The sensitivity of spirit level vials

The sensitivity of a spirit level vial can be specified in two ways. The first possibility is to state the sensitivity of a spirit level vial with the angle in degrees (°), minutes (´) or seconds (´´), by which it must be inclined to cause the bubble to move 2 mm.
1° = 60´ = 3600´´

Another common option to specify the sensitivity, is the indication in mm/m. In this case, the height difference x in mm over a length of 1 m is decisive.
Therefore, imagine the spirit level vial to be situated on a flat supporting surface which is exactly 1 m long. This means, that for a spirit level vial with a sensitivity of e.g. 0,6 mm/m, one end of this 1 m long supporting surface has to be lifted 0,6 mm to cause a bubble movement of 2 mm.

Unless it is stated otherwise, the sensitivity of a spirit level vial always refers to a bubble movement of 2 mm. For this reason, the graduation lines of precision spirit level vials are usually 2 mm apart.

Thus, the specification of the sensitivity is based either on the angle of inclination or on the height difference in mm in relation to 1 m. In both cases, the value at which the bubble moves 2 mm, is decisive.

Both indications can be converted into each other.

Conversion of the sensitivity from degrees in mm/m

In advance, minutes and seconds need to be converted in degrees via the rule of three:
1° = 60´ = 3600´´
x = sin (α) * 1000

Conversion of the sensitivity from mm/m in degrees

α = arcsin (x/1000)
Afterwards, the angle α can be converted into minutes and seconds via the rule of three.