Which spirit level do I need?

Length against the job, what the accuracy figure means, and the test that tells you whether a level is honest.

Difficulty: beginner
Time: 20 min
Read: 4 min

Short answer

Length matters more than anything else — a level only reads what it spans, so a 600mm level on a 2m run is guessing. A 1200mm and a 250mm torpedo cover most household work. Check any level by reversing it: the bubble must read the same both ways.

Levels are one of the tools where cheap genuinely costs, and it is invisible until something does not fit. The good news is that a two-second test tells you whether any level is telling the truth.

Step by step

Choose the length by what you are levelling.A level only measures across its own length. On a 2m run a short level can sit in a dip and read perfectly while the run is well out. Longer is more accurate for anything long.
Get a 1200mm as the main one.It handles doorways, worktops, shelves, radiators and most walls. If you buy one level, this is the size, and it is the one people wish they had bought first.
Add a 250mm torpedo.For appliances, brackets, pipe runs, inside cupboards and anywhere the long one will not fit. Magnetic versions are worth it for anything steel.
Consider a 1800mm or 2000mm for door frames.If you are hanging doors, fitting kitchens or studwork, a longer level is a real advantage. Not needed for occasional work.
Read the accuracy figure properly.Usually quoted as millimetres per metre — 0.5mm/m is good, 1.0mm/m is acceptable, and anything unstated should be assumed poor. Over 2 metres, 1mm/m is 2mm out, which is visible on a shelf.
Test it by reversing it.Put it on a surface, note the bubble, then turn it end for end in the same place. If the reading changes, the level is inaccurate and the true reading is halfway between. Do this in the shop.
Check the plumb vial the same way.Against a vertical surface, then rotated 180 degrees face-to-wall. Levels are often accurate in one axis and not the other.
Look for a solid box section.Extruded aluminium box section resists twisting. A thin pressed or plastic level flexes in the hand, which changes the reading, and that is where cheap levels fail.
Check the vials are not adjustable by accident.Some cheap levels have vials that move in their housing over time. Fixed, sealed, shock-resistant vials keep their calibration.
Consider a digital level for angles.Useful if you need actual angles rather than level and plumb — stair strings, ramps, roof pitches. For plain level and plumb, a good bubble level is faster and needs no batteries.
Think about a post level or a line level for specific jobs.A post level straps to a post and reads two axes at once. A line level hangs on a string across a long run. Both are cheap and each does something a normal level cannot.
Look after it.Dropping a level knocks it out of calibration and it will not tell you. Hang it rather than leaving it in a pile, and retest it after any drop.
Re-test it periodically.The reverse test takes seconds. A level that has quietly gone out is worse than no level, because you trust it.

Tips

The reverse test is the whole thing. Any level, any age, two seconds — if the bubble reads differently end-for-end, it is lying.
Buy a 1200mm first. It is the size that covers the most household jobs and short levels give confidently wrong answers over long runs.
Never trust a level after it has been dropped without retesting it.

Common mistakes

Using a short level over a long run — A level only measures across its own length. A short one can sit in a hollow and read perfectly while the whole run slopes.
Assuming a new level is accurate — Levels arrive out of calibration and go out when dropped, and they do not indicate it. Reverse-test any level before trusting it.

If it doesn't work

Shelf looks sloped despite being levelled

Cause: Inaccurate level, or one too short for the span — Fix: Reverse-test the level. Use one that spans most of the shelf, or a straight edge with a level on top.

Bubble reads differently when reversed

Cause: Level out of calibration — Fix: The true reading is halfway between the two. Some levels have adjustable vials; most should be replaced.

Reading changes depending on how you hold it

Cause: Level flexing — usually a thin or plastic body — Fix: Use a rigid box-section level and rest it on the surface rather than gripping it tightly.

No level long enough for the job

Cause: Span longer than any level you own — Fix: Rest a shorter level on a known-straight length of timber or aluminium to bridge the span.

Questions people ask

What size spirit level do I need?

A 1200mm covers most household work — doorways, worktops, shelves and walls — and a 250mm torpedo handles tight spaces. A level only measures across its own length, so short levels mislead on long runs.

How do I check if a spirit level is accurate?

Place it on a surface, note the bubble, then turn it end for end in exactly the same place. If the reading changes, it is inaccurate and the true reading is halfway between the two.