Your 3D printed parts don’t fit because printers don’t print exact dimensions. Learn the right tolerance and clearance values for FDM, SLA, and SLS, and your assemblies will snap, slide, and press together on the first try.
You designed a lid, a hinge, or a peg-in-hole joint. The CAD model looked perfect. The printed parts either refuse to mate or rattle like loose change. The cause is almost never your slicer. It is the gap between the dimensions you modeled and the dimensions the printer produced.
This guide gives you the numbers. You get clearance values by fit type, process-specific tolerance tables, a calibration test, and copy-pasteable slicer settings.
Quick Takeaways
- FDM clearance: use 0.2-0.4 mm per side for sliding fits and 0.1-0.2 mm for snug fits.
- SLA clearance: use 0.1-0.2 mm per side. Resin prints are more accurate but shrink and warp with post-curing.
- SLS clearance: use 0.3-0.5 mm per side to leave room for powder escape and surface roughness.
- Always print a calibration coupon before committing to a full assembly. One 10-minute test saves hours of reprints.
| Process | Typical Tolerance | Sliding Fit Clearance (per side) | Press Fit Interference |
|---|---|---|---|
| FDM | ±0.2-0.5 mm | 0.2-0.4 mm | 0.1-0.2 mm |
| SLA/DLP | ±0.1-0.2 mm | 0.1-0.2 mm | 0.05-0.1 mm |
| SLS (Nylon PA12) | ±0.3 mm (or ±0.3%) | 0.3-0.5 mm | 0.1-0.2 mm |
Tolerance vs. Clearance: What’s the Difference?
People mix up these terms. They describe different things.
Tolerance is the allowed deviation from a nominal dimension. A 20 mm hole with a ±0.2 mm tolerance can legally measure 19.8-20.2 mm. It describes your printer’s accuracy.
Clearance is the intentional gap you design between two mating parts. A 20 mm peg going into a 20.4 mm hole has 0.4 mm diametral clearance (0.2 mm per side). It is your design decision.
The relationship is simple. Your clearance must be larger than your combined tolerance stack, or some parts will bind. If your printer holds ±0.15 mm on each part, you need at least 0.3 mm of total gap to guarantee assembly.
The Three Fit Categories
| Fit Type | Behavior | Use Case |
|---|---|---|
| Clearance fit | Always a gap. Parts slide or rotate freely. | Hinges, sliding lids, bearings |
| Transition fit | May have a gap or slight interference. | Locating pins, snap-on caps |
| Interference (press) fit | Always overlapping. Needs force to assemble. | Permanent joints, bushings, magnets |
Why 3D Printed Parts Never Match Your CAD
Understanding the error sources lets you correct them instead of guessing.
FDM Error Sources
- Elephant’s foot: the first layers squish outward from bed heat and weight, enlarging the base by 0.1-0.3 mm.
- Hole shrinkage: holes print 0.1-0.3 mm smaller than designed. Extruded plastic bulges into the hole path, and polygonal approximation cuts into the circle.
- Thermal shrinkage: ABS shrinks about 0.4-0.7%, ASA about 0.4-0.6%, PETG about 0.2-0.4%, and PLA only 0.1-0.3%.
- Over-extrusion: a flow rate just 5% too high adds roughly 0.05 mm to every wall.
- Nozzle width: features smaller than the nozzle diameter (usually 0.4 mm) cannot be resolved.
- Mechanical error: belt stretch, loose pulleys, and uncalibrated steps/mm cause dimensional drift across the build plate.
SLA Error Sources
- Over-curing: light bleed enlarges small features and closes fine gaps.
- Post-cure shrinkage: standard resins shrink around 0.5-2% during UV curing.
- Support-side warping: surfaces touching supports distort slightly.
SLS Error Sources
- Powder adhesion: partially sintered powder around the part edge grows dimensions by 0.1-0.2 mm.
- Thermal shrinkage: PA12 shrinks about 3-3.5% during cooling. Service bureaus compensate in software, but variation remains.
Clearance Cheat Sheet by Process
Use these as starting points. Refine them with your own calibration test.
FDM Clearance Values (0.4 mm Nozzle, 0.2 mm Layer Height)
| Fit Goal | Clearance (per side) | Notes |
|---|---|---|
| Loose sliding (lids, drawers) | 0.3-0.4 mm | Tolerates poor calibration |
| Standard sliding | 0.2-0.3 mm | Good for well-tuned printers |
| Snug / tight sliding | 0.1-0.2 mm | Requires dialed-in flow and cooling |
| Rotating (axles, hinges) | 0.3-0.5 mm | Extra gap prevents friction fusion |
| Snap fit | 0.1-0.2 mm | Depends on material flexibility |
| Light press fit | -0.05 to -0.1 mm (interference) | Works best in PETG and nylon |
| Heat-set insert hole | Follow insert maker spec, typically 0.2-0.4 mm under outer diameter | Melts into the plastic |
SLA Clearance Values (50 µm Layers)
| Fit Goal | Clearance (per side) |
|---|---|
| Sliding fit | 0.1-0.15 mm |
| Snug fit | 0.05-0.1 mm |
| Moving assemblies (print-in-place) | 0.3-0.5 mm |
| Press fit | -0.05 mm |
SLS Clearance Values (PA12)
| Fit Goal | Clearance (per side) |
|---|---|
| Sliding fit | 0.3-0.4 mm |
| Print-in-place joints | 0.5-0.7 mm (to clear trapped powder) |
| Press fit | -0.1 mm |
Material Matters: FDM Filament Comparison
| Material | Shrinkage | Dimensional Stability | Recommended Clearance |
|---|---|---|---|
| PLA | 0.1-0.3% | Excellent | 0.2-0.3 mm |
| PETG | 0.2-0.4% | Good, slight stringing affects fit | 0.25-0.35 mm |
| ABS | 0.4-0.7% | Fair, warps without enclosure | 0.3-0.4 mm |
| ASA | 0.4-0.6% | Fair | 0.3-0.4 mm |
| Nylon (PA) | 1.0-2.0% | Poor, absorbs moisture | 0.35-0.5 mm |
| TPU | 0.5-1.0% | Compressible, forgiving | 0.1-0.2 mm |
Step-by-Step: Calibrate Your Own Tolerances
Published numbers are a starting point. Your printer, filament, and settings produce unique results. A tolerance test coupon gives you real data.
Step 1: Print a Clearance Test
Design (or download) a test with pegs and holes at stepped gaps. A good test uses:
- Gaps of 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40 mm
- A 20 mm nominal peg diameter
- The same layer height, material, and speed as your real project
Print it at your production settings. Changing speed or temperature later invalidates the result.
Step 2: Test Each Fit
Try the peg in each hole. Record three results:
- Binds / won’t enter: gap too tight.
- Slides with slight resistance: your snug fit value.
- Slides freely with no wobble: your standard clearance value.
- Rattles: gap too loose.
Step 3: Measure with Calipers
Use digital calipers (0.01 mm resolution) to measure printed holes and pegs. Compare to the nominal values.
Calculate your hole shrinkage offset:
offset = nominal_diameter - measured_diameter
If a 20.00 mm hole measures 19.75 mm, your offset is 0.25 mm. Add this to every hole diameter in your designs.
Step 4: Record a Personal Fit Table
Keep a note for each material and printer pairing:
| Printer | Material | Nozzle | Snug | Sliding | Loose |
|---|---|---|---|---|---|
| Example Printer | PLA | 0.4 mm | 0.15 mm | 0.25 mm | 0.35 mm |
Fix the Problem at the Source: Slicer and Firmware Settings
Before inflating clearances in CAD, remove the avoidable errors.
Calibrate Extrusion Flow
Over-extrusion is the most common cause of tight fits. Print a single-wall cube and measure the wall thickness with calipers. Adjust flow so it matches your line width.
new_flow = old_flow × (expected_wall / measured_wall)
Example: expected 0.45 mm, measured 0.48 mm, old flow 100% gives a new flow of 93.75%.
Calibrate E-Steps (Marlin)
Mark 120 mm of filament above the extruder, then extrude 100 mm:
M83 ; relative extrusion mode
G1 E100 F100 ; extrude 100 mm slowly
Measure what remains. If 20 mm moved wrongly, calculate and apply:
M92 E<new_value> ; set new steps per mm
M500 ; save to EEPROM
New value formula: current_esteps × (100 / actual_extruded).
Pressure Advance (Klipper)
Corner bulging and uneven walls ruin dimensional accuracy. Set pressure advance in printer.cfg:
[extruder]
pressure_advance: 0.04 ; typical direct drive starting point
pressure_advance_smooth_time: 0.040
Direct drive extruders typically land at 0.02-0.06. Bowden setups need 0.5-1.0. Run a pressure advance tuning tower to find your exact value.
Compensate Elephant’s Foot
Most slicers include an elephant’s foot compensation setting. Start at 0.1-0.2 mm. In Cura it is called Initial Layer Horizontal Expansion (use a negative value like -0.15 mm). In PrusaSlicer, use Elephant foot compensation under Print Settings > Advanced.
Use Horizontal Expansion for Global Correction
If all holes print small, apply a slicer-wide compensation instead of editing every model. In Cura, set Horizontal Expansion to a small negative value (-0.05 to -0.1 mm) for external size and use Hole Horizontal Expansion (positive, 0.1-0.2 mm) to enlarge holes.
Design Rules for Parts That Fit
Orient Mating Features Carefully
Layer lines make Z-axis dimensions different from X/Y. Vertical holes print rounder than horizontal holes. For critical round bores, print them vertically. For horizontal holes, add a teardrop or diamond shape to avoid sagging, or keep diameters below 8 mm.
Use Chamfers and Fillets
Add a 0.5-1.0 mm chamfer to the entry of every hole and the tip of every peg. Chamfers guide parts together and hide elephant’s foot flaws.
Design Press Fits with Slots
A solid peg in a solid hole is unforgiving. Add relief cuts or crush ribs (small 0.3-0.5 mm ridges) so the plastic deforms instead of cracking.
Keep Wall Thickness Above Four Perimeters
Thin walls flex and distort under assembly force. Use at least 1.6 mm walls (four perimeters with a 0.4 mm nozzle) around any fitted feature.
Standardize Your Hole Sizes in CAD
Create parametric variables once:
hole_clearance = 0.25 // mm per side
peg_diameter = 20.0
hole_diameter = peg_diameter + (2 * hole_clearance)
This one change makes every future design adjustable from a single value.
Practical Examples and Real-World Use Cases
Enclosures and Snap-Fit Lids
Electronics enclosures need lids that close with a click and open without breaking. Use 0.2 mm clearance on lip joints and 0.3-0.4 mm clearance around port cutouts. PETG’s flexibility makes it the preferred material for snap hooks.
Print-in-Place Hinges and Mechanisms
Print-in-place assemblies need enough gap to prevent layer fusion. Use 0.3-0.5 mm between moving surfaces on FDM and keep first-layer elephant’s foot compensation active. Orient hinge pins vertically when possible.
Bearing and Magnet Pockets
A standard 608 bearing has a 22.0 mm outer diameter. A snug FDM pocket usually measures 22.1-22.2 mm. Magnets (for example 6 × 3 mm discs) go into pockets sized 6.1-6.2 mm wide and 3.1-3.2 mm deep, then get a drop of CA glue.
Automotive and Industrial Prototyping
Rapid prototyping teams print fixtures, jigs, and mounting brackets that must fit real parts. Because stock metal components have tight machined tolerances (often ±0.05 mm), designers print with 0.2-0.3 mm clearance and verify with go/no-go test pieces before final production.
Dental and Medical Models (SLA)
Resin printers produce surgical guides and aligner molds where fit matters. Designers typically use 0.1 mm clearance on guide sleeves and verify with scanned comparison.
Threaded Parts
Printed threads need extra room. For FDM, increase the thread pitch diameter clearance to 0.3-0.4 mm and use threads of M8 or larger. Smaller threads rarely work. For anything smaller, use heat-set brass inserts.
Common Mistakes and Quick Fixes
| Problem | Likely Cause | Fix |
|---|---|---|
| Parts won’t enter holes | Hole shrinkage, over-extrusion | Add 0.1-0.2 mm hole offset, calibrate flow |
| Parts rattle loosely | Excess clearance | Reduce by 0.05 mm and retest |
| Base of part too wide | Elephant’s foot | Enable compensation, lower bed temp by 5°C |
| Hinge fused solid | Gap too small | Increase to 0.4 mm+, lower print temp |
| Tight in X, loose in Y | Mechanical skew, belt tension | Check belt tension and frame squareness |
| Fit changes between prints | Wet filament, temp variance | Dry filament, hold temperatures constant |
Frequently Asked Questions
What is the standard clearance for 3D printed parts?
For FDM printing, use 0.2-0.3 mm per side as the standard clearance for sliding fits with a 0.4 mm nozzle. For tight fits, drop to 0.1-0.2 mm. SLA allows 0.1-0.2 mm, and SLS needs 0.3-0.5 mm.
How do I make a 3D printed hole fit a screw or pin?
Add 0.2-0.4 mm to the nominal diameter of the hole in CAD. An M3 screw (3.0 mm) needs a clearance hole of about 3.2-3.4 mm on FDM printers. For self-tapping into plastic, drill or design the pilot hole at 2.5-2.7 mm.
Why are my 3D printed holes too small?
Holes print small because extruded plastic bulges inward and slicers approximate circles with straight segments. Compensate by adding 0.1-0.3 mm to hole diameters, increasing your model’s polygon count, and calibrating your flow rate.
How accurate is FDM 3D printing?
A well-tuned FDM printer holds ±0.1-0.2 mm on small parts and ±0.5% on larger dimensions. Entry-level or poorly calibrated machines can drift to ±0.5 mm or more. SLA printers typically reach ±0.05-0.1 mm.
