FDM tolerances and fits explained

Anyone who has moved from machining to FDM 3D printing has run into the same question: what tolerance can this process actually hold. The honest answer is less precise than CNC milling and more precise than most people assume, but only if you design around how the process really behaves instead of copying tolerance tables from another manufacturing method.

This guide sets the realistic baseline for FDM accuracy, explains why standard ISO fit classes were never written with layer-based manufacturing in mind, and shows how we approach tolerancing on incoming drawings before a part goes into production.

What accuracy is realistic on FDM parts

As a rule of thumb, we hold roughly plus or minus 0.2 mm on PLA, PETG and carbon-filled grades, and roughly plus or minus 0.3 mm on ABS, ASA and PC. These figures apply to typical dimensions on parts up to around 100 mm. On larger dimensions, especially anything approaching our build envelope of about 250 x 250 x 250 mm, the achievable tolerance widens because thermal shrinkage and minor warping scale with part size, not just with the process itself.

These are not marketing numbers, they are what we plan around when we quote a part and what we check on first article samples. If your application needs tighter local tolerances, for example a bearing seat or a press fit, we discuss which dimensions need to be called out specifically and whether a light secondary operation such as reaming makes more sense than chasing the number in the raw print.

Why ISO fit classes do not transfer directly

ISO 286 fit classes such as H7/g6 were developed for turned and milled metal parts with tolerances in the hundredths of a millimetre. FDM parts are built up in layers roughly 0.1 to 0.3 mm thick, extruded through a nozzle that itself has a finite width of 0.4 or 0.6 mm. The achievable dimensional control is an order of magnitude coarser than what those fit classes assume, so specifying H7 on a printed bore is meaningless. It will either be unmeasurable as toleranced, or it will force us into manual rework on every single part, which defeats the economics of 3D printing.

There is a second reason ISO classes do not translate well: they assume isotropic material behaviour and a single-step subtractive process. FDM parts shrink slightly as they cool, the amount depends on material and geometry, and the surface finish on vertical walls differs from the finish on top and bottom faces. A fit that behaves consistently in one orientation can behave differently if the part is reoriented, something that never happens with a milled bore.

Specify only the dimensions that matter

The most effective approach we see on well-designed drawings is selective tolerancing: leave general dimensions at a standard, achievable band and call out tight tolerances only on the handful of features that actually mate with something else, such as a bearing bore, a shaft diameter, a locating pin, or a mounting hole pattern that must match an existing bracket.

• Use a general tolerance note (for example ISO 2768-m adapted for FDM, or our standard band) for non-functional dimensions
• Call out tight tolerances only on true mating features with an explicit numeric value and, if relevant, which surface is the datum
• State which dimension is more important when two dimensions conflict, for example bore diameter over wall thickness
• Flag features that need a secondary operation such as drilling or reaming rather than asking the raw print to hit an unrealistic number

Recommended nominal clearance by fit type

For most mating parts printed on our machines, we design to a nominal clearance rather than trying to hit a nominal-plus-tolerance ISO fit. The table below is our starting point for quoting and design review, adjusted afterwards for material and part size.

How we handle tolerances during quoting

When a drawing arrives with tight ISO fit callouts, we do not simply reject it or silently ignore the note. We check which dimensions are actually functional, propose a clearance-based alternative where the fit allows it, and flag anything that genuinely needs machining after printing. This is part of the manufacturing assessment we do on every incoming file, not an afterthought once printing has started.