We've all experienced this. You spend hours designing two parts on your computer, making sure they have matching measurements. You send them to the 3D printer, wait excitedly, and when you finally try to put them together... they don't fit. The pin is too big, the hole is too small, and the lid won't go on properly. This is the most common problem in 3D printing useful objects, and it always leads to the same question: "How much space should I leave between parts?"
While there isn't one perfect number that works for every project, there is a step-by-step method. This guide will give you the basic rules, good starting points, and a repeatable way to find the perfect spacing for any project, on any printer, in 2025. We will move beyond guessing and give you the tools to get perfect fits every time.
Quick Answer: Starting Points
For those in a hurry, here are the general rules. Think of these as starting values for a well-set-up 3D printer. We will explain why these numbers change and how to improve them, but these values will get you close for your first designs.
| Fit Type | What It's Used For | Starting Gap |
|---|---|---|
| Loose Fit | Easy-to-remove lids, simple containers | 0.4mm – 0.7mm |
| Snug/Sliding Fit | Parts that slide together, smooth moving tracks | 0.2mm – 0.4mm |
| Press/Tight Fit | Securing pins, bearings, permanent assembly | 0.0mm to -0.2mm |
Important note: these are starting points only. The best spacing for your specific project depends on your printer, material, settings, and part shape. The rest of this guide will explain how to master these factors. A negative gap, as seen in a press fit, means the hole is designed to be smaller than the object it will hold, relying on the material bending to create a tight, secure connection.
Understanding Basic Ideas
To control spacing, we first need to define our terms clearly. These concepts are the foundation of designing parts that fit.
First, we must understand the difference between tolerance and clearance.
Tolerance is the acceptable range of size differences that naturally happen during 3D printing. It's a measure of your printer's accuracy. If you design a 20mm cube, your printer might make one that is 20.1mm or 19.9mm wide. That +/- 0.1mm difference is its working tolerance.
Clearance is the intentional space or gap you design between two parts that fit together. This is what you, the designer, actively control in your design software to get the result you want.
Fit is the final result when the parts come together, determined by the clearance you designed. A large clearance creates a loose fit, while a small or negative clearance creates a press fit.
So why don't parts print at their exact design measurements? The physical process of 3D printing creates differences. As plastic is heated and cools, it can shrink or bend. The very nature of drawing a shape with a round nozzle and stacking layers creates tiny errors. The digital model is perfect; the printed part is a physical copy. Our job is to account for these physical realities when we design.
Key Factors For Spacing
Your ideal clearance value is not fixed. It's a changing number influenced by several important factors. Understanding these factors is the key to predicting and controlling how your parts fit.
Printing Technology Matters
The technology you use is the biggest factor influencing required spacing.
FDM (Fused Deposition Modeling) printers generally need the largest spacing, typically in the 0.2mm to 0.5mm range for a snug fit. This is because of the relatively large nozzle size, heating and cooling of the plastic, and small inconsistencies in how the plastic comes out.
SLA (Stereolithography), DLP, and MSLA printers offer much higher accuracy. Because they use light to harden resin layer by layer, features are smaller and more precise. Spacing for these technologies is much tighter, often falling within the 0.05mm to 0.2mm range.
SLS (Selective Laser Sintering) also provides high accuracy, often matching SLA. Parts are made from a powder bed, which supports the model and reduces warping from heat. Typical spacing is in the 0.1mm to 0.3mm range, though the slightly rough surface finish must be considered for sliding parts.
Printer Setup is Key
A poorly set up printer is the number one enemy of tight spacing. An uncalibrated machine is an unpredictable one. Before you even think about adjusting clearances, you must ensure your printer is mechanically and electronically sound.
Key setup steps include checking E-steps (to ensure the extruder pushes the correct amount of plastic), tuning the flow rate (to compensate for different plastic properties), ensuring proper belt tightness (to prevent backlash and vibrations), and achieving a perfectly level bed. A well-tuned machine produces predictable, repeatable results, making your spacing adjustments meaningful.
Impact of Material Properties
Different materials behave differently, directly affecting the final size of your print.
Shrinkage is a major factor. Materials like ABS and Nylon shrink much more as they cool compared to PLA or PETG. A part printed in ABS will be smaller than the same part printed in PLA, requiring you to adjust your clearance values accordingly.
Plastic quality also plays a huge role. Inconsistent plastic diameter is a hidden enemy of accuracy. If your plastic varies from 1.70mm to 1.80mm, your printer's extruder cannot deliver a consistent amount of material, and your part sizes will change unpredictably.
Finally, even different colors of the same material type from the same brand can behave differently. The additives used to create colors can change the material's melting point, flow characteristics, and shrinkage rate. A part that fits perfectly in black PLA might be too tight in white PLA.
Important Slicer Settings
Your slicer software is where you translate the digital model into machine instructions, and several settings directly impact spacing.
Layer height affects vertical accuracy. While thinner layers are often associated with better detail, they can also improve the precision of curved or sloped surfaces along the Z-axis.
Print speed can trade speed for accuracy. Printing too fast can introduce vibrations and corner bulging, all of which hurt accuracy. Slower, more controlled movements generally produce parts that are closer to their intended sizes.
Flow rate, or the extrusion multiplier, is a critical setting. Over-extrusion makes every feature of your part slightly larger, which will cause both external dimensions and internal holes to be too small. Under-extrusion has the opposite effect. This setting must be calibrated for each material.
Horizontal Hole Expansion is a specific slicer feature designed to fix a common problem: small holes printing undersized. This happens because the slicer approximates a curve with a series of straight line segments, and the plastic path tends to pull inward, shrinking the hole's diameter. This setting allows you to compensate by a specific amount, enlarging all holes in the X/Y plane.
Part Shape and Orientation
How you design and orient your part on the build plate significantly impacts its final accuracy.
3D prints are anisotropic, meaning they have different properties in different directions. Parts are almost always most accurate in the X/Y plane. The Z-axis is subject to layer-by-layer variations and can be less precise.
Print orientation is critical for features like holes. A hole oriented vertically (its circle is on the X/Y plane) will be much rounder and more accurate than a hole oriented horizontally (its circle is on the X/Z or Y/Z plane). Horizontally printed holes are built from a series of stacked bridges and overhangs, resulting in a slightly squashed or polygonal shape.
A Practical Method
Now let's move from theory to a repeatable, practical workflow for finding and applying the perfect spacing for your projects.
Step 1: Calibrate and Test
First, ensure your machine is calibrated as discussed previously. Once your printer is in a known, reliable state, your next step is to test its performance with your chosen material and settings. The best way to do this is by printing a dedicated tolerance test.
A tolerance test is a small, quick-to-print model designed specifically to measure clearance. A common design features a series of pegs and holes (or slots) with progressively smaller gaps. For example, a test might have a 10mm peg and a series of holes measuring 10.7mm, 10.6mm, 10.5mm, and so on.
You can find many excellent tolerance test models on repositories like Printables. A simple and effective example is this type of clearance gauge: https://www.printables.com/model/17395-clearance-gauge.
Print this test using your standard, go-to settings for the material you want to test. Once it's finished, try to fit the peg into each slot. Find the tightest-fitting slot that still provides the type of fit you want (e.g., slides smoothly, or can be removed with a little force). The clearance value marked on that slot is your baseline clearance for that material, on that printer, with those specific slicer settings. For example, if the peg fits snugly into the 0.2mm slot, you now know that a 0.2mm clearance is a great starting point for snug-fit parts.
Step 2: Choose Your Fit
With your baseline clearance established, you can now apply it intelligently based on the desired fit. Look back at the fit types from the beginning:
For a Loose Fit, like a simple lid for a box that you want to remove easily, you would start with a larger clearance. If your test showed a snug fit at 0.2mm, you might design the lid with a 0.5mm clearance to ensure it goes on and comes off with no resistance.
For a Snug Fit, like two parts of a case that need to slide together smoothly without wobbling, you would use the baseline value you discovered. If your test part slid well at 0.3mm, use a 0.3mm clearance in your design.
For a Press Fit, where you want to securely mount a bearing or a pin, you need to reduce the clearance significantly. If your snug fit was 0.2mm, you might try a design with 0.0mm clearance (exact dimensions) or even a negative clearance like -0.05mm. This forces the material of the hole to bend slightly, gripping the object tightly.
Step 3: Design for Tolerance
This is where advanced users separate themselves. Instead of just adjusting numbers, you can design features that make your parts more forgiving of spacing variations. This is designing for the process.
Use Chamfers and Fillets. A small 45-degree chamfer on the leading edge of a pin and the opening of a hole acts as a guide. It helps the two parts align easily and can gracefully compensate for any "elephant's foot" or first-layer squish that makes the bottom of your print slightly wider. A tiny chamfer can be the difference between a part that fits and one that doesn't.
Design Flexible Features. For press-fits and snap-fits, avoid designing a solid peg to fit into a solid hole. This is an unforgiving design. Instead, introduce flexibility. For a snap-fit, design a flexible clip. For a press-fit bearing housing, design a small slit in the wall of the housing. This allows the housing to flex open slightly as the bearing is pressed in, creating a secure fit without requiring superhuman precision or a hammer.
Use Print Orientation Wisely. As mentioned, always orient critical circular holes vertically so their profile is on the X/Y build plane. This will produce the roundest, most accurate hole your printer is capable of. If you need a precise hole on the side of a part, consider printing it in two pieces and assembling, or designing the hole with a teardrop shape to support the top surface.
Follow the "One-Sided" Tolerance Rule. When designing a peg and a hole, it is best practice to apply the entire clearance value to only one of the two parts. For example, if you need a 0.3mm clearance for an M5 bolt (which has a 5mm diameter), keep the bolt model at exactly 5.0mm and design the hole to be 5.3mm. This makes troubleshooting and design changes much simpler. If the fit is too loose, you only need to adjust the hole, not both parts.
Fixing Common Problems
Even with a good process, you may run into issues. Here are some common problems and their solutions.
Problem: "My parts are always too tight and won't fit."
Solutions: This is most often caused by over-extrusion. Your first step should be to calibrate your flow rate (extrusion multiplier). You may also have significant "elephant's foot" on your first layer; using a raft or adding a generous chamfer to your design can solve this. If calibration is correct, simply increase the clearance value in your design software.
Problem: "My parts are too loose and wobbly."
Solutions: This suggests under-extrusion. Calibrate your flow rate to ensure you aren't printing with too little material. Also, check for mechanical issues like loose belts, which can cause size errors and make parts smaller than intended. If the printer is mechanically sound, decrease the clearance in your design software.
Problem: "My holes are always smaller than I designed them."
Solutions: This is a normal and expected phenomenon in FDM printing. The easiest fix is to use your slicer's "Horizontal Hole Expansion" setting. Start with a value like 0.1mm and print a small test. Alternatively, you can compensate for this directly in your design software. To get a hole that fits an M3 screw (3mm diameter), you might need to design the hole to be 3.2mm or 3.3mm. Your tolerance test print will give you a good idea of how much you need to oversize your holes.
Tolerance is a Process
Ultimately, there is no universal number for how much tolerance for 3d printing. It is a variable that depends on your specific machine, material, and settings. The key is to stop searching for a magic number and instead adopt a reliable process.
The process is: Calibrate your printer thoroughly. Test its performance with a tolerance gauge to find your baseline clearance. Design your parts intelligently, using features like chamfers and applying clearance based on the desired fit. Finally, iterate as needed.
By understanding the factors at play and using this systematic approach, you can move from frustration to confidence. You can stop guessing and start engineering parts that fit together perfectly, right off the printer.