Why do 3D printed holes come out too small?
By Layer & Loop Store · Published
Short answer
Holes in FDM 3D prints usually come out slightly smaller than modelled — commonly by a few tenths of a millimetre, and more on small or sideways holes, depending on the material, printer calibration and orientation. The plastic lines on the inside of a hole bulge inwards, curved edges are made of flat segments, and the top of a horizontal hole sags. The fix is to add clearance in the model, not to expect an exact CAD size.
Why printed holes shrink
The plastic squashes inwards
An FDM printer lays each line of plastic slightly flattened so it bonds to the layer below. On the inside of a hole, that extra width has nowhere to go but into the hole. On a large hole it hardly matters; on a 3 mm hole it's a noticeable share of the diameter.
Circles become polygons
STL files store curves as flat triangles. If a model is exported at low resolution, a round hole becomes a polygon whose flat sides sit inside the true circle, so a bolt touches the flats first. Exporting with a finer mesh resolution keeps holes rounder.
Sideways holes sag at the top
When a hole's axis lies parallel to the print bed, the top of the hole is printed over empty space. It sags a little, making the hole slightly oval and smaller in height than width. This is one reason orientation matters — see orientation and supports.
First-layer squish
The first layer is pressed firmly onto the bed so the part sticks, which can make it bulge slightly outwards and narrow the bottom edge of any vertical hole that starts at the bed.
How much clearance to design in
The values below are illustrative starting allowances, not a specification. How a hole actually prints depends on the material, the printer and its calibration, the part's orientation, the hole's size and the kind of fit you need — so treat them as a first guess and confirm with a test print where the fit matters.
| Fit you want | Illustrative starting allowance | Example (adjust after testing) |
|---|---|---|
| Bolt passes through freely | +0.2 to +0.4 mm on diameter | M3 bolt → try around 3.3–3.4 mm |
| Shaft rotates freely | +0.3 to +0.5 mm on diameter | 5 mm shaft → try around 5.3–5.5 mm |
| Parts slide together | 0.2–0.3 mm gap per side | Lid in a box, sliding drawer |
| Snug push fit | 0 to +0.1 mm on diameter | Pin in a socket — test first |
| Threaded insert or self-tapping screw | Follow the insert or screw maker's hole size | Heat-set insert, wood screw |
Very small holes — a couple of millimetres or less — can shrink noticeably or partly close up. If you need a precise small bore, it is common to print it undersized and drill it out to size afterwards.
Test before you commit
For anything with several mating holes, a quick test print saves a lot of reprinting. Many designers print a small “fit coupon” — a thin plate with the critical holes or a short section of the joint — before printing the full part. It's faster and cheaper than printing the whole object to find one hole is too tight. The prototype guide covers what else to check on a first print.
Quick checklist for holes and fits
- Add clearance to every hole that must take a bolt, pin or shaft.
- Give sliding or nesting parts a deliberate gap — never model them at exactly the same size.
- Export the STL at a fine resolution so circles stay round.
- Use a teardrop top on horizontal holes, or orient them vertically.
- Drill or ream small precision holes after printing.
- Print a test piece when the fit really matters.
The wider file checklist is in how to prepare an STL for printing.
Common questions
How much bigger should I make a hole for a 3D printed part?
There's no universal figure. As an illustrative starting point for FDM printing, many designers model a clearance hole roughly 0.2–0.4 mm larger in diameter than the bolt or shaft — for example trying around 3.3–3.4 mm for an M3 bolt — and then adjust after a test print. The right allowance depends on the material, printer calibration, orientation, hole size and the kind of fit needed.
Why are horizontal holes in 3D prints not round?
A hole whose axis lies flat has a top edge that has to bridge across empty space, so the top sags slightly and the hole prints a little oval. Making the top of the hole a pointed teardrop shape, or printing the part so the hole is vertical, gives a rounder result.
Will Layer & Loop Store adjust hole sizes in my model?
No. Layer & Loop Store prints the model exactly as supplied and doesn't alter dimensions, so build any clearance into the STL or 3MF before uploading. For a critical fit, ordering one test print before a larger quantity is the safest approach.
Want to test a fit first?
Upload a test piece or the full part for an instant price — there's no minimum on instant orders, so a single fit check is fine.
External references
General technical background comes from these sources; notes marked “From the Layer & Loop Store workshop” are our own production experience.
- Modeling with 3D printing in mind — Prusa Knowledge Base