How to Check If a Shelf Is Level Before Mounting

Published 14 April 2026

A shelf that looks straight when you're standing next to it often reveals itself as crooked the moment you load it with books or plants. Getting the level right before drilling saves you from filling holes, repainting, and redoing the whole job from scratch.

Tools and fixings needed to mount a shelf

A pencil, a tape measure, a drill or screwdriver, wall plugs and screws (or appropriate fixings for your wall type), and your phone with Spirit Level Online open in the browser.

Step-by-step: checking a shelf before mounting

Hold the shelf bracket in position. Press one bracket against the wall at the height you want and mark the screw holes lightly with a pencil. Do not drill yet.
Mark the second bracket position. Measure horizontally from the first bracket to where the second bracket needs to go. Mark its screw holes at the same height. Using a tape measure rather than eyeballing this is essential — even a 5 mm difference will show once the shelf is loaded.
Check both marks are at the same height. Measure the vertical distance from the floor (or ceiling) to each pencil mark. They should match. If they don't, adjust the lower mark upward before drilling.
Open Surface Mode on Spirit Level Online. Before fixing the brackets, lay your phone flat on the shelf itself (or on a long straight board placed across both brackets) and open Spirit Level Online in Surface Mode. The circular bubble should sit at the centre. If it drifts left or right, your bracket marks are not at the same height.
Drill, fix the brackets, and mount the shelf. Once the bubble centres, drill your holes, insert wall plugs, screw in the brackets, and place the shelf.
Final check with the shelf in place. Lay your phone flat on the mounted shelf. The bubble should sit at centre with a reading close to 0.0°. A small tolerance of ±0.5° is acceptable for most shelves — anything beyond 1° will be visible to the eye.

Tip: Uneven floors. If your floor is not perfectly flat, measuring up from it to set bracket height can introduce error. Measure down from the ceiling instead, or use the spirit level on a long straight board laid horizontally across both bracket positions to verify they are at the same height before drilling.

Floating shelves without visible brackets

Floating shelves use concealed fixings — typically metal rods that insert into holes drilled in the wall. The process is the same: mark both fixing positions at equal heights, verify with the spirit level before drilling, then drill and insert the shelf. Because you can't adjust the shelf once the rods are fixed, getting the level right before drilling is especially important.

What if the wall itself isn't straight?

Older walls often have slight bows or leans. If you're installing a long shelf, check the spirit level reading at the centre of the shelf as well as the ends — a bow in the wall can make the brackets level but the shelf still dip in the middle. In this case, shimming the centre bracket slightly forward corrects the bow.

For shelves, TV mounts, picture frames, and any other wall-mounted item, Spirit Level Online works directly in your browser — no app download, no sign-up, free on any smartphone.