A clamp grips the desk edge and needs no hole; a grommet bolts through a hole in the desktop and holds more rigidly. Most arms include both — the choice comes down to your desk’s edge, its thickness and whether there is already a hole.
A clamp closes on the edge from above and below. It installs and comes off in minutes, leaves no mark beyond the pad, and suits a rented or shared desk. It needs a free edge within its stated thickness range and nothing underneath — a frame rail or drawer an inch inside the edge leaves it nowhere to close. The HUANUO single arm in the monitor arms hub clamps desks 15 to 90 mm (0.59 to 3.54 in) thick.
A grommet passes a threaded rod through a hole in the desktop and tightens a plate underneath. It spreads the load over the plate rather than the edge, which is firmer for a heavy screen or an arm at full reach, and it works on a desk whose edge is too thin, too thick or obstructed. It needs a hole: an existing cable grommet is ideal; otherwise it means drilling.
Check both limits — they often differ. On the same arm, the grommet can take a narrower range than the clamp. The VIVO dual mount in the dual monitor mounts hub clamps tops up to 3.1 in but takes the grommet only to 1.9 in; the MOUNTUP needs a bolt hole between 0.39 and 2.36 in; the Acer asks for a hole over 10 mm. A desk that suits one fixing can fail the other.
Glass and soft tops. Several makers exclude glass and plastic desks outright, for both fixings — the HUANUO and Acer dual mounts among them. On a thin or soft top, a reinforcement plate under the clamp spreads the pressure; where the listing forbids the material, use a freestanding monitor stand instead.
What to check before buying a monitor arm lists the other measurements to take, and measuring desk thickness for a clamp explains how to measure the edge.
Last reviewed 13 October 2026