A standard LEGO brick stud has a nominal diameter of 4.8 millimetres.
That small cylindrical projection is central to the LEGO System’s famous clutch connection. LEGO’s standard grid uses a nominal spacing of about 8 millimetres from one stud centre to the next, while the brick itself is slightly smaller than a full mathematical grid square. The resulting geometry lets the underside tubes and walls grip studs securely without making pieces impossible to separate.
The 4.8 mm figure describes the stud’s nominal top diameter, not the width of a LEGO brick or the distance between studs. A one-stud-wide brick is roughly 8 mm across on the modular grid, and a standard brick is 9.6 mm high excluding its studs. Manufacturing tolerances and the exact shape of a moulded element mean caliper measurements can vary slightly.
Another common confusion is applying the measurement to every LEGO product. Duplo, Primo, Technic components and specialised elements use different dimensions, so 4.8 mm is the familiar standard System stud specification.