You designed a nice magnetic lid, dropped in the magnets, and now they either rattle around loose or refuse to seat flush. Or worse: two lids that were supposed to snap together shove each other apart, because you sealed a magnet in backwards and there is no getting it out now. Magnets are the easiest way to make a print feel finished, and also one of the easiest places to waste a whole print on a pocket that is 0.3 mm off.
The fix is almost never the magnet. It is the pocket. Get the diameter, the depth, and the polarity right and a press-fit magnet stays put with no glue, sits perfectly flush, and pulls with a satisfying click. This guide covers the sizes that actually hold, how to embed a magnet flush or fully hidden, how to never get polarity wrong again, and how to turn all of it into a part with a slider you can nudge after the first test fit.
What size magnet should you use for a 3D print?
For most maker projects, a 6 mm diameter by 3 mm thick neodymium disc (a "6x3") is the default, and it is the one worth standardizing on. It is strong enough for lids, panels, tool holders, and wall mounts, small enough to hide in a 3 to 4 mm wall, and it is the size half the functional prints on Printables and MakerWorld are built around, including most Gridfinity bins. Buy a bag of 6x3s once and you will reach for them for years.
Go smaller (6x2, or 5x1 for something delicate) when you need a lighter hold or a thinner wall. Go bigger (8x3, 10x3) when you are holding real weight, like a heavy lid or a wall-mounted tool. But unless you have a reason, design around 6x3 and keep a single size in your parts bin. It makes every pocket you draw reusable.
One thing to know before you design anything: neodymium magnets lose strength when they get hot. Standard N-grade discs are rated to roughly 80 degrees C. That matters if you print ABS or ASA in a hot enclosure right on top of one, but for the pause-and-embed method below (where the magnet only ever meets a thin, already-cooling layer) it is a non-issue.
How big should a magnet pocket be for a press fit?
Model the pocket diameter at the magnet's nominal size or a hair over: 6.0 to 6.1 mm for a 6 mm magnet. Because FDM printers render inside holes slightly undersized, a pocket you drew at 6.0 mm usually comes out around 5.8 to 5.9 mm, which is already a firm press fit that grips a 6 mm magnet without glue. If your printer runs tight and the magnet won't go in, bump the model to 6.15 mm rather than forcing it.
Depth is the other half. For a flush magnet, make the pocket exactly as deep as the magnet is thick: 3.0 mm for a 6x3. If you want the magnet to sit a touch below the surface, add 0.1 to 0.2 mm. Leave at least 1.5 to 2 mm of wall (three or four perimeters) around and behind the pocket so the magnet can't blow out the side and doesn't telegraph a bump through a thin outer skin.
Here is the whole thing as a starting-point cheat sheet for a 6x3 disc on a well-tuned FDM printer:
| What you want | Model the pocket at | Why | |---|---|---| | Press fit, no glue | 6.0 to 6.1 mm diameter, 3.0 mm deep | Hole shrinkage makes this grip on its own | | Glue-in (loose drop) | 6.2 to 6.4 mm diameter, 3.1 mm deep | Room for the magnet plus a thin glue layer | | Flush surface | Depth = magnet thickness (3.0 mm) | Magnet face sits level with the wall | | Fully hidden | 3.0 mm deep, then bridge a cap over it | Sealed inside, nothing visible | | Minimum surrounding wall | 1.5 to 2 mm all around | Stops blowout and surface bumps |
These are starting numbers, not guarantees, because the exact fit depends on your specific printer, filament, and how well your flow is dialed in. The five-minute move is to print a small test coupon with two or three pockets at 6.0, 6.1, and 6.2 mm, try your actual magnets in each, and use whichever grips right. Our guide to 3D printing tolerances explains why holes come out undersized and how to run that test fit, and the calipers guide covers measuring the magnet itself, since real discs are often 5.95 or 6.05 mm, not a clean 6.00.
Should you press-fit magnets or glue them in?
Press fit whenever you can. A correctly sized press-fit pocket holds the magnet with friction alone, which means no glue mess, no cure time, and no risk of glue wicking onto the magnet face and killing the hold. Glue only earns its place when your pocket printed too loose, when the part will see vibration or drops, or when you deliberately sized the pocket oversized for easy assembly.
If you do glue, a small dab of superglue (cyanoacrylate) or a smear of two-part epoxy on the pocket wall is plenty. Keep it off the magnet's mating face, and do not flood the pocket, because excess glue pushes the magnet proud and ruins the flush fit you were after.
| | Press fit | Glue-in | |---|---|---| | Pocket size | Nominal (6.0 to 6.1 mm) | Oversized (6.2 to 6.4 mm) | | Hold | Friction, immediate | Adhesive, needs cure time | | Mess and cleanup | None | Possible squeeze-out | | Best for | Most parts, clean builds | Loose pockets, high vibration, drops | | Risk | Too tight to seat | Glue on the face weakens pull |
How do you hide magnets inside a 3D print?
To fully seal a magnet inside a part with a smooth outer surface, use the pause-and-embed method: design the pocket to open toward the print bed side that finishes last, print until the pocket is at full depth, pause the print, drop the magnet in, and let the printer bridge a thin cap right over it. When it is done, the magnet is invisible and permanently captured, with no glue and no visible seam.
The geometry that makes this clean: put the pocket depth on a layer boundary so the pause lands exactly at the pocket's top. A 3 mm magnet at a 0.15 mm layer height is 20 layers, or 15 layers at 0.2 mm, so a 0.15 to 0.2 mm layer height gives you a clean landing and a nicer cap. Then let the printer bridge across the 6 mm opening (a trivial span for any tuned machine) with two or three solid layers on top. Most slicers can insert the pause automatically at a chosen height, or you can add an M600 or pause command by hand.
This is the same idea as an embedded nut or a captured bearing, and it is a close cousin of using heat-set inserts to put real threads in a print. In every case you are designing the part around a piece of hardware and giving the printer a clean way to capture it.
How do you get the magnet polarity right?
Before you place a single magnet, mark its poles and decide the orientation for the whole part, because two mating halves only attract if their facing poles are opposite. The reliable habit: take one magnet as your reference, mark the face that should point "out" on every pocket with a marker, and check each new magnet against the reference before it goes in. Magnets that repel your reference get flipped.
Polarity mistakes are cheap to fix with a press fit (pull it out and turn it around) and permanent with the pause-and-embed method (once it is sealed under a cap, it is staying wrong). So if you are embedding magnets under a bridge, do a dry run first: hold your loose magnets in the intended orientation against the matching part and confirm they pull together, not apart, before you commit any of them to a sealed pocket. Two minutes of checking beats a four-hour reprint.
How do you turn this into a part with an adjustable pocket?
This is where a magnet project either becomes a reusable design or a one-off you dread editing. The trick is to make the pocket a real parameter, not a number baked into a mesh you can't touch.
In Meshra, you describe the part in plain English and put the magnet dimensions right in the sentence, for example "a 60 mm square coaster holder with four 6.1 mm diameter by 3 mm deep magnet pockets, one in each corner, 8 mm in from the edges, 3 mm walls." Meshra writes real parametric CAD from that description and gives you a slider for every dimension you named, including the pocket diameter and depth. When your test print comes back and the magnet is a whisker too tight, you drag the pocket-diameter slider from 6.1 to 6.2 mm and the part regenerates instantly. That re-run is deterministic and free on every plan: it is the same program executed again with one number changed, not a new AI generation, so the pockets update while everything else stays exactly where you put it. Then export STL or 3MF for the slicer, or STEP if you want to keep editing in other CAD, all from the same part.
The payoff is that "the magnet fit" stops being a property of a frozen file and becomes a slider. You dial it in once on a test coupon, feed that exact number into the real part, and never redraw a pocket by hand again.
Frequently asked questions
What is the most common magnet size for 3D printing? A 6 mm diameter by 3 mm thick neodymium disc (a "6x3"). It is strong enough for lids, panels, and tool holders, thin enough to hide in a normal wall, and it is the size most functional prints and Gridfinity bins are built around, so a single bag covers most projects.
How much clearance should a magnet pocket have? For a press fit, model the pocket at the magnet's nominal diameter or up to 0.1 mm over (6.0 to 6.1 mm for a 6 mm magnet), because FDM holes print undersized and grip on their own. For a glue-in fit, go 0.2 to 0.4 mm oversized so the magnet drops in with room for adhesive.
Do I need glue for press-fit magnets? No, if the pocket is sized right. A correctly sized press-fit pocket holds a magnet by friction alone. Reach for glue only when a pocket printed too loose, or when the part will take real vibration or drops.
How do I embed a magnet so it is completely hidden? Print until the pocket is at full depth, pause the print, drop the magnet in, and let the printer bridge a thin solid cap over the 6 mm opening. Land the pocket top on a layer boundary (a 0.15 to 0.2 mm layer height helps) so the cap starts cleanly.
Why do my two magnetic halves push apart instead of holding? Reversed polarity. The facing poles have to be opposite to attract. Mark one magnet as a reference, check every other magnet against it before inserting, and do a dry run before sealing any magnet permanently.
Where to start
Pick one magnet size and stick with it: a bag of 6x3 discs will carry most of what you build. Print a quick three-pocket test coupon at 6.0, 6.1, and 6.2 mm, find the one that grips your magnets right, and write that number down. Then, in the builder, describe your part with that pocket diameter and depth in the sentence, and you get a parametric part with a slider for the pocket so the fit is something you tune, not something you guess. See pricing for the plans, or browse the template gallery for parts you can derive and edit right now.
Magnets make a print feel done. Size the pocket for a press fit, mark your polarity before you commit, and let a slider handle the last tenth of a millimeter instead of a reprint.
