The turn (symbol tr or pla) is a unit of plane angle measurement that is the angular measure subtended by a complete circle at its center. It is equal to 2π radians, 360 degrees or 400 gradians. As an angular unit, one turn also corresponds to one cycle (symbol cyc or c)^{[1]} or to one revolution (symbol rev or r).^{[2]} Common related units of frequency are cycles per second (cps) and revolutions per minute (rpm).^{[a]} The angular unit of the turn is useful in connection with, among other things, electromagnetic coils (e.g., transformers), rotating objects, and the winding number of curves. Subdivisions of a turn include the halfturn and quarterturn, spanning a straight angle and a right angle, respectively; metric prefixes can also be used as in, e.g., centiturns (ctr), milliturns (mtr), etc.
Turn  

General information  
Unit of  Plane angle 
Symbol  tr, pla, rev, cyc 
Conversions  
1 tr in ...  ... is equal to ... 
radians  2π rad ≈ 6.283185307... rad 
milliradians  2000π mrad ≈ 6283.185307... mrad 
degrees  360° 
gradians  400^{g} 
Because one turn is radians, some have proposed representing with a single letter. In 2010, Michael Hartl proposed using the Greek letter (tau), equal to and corresponding to one turn, for greater conceptual simplicity when stating angles in radians.^{[3]} This proposal did not initially gain widespread acceptance in the mathematical community,^{[4]} but the constant has become more widespread,^{[5]} having been added to several major programming languages and calculators.
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.
There are several unit symbols for the turn.
The German standard DIN 1315 (March 1974) proposed the unit symbol "pla" (from Latin: plenus angulus 'full angle') for turns.^{[6]}^{[7]} Covered in DIN 13011 (October 2010), the socalled Vollwinkel ('full angle') is not an SI unit. However, it is a legal unit of measurement in the EU^{[8]}^{[9]} and Switzerland.^{[10]}
The scientific calculators HP 39gII and HP Prime support the unit symbol "tr" for turns since 2011 and 2013, respectively. Support for "tr" was also added to newRPL for the HP 50g in 2016, and for the hp 39g+, HP 49g+, HP 39gs, and HP 40gs in 2017.^{[11]}^{[12]} An angular mode TURN was suggested for the WP 43S as well,^{[13]} but the calculator instead implements "MULπ" (multiples of π) as mode and unit since 2019.^{[14]}^{[15]}
A turn can be divided in 100 centiturns or 1000 milliturns, with each milliturn corresponding to an angle of 0.36°, which can also be written as 21′ 36″.^{[16]}^{[17]} A protractor divided in centiturns is normally called a "percentage protractor".
While percentage protractors have existed since 1922,^{[18]} the terms centiturns, milliturns and microturns were introduced much later by the British astronomer Fred Hoyle in 1962.^{[16]}^{[17]} Some measurement devices for artillery and satellite watching carry milliturn scales.^{[19]}^{[20]}
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 1/32 turn. The binary degree, also known as the binary radian (or brad), is 1/256 turn.^{[21]} 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 2^{n} equal parts for other values of n.^{[22]}
The number 2π (approximately 6.28) is the ratio of a circle's circumference to its radius, and the number of radians in one turn.
The meaning of the symbol was not originally fixed to the ratio of the circumference and the diameter. In 1697, David Gregory used π/ρ (pi over rho) to denote the perimeter of a circle (i.e., the circumference) divided by its radius.^{[23]}^{[24]} However, earlier in 1647, William Oughtred had used δ/π (delta over pi) for the ratio of the diameter to perimeter. The first use of the symbol π on its own with its present meaning (of perimeter divided by diameter) was in 1706 by the Welsh mathematician William Jones.^{[25]}^{[26]}
The first known usage of a single letter to denote the 6.28... constant was in Leonhard Euler's 1727 Essay Explaining the Properties of Air, where it was denoted by the letter π. Euler would later use the letter π for the 3.14... constant in his 1736 Mechanica and 1748 Introductio in analysin infinitorum, though defined as half the circumference of a circle of radius 1—a unit circle—rather than the ratio of circumference to diameter. Elsewhere in Introductio in analysin infinitorum, Euler instead used the letter π for onefourth of the circumference of a unit circle, or 1.57... . Eventually, π was standardized as being equal to 3.14..., and its usage became widespread.^{[27]}^{[28]}
Several people have independently proposed using 𝜏 = 2π, including:^{[29]}
In 2001, Robert Palais proposed using the number of radians in a turn as the fundamental circle constant instead of π, which amounts to the number of radians in half a turn, in order to make mathematics simpler and more intuitive. His proposal used a "π with three legs" symbol to denote the constant ( ).^{[30]}
In 2008, Robert P. Crease proposed the idea of defining a constant as the ratio of circumference to radius, a proposal supported by John Horton Conway. Crease used the Greek letter psi: .^{[31]}
The same year, Thomas Colignatus proposed the uppercase Greek letter theta, Θ, to represent 2π.^{[32]} The Greek letter theta derives from the Phoenician and Hebrew letter teth, 𐤈 or ט, and it has been observed that the older version of the symbol, which means wheel, resembles a wheel with four spokes.^{[33]} It has also been proposed to use the wheel symbol, teth, to represent the value 2π, and more recently a connection has been made among other ancient cultures on the existence of a wheel, sun, circle, or disk symbol—i.e. other variations of teth—as representation for 2π.^{[34]}
In 2010, Michael Hartl proposed to use the Greek letter tau to represent the circle constant: τ = 2π. He offered several reasons for the choice of constant, primarily that it allows fractions of a turn to be expressed more directly: for instance, a 3/4 turn would be represented as 3τ/4 rad instead of 3π/2 rad. As for the choice of notation, he offered two reasons. First, τ is the number of radians in one turn, and both τ and turn begin with a /t/ sound. Second, τ visually resembles π, whose association with the circle constant is unavoidable. Hartl's Tau Manifesto^{[b]} gives many examples of formulas that are asserted to be clearer where τ is used instead of π.^{[36]}^{[37]}^{[38]} For example, Hartl asserts that replacing Euler's identity e^{iπ} = −1 by e^{iτ} = 1 (which Hartl also calls "Euler's identity") is more fundamental and meaningful.
Initially, neither of these proposals received widespread acceptance by the mathematical and scientific communities.^{[4]} However, the use of τ has become more widespread.^{[5]} For example:
The following table shows how various identities appear when τ = 2π is used instead of π.^{[56]}^{[30]} For a more complete list, see List of formulae involving π.
Formula  Using π  Using τ  Notes 

Angle subtended by 1/4 of a circle  τ/4 rad = 1/4 turn  
Circumference C of a circle of radius r  
Area of a circle  The area of a sector of angle θ is A = 1/2θr^{2}.  
Area of a regular ngon with unit circumradius  
nball and nsphere volume recurrence relation 


V_{0}(r) = 1 S_{0}(r) = 2 
Cauchy's integral formula  is the boundary of a disk containing in the complex plane.  
Standard normal distribution  
Stirling's approximation  
nth roots of unity  
Planck constant  ħ is the reduced Planck constant.  
Angular frequency 
One turn is equal to 2π (≈ 6.283185307179586)^{[57]} radians, 360 degrees, or 400 gradians.
Turns  Radians  Degrees  Gradians  

0 turn  0 rad  0°  0^{g}  
1/72 turn  𝜏/72 rad  π/36 rad  5°  5+5/9^{g} 
1/24 turn  𝜏/24 rad  π/12 rad  15°  16+2/3^{g} 
1/16 turn  𝜏/16 rad  π/8 rad  22.5°  25^{g} 
1/12 turn  𝜏/12 rad  π/6 rad  30°  33+1/3^{g} 
1/10 turn  𝜏/10 rad  π/5 rad  36°  40^{g} 
1/8 turn  𝜏/8 rad  π/4 rad  45°  50^{g} 
1/2π turn  1 rad  c. 57.3°  c. 63.7^{g}  
1/6 turn  𝜏/6 rad  π/3 rad  60°  66+2/3^{g} 
1/5 turn  𝜏/5 rad  2π/5 rad  72°  80^{g} 
1/4 turn  𝜏/4 rad  π/2 rad  90°  100^{g} 
1/3 turn  𝜏/3 rad  2π/3 rad  120°  133+1/3^{g} 
2/5 turn  2𝜏/5 rad  4π/5 rad  144°  160^{g} 
1/2 turn  𝜏/2 rad  π rad  180°  200^{g} 
3/4 turn  3𝜏/4 rad  3π/2 rad  270°  300^{g} 
1 turn  𝜏 rad  2π rad  360°  400^{g} 
Rotation  

Other names  number of revolutions, number of cycles, number of turns, number of rotations 
Common symbols  N 
SI unit  Unitless 
Dimension  1 
In the International System of Quantities (ISQ), rotation (symbol N) is a physical quantity defined as number of revolutions:^{[58]}
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 𝜑 denotes the measure of rotational displacement.
The above definition is part of the ISQ, formalized in the international standard ISO 800003 (Space and time),^{[58]} and adopted in the International System of Units (SI).^{[59]}^{[60]}
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:
where {𝜑}_{tr} is the numerical value of the angle 𝜑 in units of turns (see Physical quantity#Components).
In the ISQ/SI, rotation is used to derive rotational frequency (the rate of change of rotation with respect to time), denoted by n:
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).
Revolution  

Unit of  Rotation 
Symbol  rev, r, cyc, c 
Conversions  
1 rev in ...  ... is equal to ... 
Base units  1 
The superseded version ISO 800003:2006 defined "revolution" as a special name for the dimensionless unit "one",^{[c]} which also received other special names, such as the radian.^{[d]} Despite their dimensional homogeneity, these two specially named dimensionless units are applicable for noncomparable kinds of quantity: rotation and angle, respectively.^{[62]} "Cycle" is also mentioned in ISO 800003, in the definition of period.^{[e]}
[…] I'd like to see a TURN mode being implemented as well. TURN mode works exactly like DEG, RAD and GRAD (including having a full set of angle unit conversion functions like on the WP 34S), except for that a full circle doesn't equal 360 degree, 6.2831... rad or 400 gon, but 1 turn. (I […] found it to be really convenient in engineering/programming, where you often have to convert to/from other unit representations […] But I think it can also be useful for educational purposes. […]) Having the angle of a full circle normalized to 1 allows for easier conversions to/from a whole bunch of other angle units […]
There are various other ways of finding the Lengths, or Areas of particular Curve Lines or Planes, which may very much facilitate the Practice; as for instance, in the Circle, the Diameter is to Circumference as 1 toReprinted in Smith, David Eugene (1929). "William Jones: The First Use of π for the Circle Ratio". A Source Book in Mathematics. McGraw–Hill. pp. 346–347.
3.14159, &c. = π. This Series (among others for the same purpose, and drawn from the same Principle) I receiv'd from the Excellent Analyst, and my much Esteem'd Friend Mr. John Machin; and by means thereof, Van Ceulen's Number, or that in Art. 64.38. may be Examin'd with all desireable Ease and Dispatch.
unde constat punctum B per datum tantum spatium de loco fuo naturali depelli, ad quam maximam distantiam pertinget, elapso tempore t=π/m denotante π angulum 180°, quo fit cos(mt)= 1 & B b=2α.[from which it is clear that the point B is pushed by a given distance from its natural position, and it will reach the maximum distance after the elapsed time t=π/m, π denoting an angle of 180°, which becomes cos(mt)= 1 & B b=2α.]