What length of level should I buy?

At least two thirds of the surface being aligned. Walls and joinery want 120-200 cm, doors and units 80-100 cm, electrical and plumbing 40-60 cm, and tight spaces a 20-25 cm torpedo.

What is the practical difference between 0.5 and 1.0 mm/m?

A 1.0 mm/m level reads a one-millimetre deviation over a metre as flat. Along a wall that error accumulates, so formwork, joinery and kitchen fitting call for 0.5 mm/m.

Does a drop knock a level out of true?

It can. The check is simple: read the bubble on a flat surface, then turn the level end for end in the same place. If the two readings differ, the level is out.

Is the accuracy the same upside down?

No — inverted, the permitted deviation usually doubles. For ceiling and under-beam work, choose one grade tighter than you otherwise would.

When is a digital level worth it?

When the job needs a specific fall rather than dead level: waste runs, drainage, ramps and roofs. It reads the angle in degrees or percent, and some models sound an alert at a stored target.

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Spirit Levels

Spirit levels are the largest group in this tree, and the choice comes down to two numbers: accuracy in mm/m and length. This page covers the deviation the eye can actually catch, where a magnetic or cast body earns its cost, how to pick a length from torpedo to 240 cm, and what a digital model adds. The catalogue holds over 180 products from BMI, Milwaukee, GFB, Proft, Bosch and Metresan.

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Accuracy: mm/m and the limit of the eye

A level's accuracy is stated in millimetres per metre and describes the smallest deviation readable from the bubble. 0.5 mm/m is the standard for general building work; 0.75 and 1.0 mm/m appear on economy models, and 0.3 mm/m or better belongs to precision fitting.

The figure applies in the normal position; inverted — ceiling work, under a beam — the permitted deviation usually doubles. If a ceiling line has to hold, choose one grade tighter than the job would otherwise need.

The vial itself is part of the choice: solid block acrylic vials resist heat and knocks and hold their setting, while adjustable vials can be re-aligned after a drop but may drift over time.

Accuracy	Deviation over 1 m	Typical work
0.3 mm/m	0.3 mm	Machine bases, precision fitting
0.5 mm/m	0.5 mm	Walls, formwork, joinery — standard
0.75 mm/m	0.75 mm	General site, rough work
1.0 mm/m	1 mm	Economy, intermediate checks

Choosing a length, and the body

A longer level does not read more accurately — it reads a wider surface. A 20 cm level counts a local bulge in the render as a slope; a 200 cm level gives the plane of the wall. As a rule the level should span at least two thirds of the surface being aligned.

An extruded aluminium body is the standard: light, and rigid enough to resist bending. Reinforced bodies such as Backbone add a spine inside the extrusion to limit bending after a drop. Cast bodies are heavier but the most dimensionally stable, which is why they are the workshop choice.

Magnetic models hold themselves on steel sections, box profile and pipe, which makes single-handed work possible in welding, steel erection and pipework. The magnets are recessed into the measuring face, so they do not spoil the reference plane.

Length	Where it is used
20 - 25 cm (torpedo)	Between units, on pipe, tight spaces
40 - 60 cm	Electrical, plumbing, back boxes
80 - 100 cm	Doors, windows, kitchen units
120 - 200 cm	Walls, formwork, joinery planes
240 cm	Long walls, suspended ceilings, columns

Digital inclinometers and optical levels

Digital levels show the angle in degrees or percent fall alongside the bubble. That decides the job wherever a specific fall, rather than dead level, has to be held: waste pipe runs, ramps, roofs and drainage. Some models store a target angle and sound an alert when it is reached.

Transferring a datum over distance is not a level's job. Beyond ten metres the work belongs to an optical level set or a rotary laser, and the spirit level becomes the final check only.

Percent fall: waste, drainage, ramps.
Degrees: roofs, stairs, brackets.
Audible alert: holding a line single-handed without watching the vial.
Ultrasonic / distance models: alignment and distance in one tool.

Frequently Asked Questions

What length of level should I buy?
At least two thirds of the surface being aligned. Walls and joinery want 120-200 cm, doors and units 80-100 cm, electrical and plumbing 40-60 cm, and tight spaces a 20-25 cm torpedo.
What is the practical difference between 0.5 and 1.0 mm/m?
A 1.0 mm/m level reads a one-millimetre deviation over a metre as flat. Along a wall that error accumulates, so formwork, joinery and kitchen fitting call for 0.5 mm/m.
What does a magnetic level gain me?
It holds itself on steel and iron, freeing both hands in welding, steel erection and pipework. The magnets are recessed into the measuring face, so the reference plane stays true.
Does a drop knock a level out of true?
It can. The check is simple: read the bubble on a flat surface, then turn the level end for end in the same place. If the two readings differ, the level is out.
Is the accuracy the same upside down?
No — inverted, the permitted deviation usually doubles. For ceiling and under-beam work, choose one grade tighter than you otherwise would.
When is a digital level worth it?
When the job needs a specific fall rather than dead level: waste runs, drainage, ramps and roofs. It reads the angle in degrees or percent, and some models sound an alert at a stored target.