300 DPI Is a Label, Not a Size. Here Is What Your File Actually Needs, in Centimetres

What DPI actually counts

Printful’s own help page gives the cleanest pair of definitions, and they are worth reading together because they describe two different things:

Resolution refers to the number of pixels in an image, usually written as width x height.

DPI is a printing term that tells you how many dots of ink are printed in one inch.

Pixels live in your file. Dots live on the paper. The bridge between them is the size you print at, and that bridge is the whole subject of this guide. Gelato writes the bridge out as a formula on its own help page:

Pixel = Millimeter * (Resolution / 25.4)

Gelato then works the example: a design 10 centimetres wide is “approximately 590 pixels” at 150 DPI and “about 1180 pixels” at 300 DPI. Nothing about the file changed between those two answers. Only the intended size did.

Redbubble takes the same fact and states it as bluntly as anyone in the trade:

Please also note that DPI and PPI don’t affect your final uploaded image.

And on the same page: “When you’re printing work, it’s the pixels that matter.” Redbubble does not publish a DPI requirement at all. It publishes minimum pixel counts, such as “2870 x 4100 pixels” for its smallest poster, which is the same requirement expressed in the unit that cannot drift.

The same number doing four different jobs

Here is why “300 DPI” feels confusing even when you have read all the documentation. It is not one instruction. It is four, depending on whose page you are on.

Printful sits across two of those rows at once. Its general guidance is a floor of 150: “We recommend a minimum of 150 DPI for fast processing and quality results”, and its file-prep page asks you to “Use files that are 150-300 DPI”. Its own definition page then asks for files “At least 150 DPI” and “Sized to match the actual print dimensions”, which is the only phrasing on any of these pages that names both halves of the problem in one breath.

The word is overloaded in a second way too. On Gelato’s poster page, DPI is also how the presses themselves are described: its digital press is characterised at “175 lines per inch”. That is a property of a machine, not a request about your file. Two different quantities, one abbreviation.

What you actually need, in centimetres

This is the table to keep. It is our own arithmetic, using Gelato’s formula above: pixels equal centimetres divided by 2.54, multiplied by the DPI. Nothing here is a vendor specification; it is the multiplication the vendors are implicitly asking you to do.

If you are buying a printable, this is the table that tells you whether the listing is honest. A seller offering “up to 70 x 100 cm” owes you roughly 8268 by 11811 pixels. If the file you download is 3000 pixels on the long edge, the big size in that listing is a hope, not a specification.

What happens when you stretch one file up

Sellers often ship one file and list it for a ladder of sizes. That works, within limits, and the limits are arithmetic. Take a file made properly for 21 by 30 cm at 300 DPI, which is 2480 by 3543 pixels, and print it bigger without adding a single pixel.

Those sizes are not all the same shape, so printing across them involves a crop as well as a stretch. The figures above give the limiting edge, which is the more cautious of the two numbers in each case.

The shape of that column is the point. Nothing fails suddenly. Sharpness drains away one size at a time, and the print that finally looks wrong is usually three sizes past the one where the file stopped being adequate.

Printify names the trap that hides this:

If the file resolution has been artificially increased, it won’t make the pixels any clearer or crisper.

Upscaling writes more pixels. It does not recover detail that was never captured. A 2480-pixel file resampled to 5906 pixels reports itself as 300 DPI at 50 by 70 cm and still contains exactly as much picture as it did before.

There is a smaller version of the same mistake at the other end. Printful warns that exporting through the wrong dialog “often reduces your file to 72 DPI, which can result in a grainy print”. In that case the pixels really are gone.

The 20 MB problem nobody mentions

Here is a constraint that catches sellers rather than buyers, and it is pure arithmetic. Etsy’s own help page says:

You can upload up to five digital files. The maximum size for each file is 20MB.

Now put that next to the pixel table. A 50 by 70 cm file at 300 DPI is 5906 by 8268 pixels, which is 48.8 million pixels. Uncompressed in 24-bit colour that is about 146 MB, so the file has to survive roughly a seven-to-one squeeze to pass through a 20 MB slot. For 70 by 100 cm the ratio is closer to fourteen to one. Those numbers are our own arithmetic about a ceiling, not a claim about any real file: a quiet line drawing compresses far better than a dense photograph, and a good JPEG will often make it comfortably.

It is worth knowing because the print shops downstream are not the bottleneck. Printful accepts files “up to 200 MB, with maximum dimensions of 20,000 x 20,000 px”. Printify says “The maximum file size is 100 MB for JPEG and PNG files”. Redbubble says images “above 300MB, or 13500 × 13500 pixels in size, cannot be accepted on the site”. The tightest door in the chain is the storefront, not the press.

Prodigi, incidentally, is relaxed about the format question that usually follows: “JPG files are preferred”, and it says there is no meaningful difference between a high-quality JPEG and other high-quality formats once the thing is on paper.

Do not forget the bleed

If your file goes to a shop that trims after printing, the finished size is not the size you design at. Gelato asks for one:

We require a 4 mm bleed area to be provided on all sides.

Four millimetres on all sides adds 0.8 cm to each dimension before you multiply. A 50 by 70 cm print becomes a 50.8 by 70.8 cm canvas, which is 6000 by 8362 pixels at 300 DPI rather than 5906 by 8268. It is a small difference in pixels and a total difference in outcome, because a file supplied without bleed gets either a white sliver along one edge or a crop into your picture.

Thirty seconds, two numbers

You do not need a calculator for this, only the habit of asking the right question.

1. Open the file and read its pixel dimensions. Every operating system shows them in the file information panel. Ignore whatever it says about DPI.
2. Decide the biggest size you will actually print.
3. Divide the long edge in pixels by the long edge in centimetres, then multiply by 2.54. That is your real DPI at that size.
4. Compare it against 150. Above it you are fine for a poster. Near 300 you are fine for anything.

Prodigi gives the seller’s version of the same advice in one sentence: “If you plan to offer your product in multiple sizes, it’s a good idea to set up your artwork based on the largest size you intend to offer.” Build for the top of the ladder and every rung below it comes free.

Sources

• Printful, “What is DPI, resolution, and actual print file size”: help.printful.com
• Printful, “Is there a difference in quality between 150 and 300 DPI”: help.printful.com
• Printful, “How should I prepare my print file for the best results”, on the 150 to 300 range, the 72 DPI export trap and the 200 MB library limit: help.printful.com
• Gelato, “How can I convert the size of my design from centimeters/inches to pixels”, the source of the formula used in this guide, 29 October 2025: support.gelato.com
• Gelato, “What are the design requirements for PDF uploads”, on the 150 to 300 range and the 4 mm bleed, 30 October 2025: support.gelato.com
• Gelato, “Optimizing print results for posters”, on the 150 target and the 175 lines per inch press figure, 30 October 2025: support.gelato.com
• Prodigi, image FAQ, on 300 dpi being ideal, JPG preference and sizing for the largest offered size: prodigi.com
• Printify, “How do I get a high-quality design file”, on artificial resolution increases: help.printify.com
• Printify, “What type of print files does Printify require”, on the 100 MB limit: help.printify.com
• Redbubble, “Dimensions & Format”, on DPI not affecting the upload and on minimum pixel counts, updated 4 August 2026: help.redbubble.com
• Etsy, “How to Manage Your Digital Listings”, on the five-file and 20 MB limits: help.etsy.com

Every pixel figure in the two tables is our own arithmetic from Gelato’s published formula, rounded to whole pixels. The compression ratios in the Etsy section are arithmetic about a limit, not measurements of any real file.

Pin this guide

Worth keeping if you buy printables or sell them. The pixel table is the one to have open while you are reading a listing, because it turns a vague promise about size into a number you can check before you pay.

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