How Accurate Is 3D Printing? Tolerances You Can Expect

Will a 3D printed lid fit the jar? Will the bracket line up with the holes on the wall? Accuracy is the question behind almost every functional order, and the answer is more nuanced than a single number. This article gives you realistic figures for FDM and resin printing, explains where the error comes from, and shows how to tell Fantasy Lab 3D what matters so your part fits the first time.

Accuracy, tolerance and resolution are three different things

People often use these words interchangeably, so it helps to separate them.

• Resolution is the smallest feature a printer can produce: the layer height and the nozzle or pixel size. A 0.4 mm nozzle cannot print a 0.2 mm wall.
• Accuracy is how close the printed dimension is to the digital one. A 50 mm cube that comes out 50.15 mm is accurate to 0.15 mm.
• Tolerance is the amount of error you can accept. A decorative vase has a tolerance of "nobody will ever measure it". A peg that must slide into a hole might have a tolerance of 0.1 mm.

A printer can have fine resolution and mediocre accuracy at the same time, which is why the layer height in our layer height guide tells you about surface smoothness, not about whether the part will fit.

Realistic numbers

Process	Typical accuracy on a 100 mm part	Smallest reliable hole	Smallest reliable wall
FDM, 0.4 mm nozzle	Around ±0.2 to ±0.5 mm	About 2 mm	0.8 mm (two perimeters)
FDM, tuned and calibrated for the material	Around ±0.1 to ±0.2 mm	About 1.5 mm	0.8 mm
Resin (SLA/MSLA)	Around ±0.05 to ±0.15 mm	About 0.5 mm	0.3 to 0.5 mm

These are typical figures for well-maintained desktop machines, not marketing claims. The error is not uniform: it is usually larger in the horizontal plane than in height, larger on big parts than on small ones, and larger on the first layer than anywhere else. Resin is the more precise process, which is one reason it is chosen for miniatures and jewellery masters, as covered in FDM vs resin printing.

Where the error comes from

• Shrinkage. Plastic contracts as it cools. PLA shrinks little, ABS and nylon more. A studio compensates by scaling the model slightly, but no compensation is perfect.
• Holes print small. Molten plastic laid in a circle is pulled inward by surface tension, so a 10 mm hole often comes out at 9.7 to 9.8 mm. Experienced modellers design holes oversize or ask for them to be drilled or reamed after printing.
• Elephant foot. The first layer is squished onto the bed for adhesion and bulges outward by a tenth of a millimetre or two. A chamfer on the bottom edge hides it.
• Overhangs and bridges. Plastic printed in mid-air sags, so the underside of an arch or a horizontal hole is the least accurate surface on any print.
• Moisture and temperature. Wet filament and a cold room both change how the plastic flows.

How to specify a part so it fits

The single most useful thing you can do is tell us what the part touches. "A cap for a 42 mm tube" tells us far more than a bare STL file, because it tells us which dimension matters and which direction the error may go. From there:

1. Say which fit you need. Loose (a lid you lift off), sliding (a drawer), snug (a press-fit peg) or threaded. Each needs a different clearance, usually between 0.1 and 0.4 mm per side on FDM.
2. Send the real measurements, not the catalogue ones. Tubes, bottles and pipes vary. How to measure for a custom part walks through the caliper routine.
3. Let us test-fit. For anything that mates with another object, we print a small test ring or a slice of the part first, check it against your measurement, and adjust before running the full print.
4. Accept post-machining for critical holes. A 3D printed hole that is then reamed with a drill bit of the right size is round and accurate to a few hundredths of a millimetre. Printing it "close" and finishing it is standard practice.

What accuracy is enough for what

• Decorative objects, figurines, vases, book nooks: any FDM setting is fine. Nobody measures a dragon.
• Enclosures, phone stands, organisers: standard FDM with a small clearance designed in.
• Replacement parts, gears, snap-fit lids: calibrated FDM with a test-fit, or resin for small parts. Our post on 3D printing replacement parts shows typical examples.
• Jewellery masters, dental models, tiny mechanisms: resin.
• Bearing seats, precision shafts, anything below ±0.05 mm: 3D printing is the wrong tool; machining is the right one, and we will tell you so.

Fantasy Lab 3D measures functional parts with a digital caliper before shipping and includes the measured figures if you ask. Accuracy is a conversation, not a specification sheet, and the conversation costs nothing.

Ready to order?

Send Fantasy Lab 3D your model or your measurements, tell us what the part has to fit, and we will recommend the process, print a test piece where it makes sense and quote the job. Use the contact section or DM us on Instagram @fantasylab3d.almaty.