DPI, PPI and LPI:
the three numbers

Three numbers, all ending in "per inch"

Almost every resolution argument in screen printing comes from mixing up three numbers that sound identical and describe completely different things.

They stack in that order: your pixels become printer dots on a film, which form halftone dots that the mesh has to hold. Each step is coarser than the one before it, and the last step is the tightest constraint by far. A screen holds perhaps 35 halftone dots per inch. Your image has 300 pixels per inch. There is an enormous amount of detail in your file that the screen will never see — which is exactly why 300 PPI is plenty and why chasing more is wasted effort.

Your artwork: 300 PPI at final print size

The phrase that matters is at final print size. A file is not "300 DPI" on its own; resolution only means something once you say how big it will be printed. The same 3000-pixel image is 300 PPI on a 10″ print and 150 PPI on a 20″ one.

So work backwards from the print: multiply your print width in inches by 300.

Resampling a small file up to these dimensions does not create detail — it invents pixels by averaging the ones you have. The edges stay soft, and on a screen print soft edges are unmistakable. If the source is genuinely small, redrawing beats upscaling.

Where 300 comes from

The classical rule for halftone reproduction is that image resolution should be about twice the line frequency. At 45 LPI that is only 90 PPI, so 300 looks wildly generous — and for the halftone areas it is. The reason 300 is still the standard is everything that is not halftone: hard edges, type, thin linework and logos. Those are reproduced at the film's full resolution, not at the halftone frequency, and that is where extra pixels earn their keep.

In other words: 300 PPI protects your edges, not your dots.

Vector files have no resolution

An AI, EPS, PDF or SVG file stores shapes as mathematics rather than as pixels, so it can be printed at any size with perfectly hard edges. For type, logos and flat-colour illustration, vector is simply the better starting point — there is no resolution to get wrong.

Two practical notes. First, a vector file can still contain a placed photograph, and that photo has all the usual resolution limits; "it is an AI file" does not automatically make it high resolution. Second, PrintFilmReady takes raster files only for now, so if you are working in vector, export to PNG at your final print size and 300 PPI or more, and separate from that.

Film output DPI: 360 or 720

This is a different number from your artwork PPI: it is how finely the printer lays ink onto the transparency. It matters because it sets how many tonal steps a halftone dot can grow through — the number of printer pixels that fit inside one halftone cell.

Grey levels = (DPI ÷ LPI)² + 1

The eye separates roughly 60 to 100 steps in a smooth gradient, and a textile screen holds fewer than that. So 360 dpi is comfortable for ordinary shirt work. Move to 720 dpi when you are running fine mesh at high frequency and the design has smooth gradients — at 65 LPI, thirty-one steps can band visibly.

LPI and mesh: the number that actually limits you

A halftone dot has to bridge the threads of the mesh to survive washout. Too fine for the fabric and the small dots fall straight through. That is why line frequency is not a taste decision: your mesh sets the ceiling.

You will see the rule quoted as "mesh should be 3.5 to 4 times your LPI", and you will also see 4.5 or 5 recommended. Both are honest — they describe different appetites for risk.

A lower ratio means smaller dots for the same mesh: more detail, less margin for error. Plenty of shops run 156 mesh at 45 LPI every day and get clean results, because their film density and exposure are consistent. A printer whose highlights keep vanishing is nearly always at the pushed end of this table without the process control to back it up.

PrintFilmReady's presets sit at the conservative end deliberately: the first failure a self-taught printer meets is washed-out highlights, and that failure costs a screen. Once you have run a step wedge and know exactly which dots your setup holds, pushing the frequency is a reasonable next step.

What too little resolution actually looks like

• Soft, stepped edges on type and linework — the classic symptom, and the most visible on a printed shirt.
• Blocky halftone patterns where a gradient should be smooth, because the separation had too few pixels to work from.
• Ragged separations. Colour boundaries in a low-resolution or heavily compressed JPEG are full of blended pixels, which end up assigned to the wrong ink and leave fringes along every edge.

That last one is worth stressing: JPEG compression damages colour edges even in a file with plenty of pixels. For separation work, PNG is the safer raster format.

Before you separate: a thirty-second check

1. Decide the print size first. Everything else follows from it.
2. Check pixels ÷ inches ≥ 300. A 2400 px design at 10″ is 240 PPI — usable for bold shapes, marginal for fine type.
3. Prefer PNG over JPEG for anything you intend to separate.
4. Pick the mesh you will actually print with, and let the line frequency follow from it.
5. Choose 360 dpi film output unless you are running fine mesh with gradients.

PrintFilmReady does step two for you: enter the print width and it tells you the effective resolution and warns you when it drops too low. It is a small thing that prevents the most expensive mistake in this list — finding out after the screen is burned.

Common questions

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