A common rule of thumb for screen printing traps is an overlap of 0.75 to 1.5 points, about 0.26 to 0.53 mm, with the lighter color spread under the darker one; detailed art with thin lines needs less, and the right number for your shop comes from a registration test on your own press. The confusion usually starts with units, because separation advice is written in points, inches, millimetres or Photoshop pixels. Underbase, a Windows app from Automata Labs, spreads or chokes any ink in 0.05 mm steps, traps every ink under the darkest in one click, and shows each amount in millimetres and in pixels at your artwork’s resolution, so the number you choose is the number that reaches the film.
This guide explains spread and choke, converts the common numbers between units, and gives a simple test for finding your own trap.
Set traps in millimetres, see them in pixels, and send film that fits your press.
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What is trapping in screen printing?
Trapping is a deliberate overlap between colors that touch, so small registration movement on press does not open a gap where the shirt shows through. Pro Ad Co, a US print shop, describes it on its trapping page as “an overlap of colors to whatever degree is tolerable,” used to compensate for registration issues between successive screens.
There are two ways to make the overlap:
| Term | What changes | When it is used |
|---|---|---|
| Spread | An element is enlarged slightly so it overlaps its neighbor | The usual choice: the lighter ink grows under the darker ink |
| Choke | An element is reduced slightly so its background overlaps it | Keeping a base or light ink inside the edge of a darker shape |
Pro Ad Co’s general rule is that “lighter colors should be spread into and under darker colors,” because the darker ink hides the overlap. The white underbase on a dark garment is the classic choke: it is pulled in from the edges so it never peeks out from under the top colors. Our screen printing separation terms glossary defines these and related words.
How much trap is typical?
Pro Ad Co gives a general range: trap widths “should be between .75 points (.01”) and 1.5 points (.02”),” with smaller traps for graphics with a lot of detail or very thin lines and bigger traps for simple graphics with large open areas.
Treat that as a starting range, not a standard. Pro Ad Co’s own page explains its context: it typically uses translucent inks, and it describes the substrate being pulled through the press and stretching as more screens are used. Garments, platens, mesh, off-contact and press condition all change how much movement you need to cover. A tight automatic press with good registration needs less than a manual press with worn hinges.
For the white underbase, Thomas Trimingham’s article in Impressions, Your Underbase is Showing, describes trimming a solid underbase in Photoshop with a centered stroke of two to four pixels on the underbase channel, “just enough to allow the top color to slightly move.” The article also warns that contracting the underbase on art with tiny lines can remove it in places, so fine detail needs a lighter touch.
How do points, inches, millimetres and pixels convert?
A point is 1/72 of an inch, or about 0.353 mm. Pixels depend on resolution: at 300 pixels per inch, one pixel is about 0.085 mm. Here are the common amounts side by side:
| Amount | Inches | Millimetres | Pixels at 300 ppi | Pixels at 600 ppi |
|---|---|---|---|---|
| 0.5 pt | 0.007 in | 0.18 mm | 2.1 px | 4.2 px |
| 0.75 pt | 0.010 in | 0.26 mm | 3.1 px | 6.3 px |
| 1 pt | 0.014 in | 0.35 mm | 4.2 px | 8.3 px |
| 1.5 pt | 0.021 in | 0.53 mm | 6.3 px | 12.5 px |
| 2 pt | 0.028 in | 0.71 mm | 8.3 px | 16.7 px |
Two conversions catch people out:
- A centered stroke only moves the edge by half its width. A 4 px centered stroke on an underbase channel trims about 2 px from the edge. At 300 ppi, the two to four pixel stroke from the Impressions article works out to roughly 0.08 to 0.17 mm of choke.
- Pixels change meaning when the art changes resolution. Two pixels on a 150 ppi customer JPEG is twice the physical distance of two pixels on a 300 ppi file. That is why a trap you remember in pixels from one job can be wrong on the next. Our guide to effective ppi explains how resolution changes when art is scaled.
How do you find the right trap for your press?
Measure instead of guessing. A short test tells you how much your press actually moves:
- Print a registration test. Use a design with colors that kiss (touch without overlapping) and a white base under them, on the garment and ink you print most. The Impressions article recommends test prints with high-contrast colors and kiss registration to see where the press needs adjustment before you compensate in the art.
- Fix the press first. If gaps show up in one direction only, adjust registration, off-contact or pallets before you change the artwork.
- Measure the worst gap. With a loupe and a ruler, note the largest gap or shift that remains when the press is set as well as it gets.
- Set the trap just above it. Make the trap a little larger than the worst remaining movement, and keep it smaller on art with fine lines.
- Write it down per press. Record the number on your job tickets so the next separation starts from it.
The aim is the smallest trap that covers your real movement. Too little and gaps appear; too much and you get dark outlines where a light ink overprints a darker one, or a heavier ink build at the edges.
Put your tested trap and choke into every job, measured in millimetres on the film.
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How does Underbase handle trap and choke?
Underbase is a Windows app for limited-color screen printing separations. It opens the art customers send (PNG, TIFF, JPEG, BMP, GIF, WebP, SVG, PDF and PDF-compatible AI files), finds the inks, and writes film. For trapping it gives you:
| Control | What it does |
|---|---|
| Spread or choke per ink | Moves one ink’s edge from -3 to +3 mm in 0.05 mm steps |
| Apply to all inks | Spreads every ink except the darkest by the default trap, 0.15 mm unless you change it, so lighter colors tuck under the darkest |
| Underbase choke | Pulls the white base in from 0 to 3 mm in 0.05 mm steps, 0.25 mm by default |
| Units | Shows each amount in millimetres and in pixels at your artwork’s resolution, in inches or millimetres |
| Output dots preview | Shows the plate as the film will carry it, at the output resolution, before you burn a screen |
Compare the defaults with the table above: 0.15 mm is about 0.43 pt, at the tight end, and 0.25 mm of choke is about 0.71 pt. If your registration test says your press needs more, change the number once and use it on every job.
Underbase also counts every screen against your plate budget, suggests a print order (light to dark, black next to last, white on top as the highlight white), and writes 1-bit film TIFFs at 600, 720, 1200 or 1440 dpi, a true-size film PDF with registration marks, and a production ticket as PDF and JSON. It runs entirely on your PC with no account and no upload. The white underbase for dark shirts page covers the base in more detail.
Where should you not trap?
Trapping is not always the answer:
- Black outlines over flat colors. If the art already has a dark keyline, spread the colors under the line rather than adding another overlap. Pro Ad Co notes that an ideal trap splits an outline at its thinnest point, leaving room on both sides.
- Halftone fades. Fading areas need careful underbase work, not a blanket choke. The Impressions article suggests a value chart and test swatches for halftone underbases.
- Tiny type and hairlines. A trap the width of a stroke changes the letterform. Reduce or skip it on small detail.
Frequently asked questions
Is 1 pt of trap too much?
Not for many simple designs. It sits inside the 0.75 to 1.5 pt general range Pro Ad Co gives. On fine detail it can be too much; test on your press.
What is the difference between spread and choke?
A spread enlarges an element so it overlaps its neighbor. A choke shrinks an element so the surrounding color overlaps it. Lighter inks are usually spread under darker ones; the white underbase is usually choked.
How many pixels is 0.25 mm?
About 3 pixels at 300 ppi and about 6 pixels at 600 ppi.
Does Underbase do simulated process separations?
No. Underbase handles spot and index separations for limited-color art. Photographic simulated-process work needs a Photoshop-based tool.
Try Underbase
Underbase is a one-time purchase in the Microsoft Store with a free trial. Separate a recent job with your tested trap and choke, and compare the film with what went to press. More detail is on the Underbase product page.
Separations, underbase, traps and film on your PC, with no Photoshop and no subscription.
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Photoshop is a trademark of Adobe. Underbase is not affiliated with Adobe.
Last updated 20 October 2026. Published by Automata Labs, a software company in Canada.