Both solve the same core problem: your monitor is too low, so you spend the day looking down and your neck pays for it. But they solve it in very different ways, and picking the wrong one is a common and avoidable waste of money.
The short answer
Get a riser if you have one monitor, you never move it, and you want the cheapest fix that works.
Get an arm if you have two or more monitors, you share the desk with someone taller or shorter, or you want the desk space underneath back.
What a riser actually does
A riser is a platform. It lifts the screen a fixed amount and gives you a shelf underneath, which most people use for a keyboard or a laptop.
The advantages are real: it is cheap, there is nothing to install, and nothing can fail mechanically. The limitation is equally real: the height is fixed. If it lifts your monitor 10 cm and you needed 14, there is no adjustment. Some risers stack, but stacked risers wobble.
The other thing people underestimate is the footprint. A riser occupies the same desk area as the monitor stand it replaces, plus a bit more. If your desk is small, you have not gained space, you have just moved it upward.
What an arm actually does
An arm clamps to the back edge of the desk and holds the monitor out over the surface. The monitor stand comes off entirely.
That clamp is the whole point. It means the area under and behind the monitor becomes usable desk again, which on a small desk is the single biggest improvement you can make. It also means the monitor moves: height, depth, tilt, rotation. If two people use the same desk, or you alternate between sitting and standing, an arm is the only option that adapts.
What people get wrong about arms
Three things, consistently:
- Desk thickness. Most clamps handle desks between roughly 10 and 90 mm. Measure before you buy. A desk thicker than the clamp opening makes the arm useless.
- Weight range. Every gas-spring arm has a minimum and a maximum. Under the minimum the arm drifts upward on its own. Over the maximum it sags. A 27-inch monitor is typically 4 to 7 kg; check your specific model rather than guessing.
- VESA mounting. The arm attaches to a standardised bolt pattern on the back of the monitor, usually 75x75 or 100x100 mm. Most monitors have it. Some thin consumer models do not, and then you need an adapter bracket.
The case nobody mentions
If you have a curved ultrawide, an arm is not a luxury. Ultrawide stands are wide and deep, and they eat the middle of your desk. Getting that back changes how the whole surface works.
How to decide in one minute
Measure the gap between where the top of your screen is now and your eye level when sitting straight. If the gap is under 8 cm and you only have one monitor that never moves, a riser is fine and you should not spend more.
If the gap is bigger, you have two screens, or the desk is crowded, the arm is worth it, and you will notice the difference the same day.
Whatever you choose, the target is the same: the top of the screen roughly at eye level, and the screen about an arm's length away. Everything else is detail.