Start at the edge, not the brand name
An arm's fit is settled in two places, and buyers tend to check only one. The monitor side is the famous half, the VESA holes and the weight rating, which our guide to whether an arm fits your monitor walks through. The desk side is the half that turns up in the box and then refuses to go on. It comes down to something you can settle with a tape measure before you spend anything: can the arm actually take hold of your desk. No brand name rescues a desk it cannot grip, and an inexpensive arm sits happily on a good edge, so the first question to answer is about your desk rather than the arm.
The edge has to be flat, square and free
A clamp works by squeezing the top between a plate that rests on the surface and a pad that winds up to meet it from below, so it needs a stretch of edge it can close around. Three things get in the way. A rounded or heavily bevelled front gives the upper plate nothing level to press against. A desk pushed hard back to a wall hides the very edge the clamp wants, because the arm normally mounts at the rear. And a drawer, a deep apron rail or a strengthening batten running beneath the lip can leave no room for the lower jaw to swing in, even when the surface above looks perfect. So before any measuring, reach under the back of the desk where the arm will go and feel for a clear, solid, level strip two or three centimetres deep. Find that and a clamp is probably your mount.
Measure the thickness, then read the arm's own range
A clamp only opens so far, and every arm prints the span it covers. Measure your top's thickness at the exact place the arm will sit, and hold that figure against the arm's stated range instead of trusting it to cope. Ergotron, to take one published example, lists its LX desk mount clamp for tops up to 2.4 inches (60 mm); other arms stop short of that or reach well past it, so the only number that counts is the one on the arm you mean to buy. Aim to land inside the range with a little to spare rather than right at the ceiling of it, the same instinct you use on the weight rating. A chunky worktop thicker than the jaw will open, or a wafer-thin floating shelf with next to nothing to pinch, are the two ends that send people to the other mount.
When the edge says no: grommet, glass and a way round
If the edge will not take a clamp, the usual route is a grommet mount, which most of the better arms pack in alongside the clamp. Rather than pinch the rim, it goes through the top: you drill a hole, or use an existing cable port, drop the arm's post down it, and lock it with a plate from underneath. Ergotron's LX, staying with the same example, asks for a hole 8 to 51 mm across, roughly a third of an inch up to two, and a top no thicker than 57 mm for that method. It answers a desk that is too thick, has no reachable rim, or stands flush to a wall, as long as you can drill it and get a hand beneath to tighten the plate. Glass is its own problem: a bare metal clamp bears down on a small patch, which a glass top takes far less kindly than wood, so a glass desk wants an arm sold with a glass-safe clamp or cushioned pad, and you confirm the listing says so before ordering. And if the desk can accept neither a clamp nor a drill, you have not run out of ways to raise the screen, only moved to a different one. A monitor riser or a weighted free-standing stand lifts it without touching the desk at all. Once you know the mount your desk will take, our best monitor arm guide rounds up three that publish both their clamp and grommet fittings.
FAQ
Q: How thick a desk can a monitor arm clamp onto? A: There is no one answer, because the clamp's opening is a per-model figure, not an industry standard. Read the range printed on the arm you are weighing up and measure your desktop where the arm will actually grip, since a top can be thicker at a front rail than across the main span. Then aim to sit inside that range with room to spare rather than right at its limit.
Q: Can I put a monitor arm on a glass desk? A: Only with an arm meant for one. A standard metal clamp concentrates its force on a small area, which a glass top can shrug off until the day it does not, so makers sell glass-specific clamps and padded plates for the job. Look for an arm whose listing names glass support, or use a grommet mount if the glass already carries a hole; do not force a plain clamp onto a glass edge and hope it holds.
Q: What if my desk stands against a wall, or has a drawer under the front edge? A: Both take away the clear rim a clamp needs, so the grommet route is usually the fix: a hole through the top and a plate below reach a spot the clamp cannot. If drilling is off the table, say in a rented flat or on a desk you would rather not cut, that points you away from an arm altogether. Our riser or monitor arm comparison weighs up the no-mount route that leaves the desk untouched.