spec before you export a hundred files, because adding bleed later means re-exporting every one.