Print-ready t-shirt files: format, transparency and resolution

A print-ready t-shirt file is a PNG with a transparent background at 300 DPI at the size it will be printed, so a 28 × 36 cm print is 3307 × 4252 pixels. Screen printing, vinyl cutting and embroidery work best from vector artwork instead. The file holds only the design: no background, no shirt colour, nothing outside the print area.

A print file is an image whose size matches the area it will be printed on. Two numbers decide whether it prints sharp: the physical size of the print, in centimetres or inches, and its resolution, in dots per inch (DPI). Get both right and the printer can use the file as it is. Get either wrong and you either reprint, or resize the file by hand for every order.

Resolution: 300 DPI at the printed size

DPI is how many pixels of the image land on each inch of fabric. Most direct-to-garment (DTG) and direct-to-film (DTF) printers ask for 300 DPI at the final print size, so the pixels a file needs follow from the print size:

pixels = centimetres ÷ 2.54 × 300

A file at the right pixel size but labelled 72 DPI is the same file: DPI in the file is only a label, and printers go by pixels and the size you tell them. What matters is that the pixels cover the print area at 300 per inch. The size of each placement, from a full front to a left chest logo or a sleeve, in inches, centimetres and pixels, is in what size a t-shirt design should be.

Why customer photos print blurry

A customer’s image has a fixed number of pixels. The bigger it is placed on the shirt, the fewer pixels land on each inch:

effective DPI = image width in pixels ÷ (printed width in cm ÷ 2.54)

A 1200-pixel-wide photo printed 28 cm wide gets about 109 DPI and will look soft. The same photo printed 15 cm wide gets about 203 DPI and holds up well. Below about 150 DPI at the printed size, softness becomes visible at a normal viewing distance; logos with fine lines and small text suffer first.

So the useful check isn’t “is this a big file?” but “is this file big enough at the size it is placed?” A product designer should make that check while the customer is still designing, when they can still pick a smaller size or a better file, not after the order arrives. For an image you already have, the image print size calculator does the sum.

Transparency and the shape of the print

DTG and DTF print every pixel that isn’t transparent. A logo on a white rectangle prints as a white rectangle on a coloured shirt. Print files for these methods are therefore PNG with a transparent background, cropped or placed exactly within the print area.

JPEG has no transparency, so a JPEG photo prints as a rectangle: fine for a photo print, wrong for a logo. If a customer uploads a logo as JPEG on a white background, the white will print unless it is removed.

What each printing method needs

• DTG (direct to garment): a full-colour PNG at 300 DPI at print size, transparent background. On dark garments the printer lays a white underbase under everything that isn’t transparent.
• DTF (direct to film): the same: a transparent PNG at 300 DPI. Very thin lines and isolated single pixels may not transfer well, so keep details at least about 0.5 mm thick. DTF print file requirements has the full list.
• Screen printing: one screen per ink colour. The printer separates the artwork into spot colours, so a PNG works as artwork, but vector files (SVG, PDF, AI) or artwork with few flat colours separate more cleanly. Photographs need halftoning.
• Vinyl or heat-transfer cutting: a cutter follows vector outlines; raster artwork has to be traced first, which suits simple shapes and text.
• Embroidery: artwork is digitised into stitches by a specialist; simple, bold shapes work best.

Colour on screen and on fabric

Screens mix light (RGB); printers lay down ink (CMYK, plus white on dark shirts). Very saturated screen colours, especially neon greens, electric blues and hot pinks, can print duller than they look. The shirt colour also shows through thin ink. If colour accuracy matters to a customer, a printed sample is the only reliable proof. Which colour mode each print method asks for, and why converting to CMYK yourself makes things worse, is in RGB or CMYK for t-shirt printing?

How PrintBlob produces the files

PrintBlob Pro renders each printed side, when the customer adds the design to the cart, as a transparent PNG at exactly the print area’s size at 300 DPI: a 28 × 36 cm print area becomes 3307 × 4252 pixels. Beside it, it makes a vector PDF and an SVG whose page is the print area’s size, with typed text as outlines in the customer’s font and the customer’s images embedded as they uploaded them, for screen printing, cutting and printers that ask for vector files. Once you upload the ICC profile your printer gives you, the print queue also makes each side’s PDF in CMYK through it. The files are attached to the order, ready to download; the free PrintBlob keeps each order’s design, so Pro also makes the files of orders placed before it. While the customer designs, the designer shows the effective DPI of a selected image at its current size and warns below 150 DPI. You set each side’s print size once, in the mockup. How a print shop takes orders this way is in a print shop taking custom orders online.

Questions shop owners ask

Try the designer and add a design to the cart to see the print file it makes.