A spirit level is an instrument designed to indicate whether a surface is level.

Spirit levels feature a slightly curved glass tube which is incompletely filled with a liquid, usually coloured 'spirit' (a synonym for ethanol), leaving a bubble in the tube. Ethanol is used because of its low freezing point, −114°C, which prevents it from freezing in cold weather. Most commonly spirit levels are employed to indicate how horizontal or how vertical a surface is. A spirit level often has a wide body, and looks like a short plank of wood, to ensure stability and that the surface is being measured correctly. Embedded in the middle of the spirit level is a small window where the bubble and the tube is mounted. Two notches designate where the bubble should be if the surface was level. Often also a indicator for a 45 degree inclination is included.

Some are also capable of indicating the level of a surface between horizontal and vertical to the nearest degree. The crudest form of the spirit level is the bull's eye level : a circular flat-bottomed device with the liquid under a slightly convex glass face which indicates the center clearly. It serves to level a surface in two perpendicular directions, while the tubular level only does so in the direction of the tube.

The sensitivity of a level is given as the angle, in seconds of arc, by which the level has to be tilted to move the bubble by one graduation unit. For precision levels it is as little as 5″.

The spirit level was invented by Mechisedech Thevenot (born in either 1620 or 1621, died 1692). Thevenot was an amateur scientist and patron of many scientists and mathematicians. He was wealthy and well-connected, later becoming the Royal Librarian to King Louis XIV of France. Thevenot invented the instrument some time before February 2, 1661. This date can be very accurately established from Thevenot's correspondence with scientist Christian Huygens. Within a year of this date the inventor circulated details of his invention to others, including Robert Hooke in London and Vincenzo Viviani in Florence. It is occasionally argued that these bubble levels did not come into widespread use until the beginning of the eighteenth century, the earliest surviving examples being from that time, but Adrien Auzout had recommended that the Académie Royale des Sciences take "levels of the Thevenot type" on its expedition to Madagascar in 1666. It is very likely that these levels were in use in France and elsewhere long before the turn of the century.

Thevenot is often confused with his nephew, the traveller Jean de Thevenot (born 1633, died 1667). There is evidence to suggest that both Huygens and Hooke later laid claim to the invention, although only within their own countries.

Often the term spirit level is used to refer to a leveling instrument as used in surveying to measure height differences over larger distances. It consists of a spirit level in the above sense, mounted on a measurement telescope containing cross-hairs, itself mounted on a tripod. The observer reads height values off two staffs, one in front of him and one behind him, to obtain the height difference between the ground points on which the staffs are resting. By repeating, height differences can be measured cumulatively over large distances.

There are different types of spirit levels for different uses:

• Carpenter's level
• Engineer's precision levels
• Electronic levels

See also

• Laser line level
• Physical geodesy
• Theodolite