Checking a spirit level: how to test if it is still accurate

Short answer: lay the spirit level on a flat, stable surface and see where the bubble is. Mark the position, turn the level 180 degrees so left and right swap, place it in exactly the same spot and look again. If the bubble is in the same place both times, your level is accurate. If the bubble clearly differs, it no longer measures true.

👷 Calling in a professional is never unnecessary. For tiling in a bathroom, a kitchen that has to stand level or a frame that must fit exactly, a professional works with checked measuring tools and experience. A small measuring error quickly becomes a visible difference across a whole wall.

Why check your spirit level?

A spirit level looks indestructible, but a fall from a ladder, a knock in the van or use as a lever can shift the vial or the profile slightly. You cannot see that from the outside. A deviation of 1 mm per metre sounds small, but on a 3 metre wall a row of tiles is then already 3 mm off. So check your level:

• after a fall or hard knock;
• before a job where precision really matters, such as tiling or fitting a kitchen;
• when the result does not match what you expect, for example a shelf that looks crooked.

How to test the horizontal vial

1. Find a flat and stable surface, such as a workbench or table. It does not have to be exactly level.
2. Lay the level down and use a carpenter pencil to mark both ends, so you can place it in exactly the same spot later.
3. Read the bubble and note where it sits relative to the lines on the vial.
4. Turn the level 180 degrees so the left end is now on the right, and place it exactly between your marks.
5. Read again. If the bubble is in the same place, the vial is accurate. If it is on the other side, it deviates.

💡 If your surface is slightly sloped, that actually helps: the bubble is then not in the middle, and you can easily see whether it sits at the same distance from the centre after turning.

How to test the vertical vial

1. Hold the level vertically against a door post or wall and draw a pencil line along the side.
2. Read the vertical vial.
3. Turn the level around so the same side stays against the line but the front now faces backwards, and read again.
4. Compare. The same position means the vial is accurate.

The level is not accurate: what now?

On most spirit levels the vials are fixed and cannot be adjusted. Then you replace the level. Choose the length that suits your work:

• A 40 cm spirit level or 60 cm spirit level for shelves, cabinets and small jobs.
• An 80 cm spirit level or 100 cm V-series spirit level for doors, frames and tiling.
• A 120 cm spirit level or 200 cm spirit level to check a straight line over longer distances, for example on a floor or a long row of tiles.
• A 60 cm magnetic spirit level stays put on steel profiles and frames, leaving both hands free.

If you often set out long lines, for example when tiling, a cross line laser with green lines is pleasant to work with. Check a laser regularly too, following its manual. Read how to set out a tiled wall in our blog on tiling a wall.

How to keep your spirit level accurate

• Do not use it as a hammer, crowbar or straightedge to cut along.
• Lay it flat in your toolbox or hang it up, not underneath a pile of heavy tools.
• Keep the measuring faces clean: a crust of adhesive or plaster under the level already causes a deviation.
• Always measure with the same side when you extend a line. Read more about precise measuring in our blog on tape measures and measuring tools.

Working safely

If you drill after measuring, wear safety goggles, use work gloves with nitrile coating when carrying materials, and use the right universal expansion plug for the wall. Double-check distances with a tape measure with auto-lock before drilling.

How accurate does a spirit level need to be?

That differs per type and is often stated in the product information, for example in millimetres per metre. For tiling and kitchens an accurate level matters more than for a garden gate.

Can I check a cross line laser myself?

Yes, with a similar reversal test against a wall. Follow the laser's manual for the right method.

My level is only off on one side. Is it still usable?

Then only use the vial that is accurate and replace the level as soon as you do precision work.

Two minutes of testing prevents a crooked shelf or row of tiles. Turn, compare and only trust your level when it shows the same both times.

Read also: Cross-line laser or spirit level: how to tile and measure truly straight

Where can you find everything for this job?

At Saniso, the online store for DIYers and professionals, you will find tools, materials and personal protective equipment for your job.

• Browse our construction tools and our workwear and safety.
• Create a customer account and order easily.
• Working on a bigger project or want to hand over the job? Request a project quote.
• Book a showroom visit, at least 24 hours in advance.
• Not sure which material suits your job? Get in touch with us.

In doubt about the approach? Feel free to call in a professional.

Saniso – Build your home with Saniso.