Lego dimensions
Horizontal dimensions
The Lego system is designed around a square 8mm grid. Simple bricks are nominally multiples of 8mm per side, but with the sides then "pulled back in" by a tenth of a millimetre to allow a tiny amount of clearance between bricks, and to allow room for the bricks to flex. When one Lego brick is clipped to another, its walls flex out fractionally to accommodate the stud, providing a springy tension, and giving more grip (referred to by Lego Group as the brick's ability to "clutch"), and there needs to be a tiny space between adjacent bricks to allow for this flexing.
Vertical dimensions
Oddly, the vertical size of the bricks is NOT based on an 8mm grid: the standard single-stud brick is not a cube.
The height of a single plate (not including the stud) is 3.2 mm, and the height of a standard brick (not including the stud) is exactly three plates, or 3 × 3.2mm = 9.6mm.
Stud height is half a plate's body height, or 1.6mm.
Stud width
There has been some disagreement about the standard width of a Lego stud.
- One line of thought says that the stud width looks like 5mm, giving a simple, easily-memorable definition of "5mm studs on a 8mm grid".
- Others have pointed out that the other main dimensions of Lego bricks and plates are multiples of 1.2mm, and have argued that the nominal width of a stud therefore ideally ought to be 4×1.2mm = 4.8mm
Which is correct?
After attacking a pile of miscellaneous Lego bricks with a digital micrometer, the real-world stud size seems to be somewhere between the two: ~4.9mm. Almost every brick stud we measured, from 1960s sets to modern sets, was reported by the micrometer as being 4.88mm or 4.89mm (with an occasional 4.87 and a very occasional 4.90).
The same micrometer readings gave a consistent ~15.9mm and ~31.9mm for the widths of standard bricks (16mm and 32mm, minus a tenth of a millimetre for clearance). So the official sizes are correct for the rest of the brick, but the stud size appears to be deliberately oversized "pragmatically" away from the official dimensions in order to force the mating brick's walls to flex to accommodate the stud, which then provides grip. If one was making Lego-compatible bricks in a more rigid metal or ceramic, and assuming that the wall's flexure would be minimal, one would probably have to resize the studs back down to something closer to the official 4.8mm
- Patent number WO2019106129 (A1) ― 2019-06-06 "TOY BUILDING BRICKS MADE OF BIOPOLYMERIC MATERIAL", by LEGO AS, re-patents the idea of the Lego brick, but specifically made out of more environmentally friendly, renewable materials. This 2019 patent gives the nominal dimensions of a standard Lego brick, with the studs quoted as having a width of 4.8mm.
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