Choose magnets that fit the model and suit its intended mounting surface. Diameter and thickness alone do not determine how much weight a finished magnet can hold.
Measure first, then add clearance
Physical magnet dimensions describe the purchased item. Cavity dimensions describe the space printed around it. Total diameter clearance is added once across the diameter. Depth clearance is extra vertical space, with any layer-alignment adjustment shown separately.
Plastic changes the magnetic gap
The bottom skin separates the magnet from the mounting surface. Extra cavity depth can allow movement inside the print and increase the gap further. Greater separation reduces attraction. A vertical magnet also depends on friction to resist sliding, so a manufacturer’s direct pull rating is not a vertical load rating.
Read K&J Magnetics’ explanation of pull force, friction, and separation.
What automatic placement means
Without a measured holding load, the automatic count is a geometry-based suggestion that responds to artwork area, cavity size, and separate pieces. It is not a claim of sufficient holding strength. Each cavity must fit inside existing material, avoid holes, and keep the selected surrounding and inter-cavity wall.
Use a matching load test
Test a magnet vertically on the actual mounting surface through the same plastic separation, including possible cavity play. Enter the measured load per magnet and a margin multiplier. The tool estimates solid-plastic and magnet mass using the displayed density assumptions. It can suggest more magnets or warn if the resulting allowance is insufficient.
The estimate assumes the magnets share the load. Shape, leverage, uneven contact, printing variation, and installation can change the result. Verify the complete finished print rather than using this tool to rate load-bearing fixtures.
Buy for consistency
Look for clear dimensions, magnet material and grade, magnetization direction, tolerances, and temperature information. Check polarity before insertion. For printers, prioritize the filament capability and pause workflow you can verify on the actual machine.