Using a spirit level properly

Learn to read a spirit level correctly and trust the result, not just glance at it.

SL-01Know what you're looking atA spirit level is a straight bar with one or more small glass or plastic tubes set into it, called vials. Each vial is filled with liquid and has a bubble in it, and two lines marked on the tube. The bubble sitting exactly between those two lines means level (or plumb, if it's the vertical vial). Most levels have a horizontal vial for flat surfaces and a vertical one for upright surfaces like walls or door frames.

SL-02Check the level itself is accurate firstBeginners assume the tool is always right, but a dropped or cheap level can be out of true. Place it on a flat surface and note where the bubble sits, then rotate the level 180 degrees end to end and put it back in exactly the same spot. If the bubble sits in the same place both times, the level is accurate. If it shifts, the level is lying to you and needs replacing or allowing for.

SL-03Read the bubble properlyGet your eye level with the vial, not looking down at an angle, because looking from above or the side makes the bubble appear off when it isn't. The bubble should sit exactly centred between the two lines, not just touching one of them. If you're not sure, move your head until the two lines and the bubble all look symmetrical.

SL-04Check something is level (horizontal)Lay the level flat along the surface you're checking, such as a shelf, and use the horizontal vial. Let the level sit under its own weight rather than pressing down on one end, as pushing changes the reading. Read the bubble as in the step above, and if it's off-centre, that end needs raising or lowering until it centres.

SL-05Check something is plumb (vertical)Hold the level flat against the upright surface, like a door frame or a length of wood you're fitting standing up, and use the vertical vial. Hold it straight against the surface without tilting the top or bottom towards you, since a slight tilt gives a false reading. Adjust the item until the bubble centres, then recheck once you've fixed anything in place, since tightening a screw or fixing can nudge things out slightly.

SL-06Avoid the mistakes that throw the reading offDon't rest the level on a lump of debris, old filler, or dust, as even a small bump under one end will give a false result. Don't press down harder on one end than the other, and don't check a short section only, as a small dip can hide over a longer run. For anything longer than your level, such as a whole wall or worktop, use a straight batten alongside the level to bridge the gap, or check in stages and mark each point.

SL-07Confirm you've actually got it rightOnce the bubble reads centred, flip the level end to end on the same spot without moving the object underneath it. If the bubble still centres in the same position, your surface really is level or plumb, not just your level giving a lucky-looking reading. If the bubble moves when you flip it, trust the flipped reading and adjust again, because this trick cancels out any small inaccuracy in the tool itself.

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