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The turn (symbol tr or pla) is a unit of plane angle measurement that is the measure of a complete angle—the angle Subtended angle by a complete circle at its center. One turn is equal to , 360 degrees or 400 . As an angular unit, one turn also corresponds to one cycle (symbol cyc or c) or to one revolution (symbol rev or r). Common related units of frequency are cycles per second (cps) and revolutions per minute (rpm). The angular unit of the turn is useful in connection with, among other things, electromagnetic coils (e.g., ), rotating objects, and the winding number of curves. Divisions of a turn include the half-turn and quarter-turn, spanning a straight angle and a right angle, respectively; metric prefixes can also be used as in, e.g., centiturns (ctr), milliturns (mtr), etc.
In the ISQ, an arbitrary "number of turns" (also known as "number of revolutions" or "number of cycles") is formalized as a dimensionless quantity called rotation, defined as the ratio of a given angle and a full turn. It is represented by the symbol N.
Because one turn is radians, some have proposed representing with the single letter 𝜏 (tau).
Using , the turn can be divided in 100 centiturns or milliturns, with each milliturn corresponding to an angle of 0.36°, which can also be written as 21′ 36″. A protractor divided in centiturns is normally called a "percentage protractor". While percentage protractors have existed since 1922, the terms centiturns, milliturns and microturns were introduced much later by the British astronomer Fred Hoyle in 1962. Some measurement devices for artillery and satellite watching carry milliturn scales.
Binary fractions of a turn are also used. Sailors have traditionally divided a turn into 32 compass points, which implicitly have an angular separation of turn. The binary degree, also known as the binary radian (or brad), is turn. The binary degree is used in computing so that an angle can be represented to the maximum possible precision in a single byte. Other measures of angle used in computing may be based on dividing one whole turn into equal parts for other values of .
| + Conversion of common angles | ||||
| 0 turn | 0 rad | 0° | 0g | |
| turn | rad | rad | 5° | g |
| turn | rad | rad | 15° | g |
| turn | rad | rad | 22.5° | 25g |
| turn | rad | rad | 30° | g |
| turn | rad | rad | 36° | 40g |
| turn | rad | rad | 45° | 50g |
| turn | 1 rad | 57.3° | 63.7g | |
| turn | rad | rad | 60° | g |
| turn | rad | rad | 72° | 80g |
| turn | rad | rad | 90° | 100g |
| turn | rad | rad | 120° | g |
| turn | rad | rad | 144° | 160g |
| turn | rad | rad | 180° | 200g |
| turn | rad | rad | 270° | 300g |
| 1 turn | rad | 2 rad | 360° | 400g |
N is the number (not necessarily an integer) of revolutions, for example, of a rotating body about a given axis. Its value is given by:where denotes the measure of rotational displacement.
The above definition is part of the ISQ, formalized in the international standard ISO 80000-3 (Space and time), and adopted in the International System of Units (SI).
Rotation count or number of revolutions is a quantity of dimension one, resulting from a ratio of angular displacement. It can be negative and also greater than 1 in modulus. The relationship between quantity rotation, N, and unit turns, tr, can be expressed as:
In the ISQ/SI, rotation is used to derive rotational frequency (the Derivative of rotation with respect to time), denoted by :
The SI unit of rotational frequency is the reciprocal second (s−1). Common related units of frequency are hertz (Hz), cycles per second (cps), and revolutions per minute (rpm).
The superseded version ISO 80000-3:2006 defined "revolution" as a special name for the dimensionless unit "one", which also received other special names, such as the radian. Despite their dimensional homogeneity, these two specially named dimensionless units are applicable for non-comparable kinds of quantity: rotation and angle, respectively. "Cycle" is also mentioned in ISO 80000-3, in the definition of period.
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