Desk
Monitor arms: the four numbers that decide whether it works
VESA pattern, load range, clamp jaw depth and usable reach determine whether a monitor arm holds position or drifts, sags and dents your desk.
A monitor arm either holds the screen where you left it or it becomes a daily annoyance that creeps downward every afternoon. The difference is rarely build quality. It comes down to four measurements printed on the box and routinely ignored.
The VESA pattern, and monitors that don’t have one
VESA’s mounting interface standard defines a bolt pattern on the back of the display. Desktop monitors almost always use MIS-D: 100 x 100 mm or 75 x 75 mm, both with M4 screws. Larger displays step up to 200 x 200 with M6 or M8 hardware.
Screw length causes more trouble than the pattern does. A curved or recessed back needs the mount held off the panel with spacers, and a screw that bottoms out before it clamps leaves the display hanging on thread tips. Screws are also often omitted, or supplied in one length.
Plenty of monitors, especially thin all-in-one styled ones, ship with no VESA holes. The fix is an adapter plate: a frame that grips the monitor’s top and bottom edges and presents a standard pattern behind it. They work, but they add depth and weight and push the centre of mass further from the arm, so budget for that against the load rating.
The load rating is a range, and the bottom of it is real
A sprung arm is not rated “up to 8 kg”. It is rated for something like 3 to 8 kg, and the minimum binds as hard as the maximum.
The mechanism is a counterbalance: a spring pushes up with a force the tension adjustment sets, the monitor pulls down, and when the two match the arm floats. Hang a 2 kg monitor on an arm whose spring cannot wind below the force needed for 3 kg and the arm wins: the screen drifts upward, or springs to the top the moment you let go. Tightening the adjustment will not help; you have run out of travel. Overloading fails the other way, with the screen sinking under its own weight.
Weigh what you are mounting, including any adapter plate and anything clipped on top. Makers publish monitor weight with and without the factory stand; you want the figure without. If your display falls below an arm’s minimum, the fixes are a lower-rated arm, a friction arm instead of a sprung one, or added counterweight.
Clamp jaw depth, desk thickness and the grommet alternative
C-clamps have a stated jaw capacity, commonly 10 to 60 mm. Measure at the edge where the clamp will sit rather than trusting the desk’s advertised thickness, because bullnose edging, a thicker front rail or an underside apron can put the real number outside the range. The clamp also needs access above and below, so a desk with a frame rail, cable tray or drawer along the back edge may have no legal clamping point.
Grommet mounting is the alternative. The post passes through a hole and clamps from underneath with a plate, spreading load into a horizontal plane instead of pinching an edge. It handles thick or awkwardly profiled tops, but needs an existing cutout of the right diameter or one you drill, which is permanent.
Stated reach and usable reach
Reach is quoted from the centre of the post to the VESA plate, arm straight out. You will not get that. Subtract the depth of the monitor and anything behind it, then the distance the post sits behind the desk edge. Then account for tilt: angling the screen swings its top or bottom through an arc, so the panel effectively moves toward you and eats into what is left.
Minimum reach matters too. Some arms fold to a large closed length, so a shallow desk cannot pull the screen close enough.
The failure modes
Sag is the common one. Gas struts lose pressure as seals age, and coil springs take a small permanent set. Both show as a screen sitting slightly lower each month. An arm whose tension adjustment still has travel can be rewound; a spent sealed strut cannot, which is why a serviceable tension screw matters.
Desk damage is the other. A clamp concentrates force onto a small footprint at the edge, exactly where particleboard, MDF and hollow-core tops are weakest. Hollow-core desks are two thin skins over a cardboard honeycomb, and a clamp tightened onto one can dish the surface or crush the void beneath, and laminate takes a permanent impression well before anything structural gives. A wide clamp footprint, a rubber pad and a load-spreading plate underneath all help; on a hollow-core top the grommet route is often the only sound answer.
What to look for
- The VESA pattern and screw size stated explicitly, and whether screws and spacers are included
- A load rating given as a range, with your monitor’s weight excluding its stand inside it
- A tension adjustment described as user-accessible, with the method stated
- Clamp jaw capacity in millimetres, checked against your measured desk edge, not its nominal thickness
- A grommet option in the box, with the required hole diameter given
- Stated reach alongside the arm’s folded minimum, not just the maximum
None of this requires judging a brand. It is printed numbers matched against measurements you can take with a tape and a kitchen scale. Arms that fail in service usually failed on paper first, where someone read the maximum load rating as the only limit.