Upscale Image for Print on Demand: Sizes, DPI and a Mac Workflow

Printful’s guide calls 300 DPI the best resolution for paper prints and 150 DPI for other products, and it says to avoid designs under 150 DPI. Printify asks for a minimum of 300 DPI at physical print scale. Check the exact figure for your product before you upload.

DPI only means something once you know the print size. A file is a grid of pixels, and DPI is how many of those pixels land on each inch of the product. The pixels you need are the print area in inches times the DPI. A 15 by 18 inch area needs 4500 by 5400 pixels at 300 DPI, which is the canvas Printify suggests for a shirt, and 2250 by 2700 at 150 DPI.

The two Printful pages do not agree with each other. The graphics guide says all files must be at least 300 DPI for paper prints and 150 DPI for everything else, while its digital print file page says the minimum accepted for paper products is 75 DPI and strongly recommends 300. Treat 300 as the target and 150 as the floor for non-paper products. The guide also warns against going over 600 DPI, so a bigger file is not a better one.

Printful accepts PNG and JPG/JPEG, wants the sRGB color profile, and caps a print file at 200 MB. Requirements differ by product, so open the product in Printful’s Design Maker and read the file guidelines for each print area before you size anything.

Redbubble image size works the same way, but this page does not quote Redbubble’s numbers because they could not be checked on Redbubble’s own pages. Take the pixel figure from Redbubble’s upload screen for your exact product, then use the pixels-equal-inches-times-DPI arithmetic above to see how far your file is from it.

An image is too small for print when it has fewer pixels than the print area needs at the platform’s DPI. A 1500 by 1800 pixel file stretched across 15 by 18 inches is 100 DPI, below Printful’s 150 DPI floor, which is under Printful’s guidance to avoid designs below 150 DPI.

Divide the pixels you have by the inches you want. 1500 pixels over 15 inches is 100. Divide the pixels you need by the pixels you have to get the scale: 4500 over 1500 is 3x. If the answer is well over 4x, the source is probably too small for that print area, so choose a smaller print size or find a larger source.

Printful will also enlarge some designs for you: its Smart Image Tool adjusts uploads in the 38 to 74 DPI range when it generates print files. The page states no scale or size limit. Upscaling yourself lets you inspect the file before an order depends on it.

Choose the smallest scale that reaches your target pixels, and save PNG. Smol upscales 2x, 3x or 4x to PNG, JPG or WebP. Quality sliders belong to Compress, so on an upscale the choices that matter are the scale and the format.

PNG is the right export for print masters. Printify says PNG-24 with transparency for art on a clear background, and JPEG cannot store transparency at all. The cost is file size: a 4x PNG holds sixteen times as many pixels as the original, so expect a large file. Printful’s cap is 200 MB per print file, so check the output size of large files.

Should I compress the upscaled file afterward?

Usually not. Smol has no lossless mode for images: Compress always re-encodes, so running it on a print master trades quality for bytes. Compress the copies that go to web listings, mockups and email, and leave the upscaled PNG alone unless it is over a cap. If you do compress a master, set a high quality, compare the result at 100 percent zoom, and keep the uncompressed original.

For those web copies, Smol made 24 small test PNGs 69.2% smaller (PNG to PNG at quality 75, default settings). Those are 768 by 512 pixel photographs, so your own artwork will land elsewhere; the method is on the benchmark page. Smol’s image defaults also resize to fit inside 2000 by 2000 pixels unless you change it, which would undo an upscale, so raise or switch off the resize whenever you compress a large file.

Work out the scale once per size group, then drop each group in. Folders and mixed batches are accepted, and each file shows its own progress.

1. 1

   Find the pixels you need

   Take the print area from your platform’s product page, multiply inches by 300 (or 150 where that is the floor), and write down width and height.

2. 2

   Group designs by the scale they need

   Files near 1500 px wide go through at 3x, files near 1125 px at 4x, files near 2250 px at 2x.

3. 3

   Drop the folder into Smol and choose Upscale

   Pick the scale, set the output to PNG, and run it. By default the result is saved as a copy beside the original.

4. 4

   Open one result at 100 percent

   Look at edges, text and faces. If something looks smeared, the source did not have enough information to enlarge; a cleaner source file is the fix.

5. 5

   Check pixel size and file size

   In Finder, Get Info shows dimensions. Confirm they meet the table above and sit under the platform’s file cap.

6. 6

   Upload the PNG, then check the platform’s preview

   Check the resolution feedback on the platform’s own print area preview, which is the last word on whether the file passed.

Several free options exist, and the right one depends on how much control you want. Start with what is already in your seller dashboard before adding a tool.

For the file-handling side of a store, see our guides on every method of compressing images on a Mac and the sips command. Sellers who work from AI generations will find more on upscaling AI art, and artists preparing portfolios can read Smol for illustrators.

Smol enlarges pixels; it does not draw new artwork. Reach for something else in these cases.

• The art is a logo or line drawing you will print large and often. Redraw it as a vector in a drawing app so it scales without limit. Smol does not write SVG.
• The source is a tiny thumbnail, a 72 DPI web JPEG or a screenshot with small text. Printify’s advice applies: you cannot add detail that was never there. Find or recreate a larger source.
• You need to fix artifacts, remove a background or edit text inside the picture. Smol upscales and compresses; it is not an editor.
• You need CMYK separations or a color-managed proof. Printful asks for sRGB, but a print shop that wants CMYK is outside what Smol does.
• You use Windows or Linux. Smol runs on macOS 13 or later on Apple Silicon, or macOS 14 or later on Intel.
• You upscale one file a month. Preview or Printful’s own tool costs nothing.

Solo is $29 once for one Mac. Team is $49 once for three Macs, and Enterprise is $20 a seat for 5 to 500 seats. There is no subscription and no per-file quota. Presets are stored on each Mac, so a shop with two designers writes down its scale and format and recreates the preset on each machine.