How to hollow an STL for resin printing: wall, drain holes, plugs

Resin figures in this guide were measured on 7 November 2026 with our own hollowing tool on seven public test scans scaled to 120 to 150 mm. Recommendations from Prusa, Formlabs and Lychee were checked on their own pages, linked at the end.

To hollow an STL for resin printing, keep an outer wall of about 2 to 2.5 mm, remove everything inside it, and drill at least two drain holes into the empty space so that liquid resin can run out and air can get in. On a compact model such as a bust that cuts the resin by three quarters. You can do it in a slicer with a hollowing function, or hollow the STL online with drain holes and plugs in one step and check the wall in a section view before you download.

How much resin does hollowing save?

More than most people expect on chunky models, and less than they hope on thin ones. Prusa's own example goes from 237 ml of resin solid to 81 ml hollowed, and Formlabs reports a 77 percent saving on its demonstration part with a 2 mm wall. Our measurements on seven well-known test scans land in the same range for compact shapes and well below it for slender ones:

The pattern is simple. A part only has an inside if it is thicker than two walls. A bust is one big rounded volume, so almost everything goes. A dragon is wings, tail and claws, most of which are thinner than 4 or 5 mm and stay solid. That is correct behaviour, not a failure, and it is why the same wall gives such different savings.

Choosing the wall thickness

Prusa sets the minimum at 1 mm. Lychee's guidance is a useful map of the range: 1.5 to 2 mm for small decorative pieces and miniatures, 2.5 mm, its default, for most prints, and 3 to 4 mm for parts that must be strong or take stress. Formlabs points out the trade-off plainly: thinner walls use less material, and a thin shell makes a large model fragile.

• Figures and busts up to about 15 cm: 2 mm is usually enough and saves the most.
• Larger or heavier pieces, and anything that will be handled: 2.5 to 3 mm.
• Functional parts, mounts, anything screwed or clipped: 3 mm or more, or print it solid.

The saving shrinks slowly as the wall grows, so the safer choice is cheap. On the Nefertiti bust the step from 2 to 2.5 mm adds 14 ml of resin, about 4 percent of the solid volume; on the Max Planck bust it adds 11 ml. A cracked print costs all of that resin again, 50 to 80 ml on these busts.

Drain holes: at least two

A hollow part with no hole is a sealed bag of liquid resin. Prusa warns that without drainage holes the uncured resin ends up trapped inside the finished print, and both Prusa and Formlabs ask for at least two holes. Formlabs wants one of them as close to the build platform as possible, plus more wherever the cavity forms a cup. Two holes matter because one lets resin out while the other lets air in. Size, placement and the suction-cup effect have their own guide: drain holes for hollow resin prints.

Every separate cavity needs its own holes. A figure with a thin neck can end up with two cavities, one in the head and one in the body, joined by solid material. A hole in the body does nothing for the head.

Plugs, washing and curing

Drain holes leave openings in the surface. If they are on the base nobody sees them; if they are not, print a plug for each one. A plug cut from the same wall with a small clearance, about 0.15 mm all round, slides in and can be fixed with a drop of resin and a short second cure, which closes the surface again. Formlabs notes that the holes also let the washing liquid flow through the part, so rinse the inside as well as the outside and let it drain fully before curing.

Hollowing an STL online, step by step

1. Make sure the model is a closed solid. A scan or a download with holes can be closed with STL Repair; a model from an AI generator is better rebuilt with Make AI Model Printable.
2. Check the size. If the part comes in far too small, it was probably made in inches or metres; resize the STL to an exact size first.
3. Open the hollowing tool, choose the wall thickness and the hole diameter, and mark which side is the base.
4. Slide the section plane through the result. You should see an even band of wall all round and the holes opening into the cavity.
5. Move or add holes by clicking on the model if the automatic ones land somewhere visible, then download the hollowed model with its plugs.

The tool reports the solid and hollow volume, the thinnest wall actually left and the share of the surface that stayed solid. If no part of the model is thick enough to hollow, you get a message instead of a file, and nothing is charged. Before printing, open the result in your slicer and estimate the job once more; the STL analyzer gives the volume, weight and print cost from the same file.

Hollowing in the slicer or in the file

Most resin slicers can hollow: PrusaSlicer, Lychee and Chitubox all have it, with wall thickness and drain hole tools. That is the quickest route when you print the file yourself on one printer. The hollow model then lives inside the slicer project, and every time you load the original STL again, you hollow it again.

Hollowing the file itself is better when the model goes to someone else, a print service or a second slicer, when you want to check the wall in a section before committing resin, or when the slicer struggles with a large sculpt. The result is an ordinary STL with the cavity, the holes and the plugs already in it, so any slicer prints it the same way.

Common mistakes

• Holes only on the top of the part, far from the build platform: resin pools where it cannot leave.
• One hole per part when a thin neck split the inside into two cavities.
• A wall chosen for a miniature on a 25 cm statue; it saves resin and then cracks.
• Curing before the inside has drained and been rinsed; liquid resin inside keeps leaking for days.