Spirit level: what it does and when to reach for it

Learn to read a spirit level correctly so shelves, pictures and fixings sit truly level or plumb.

SL-01Understand what the bubble is telling youA spirit level is a sealed tube of liquid with a bubble inside, set into a straight bar. When the surface is level, the bubble sits exactly between two marked lines. If the bubble drifts to one side, that side is higher.

SL-02Pick the right length of level for the jobA short torpedo level (around 230mm) is handy for small jobs like checking a single bracket. A longer level (600mm to 1200mm) gives a far more accurate reading over a shelf or worktop, because small errors show up more clearly over a longer distance. For anything wider than the level itself, you will need to check in stages, moving the level along and checking it lines up each time.

SL-03Hold it flat against the surface, not at an anglePlace the level directly on the surface you are checking, and make sure the whole length of it is touching, not just the ends. Get your eye level with the bubble itself, not looking down at it from above, or it will look level when it is not. Beginners often glance at the bubble from an angle and misread it.

SL-04Check level for anything horizontalUse the level to check shelves, worktops, curtain poles and anything else meant to run flat side to side. Lay the level along the surface and read the bubble in the middle vial. If it is off, prop up or adjust the low side until the bubble sits dead centre between the lines.

SL-05Check plumb for anything verticalMost spirit levels have a second vial at one end for checking vertical lines, called plumb. Hold the level flat against a door frame, pipe, or the edge of a cupboard, standing it up rather than laying it flat. Read this end vial the same way: centred bubble means truly vertical.

SL-06Watch for the mistake that throws readings offThe most common error is resting the level on a bump of old filler, a screw head, or dust and debris, which tilts it slightly without you noticing. Always run a hand along the surface first and clear anything in the way. A level balanced on debris will lie to you confidently.

SL-07Prove your level is accurate with the flip testLevels can go out of true if dropped. To check, place it on a flat surface and note where the bubble sits, then turn the level end to end (rotate it 180 degrees) and place it in exactly the same spot. If the bubble sits in the same position both times, the level is accurate. If the reading changes, the level is faulty and should not be trusted.

SL-08Know when you actually need oneReach for a spirit level whenever something needs to look right to the eye over a distance: shelves, mirrors, pictures in a row, curtain poles, or tiling lines. For a single small picture hook, it matters less, but for anything spanning more than about 300mm, a level will catch a lean you cannot see by eye.

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