Turn (angle)

Revolution
Revolution
Unit of	Rotation
Symbol	rev, r, cyc, c
Conversions
1 rev in ...	... is equal to ...
Base units	1

Rotation
Rotation
Other names	number of revolutions, number of cycles, number of turns, number of rotations
Common symbols	N
SI unit	Unitless
Dimension	1

Turn
Turn
	Counterclockwise rotations about the center point where a complete rotation corresponds to an angle of rotation of 1 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	400g

One turn (symbol tr or pla) is a unit of plane angle measurement equal to $2π$ radians, 360 degrees or 400 gradians. Thus it is the angular measure subtended by a complete circle at its center.

Subdivisions of a turn include half-turns and quarter-turns, spanning a semicircle and a right angle, respectively; metric prefixes can also be used as in, e.g., centiturns (ctr), milliturns (mtr), etc.

As an angular unit, one turn also corresponds to one cycle (symbol cyc or c) or to one revolution (symbol rev or r).

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 the full turn. (See below for the formula.)

Common related units of frequency are cycles per second (cps) and revolutions per minute (rpm).

History

The word turn originates via Latin and French from the Greek word τόρνος (tórnos – a lathe).

In 1697, David Gregory used $.mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num,.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0 0.1em}.mw-parser-output .sfrac .den{border-top:1px solid}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px} π/ρ$ (pi over rho) to denote the perimeter of a circle (i.e., the circumference) divided by its radius. 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. Euler adopted the symbol with that meaning in 1737, leading to its widespread use.

Percentage protractors have existed since 1922, but 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.

Unit symbols

The German standard DIN 1315 (March 1974) proposed the unit symbol "pla" (from Latin: plenus angulus 'full angle') for turns. Covered in DIN 1301-1 (October 2010), the so-called Vollwinkel ('full angle') is not an SI unit. However, it is a legal unit of measurement in the EU and Switzerland.

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. An angular mode TURN was suggested for the WP 43S as well, but the calculator instead implements "MUL $π$ " (multiples of $π$ ) as mode and unit since 2019.

Subdivisions

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″. A protractor divided in centiturns is normally called a "percentage protractor".

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. 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 $n$ .

The notion of turn is commonly used for planar rotations.

SI multiples of turn (tr)
Submultiples			Multiples
Value	SI symbol	Name	Value	SI symbol	Name
10⁻¹ tr	dtr	deciturn	10¹ tr	datr	decaturn
10⁻² tr	ctr	centiturn	10² tr	htr	hectoturn
10⁻³ tr	mtr	milliturn	10³ tr	ktr	kiloturn
10⁻⁶ tr	μtr	microturn	10⁶ tr	Mtr	megaturn
10⁻⁹ tr	ntr	nanoturn	10⁹ tr	Gtr	gigaturn
10⁻¹² tr	ptr	picoturn	10¹² tr	Ttr	teraturn
10⁻¹⁵ tr	ftr	femtoturn	10¹⁵ tr	Ptr	petaturn
10⁻¹⁸ tr	atr	attoturn	10¹⁸ tr	Etr	exaturn
10⁻²¹ tr	ztr	zeptoturn	10²¹ tr	Ztr	zettaturn
10⁻²⁴ tr	ytr	yoctoturn	10²⁴ tr	Ytr	yottaturn
10⁻²⁷ tr	rtr	rontoturn	10²⁷ tr	Rtr	ronnaturn
10⁻³⁰ tr	qtr	quectoturn	10³⁰ tr	Qtr	quettaturn

Unit conversion

The circumference of the unit circle (whose radius is one) is $2 π$ .

One turn is equal to $2 π$ (≈ 6.283185307179586) radians, 360 degrees, or 400 gradians.

Conversion of common angles
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
1/2 $π$ 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	2 $π$ /5 rad	72°	80^g
1/4 turn	𝜏/4 rad	$π$ /2 rad	90°	100^g
1/3 turn	𝜏/3 rad	2 $π$ /3 rad	120°	133+1/3^g
2/5 turn	2𝜏/5 rad	4 $π$ /5 rad	144°	160^g
1/2 turn	𝜏/2 rad	$π$ rad	180°	200^g
3/4 turn	3𝜏/4 rad	3 $π$ /2 rad	270°	300^g
1 turn	𝜏 rad	2 $π$ rad	360°	400^g

Proposals for a single letter to represent 2π

In 1746, Leonhard Euler first used the Greek letter pi to represent the circumference divided by the radius of a circle (i.e., $π$ = 6.28...).

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 ().

In 2008, Thomas Colignatus proposed the uppercase Greek letter theta, Θ, to represent 2 $π$ . 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. It has also been proposed to use the wheel symbol, teth, to represent the value 2 $π$ , 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 2 $π$ .

In 2010, Michael Hartl proposed to use the Greek letter tau to represent the circle constant: $τ = 2 π$ . He offered two reasons. First, $τ$ is the number of radians in one turn, which allows fractions of a turn to be expressed more directly: for instance, a 3/4 turn would be represented as $3 τ / 4$ rad instead of $3 π / 2$ rad. Second, $τ$ visually resembles $π$ , whose association with the circle constant is unavoidable. Hartl's Tau Manifesto gives many examples of formulas that are asserted to be clearer where $τ$ is used instead of $π$ , such as a tighter association with the geometry of Euler's identity using $e iτ = 1$ instead of $e iπ = -1$ .

Initially, neither of these proposals received widespread acceptance by the mathematical and scientific communities. However, the use of $τ$ has become more widespread, for example:

In 2012, the educational website Khan Academy began accepting answers expressed in terms of $τ$ .
The constant $τ$ is made available in the Google calculator, Desmos graphing calculator and in several programming languages such as Python, Raku, Processing, Nim, Rust, GDScript, UE Blueprints, Java, and .NET.
It has also been used in at least one mathematical research article, authored by the $τ$ -promoter Peter Harremoës.

The following table shows how various identities appear if $τ = 2 π$ was used instead of $π$ . 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	$π / 2$ rad	$τ / 4$ rad	$τ / 4 rad = 1 / 4 turn$
Circumference $C$ of a circle of radius $r$	$C = 2 π r$	$C = τ r$
Area of a circle	$A = π r 2$	$A = 1 / 2 τ r 2$	The area of a sector of angle $θ$ is $A = 1 / 2 θr 2$ .
Area of a regular $n$ -gon with unit circumradius	$A = n / 2 sin 2π / n$	$A = n / 2 sin τ / n$
$n$ -ball and $n$ -sphere volume recurrence relation	$V n (r) = r / n S n -1 (r)$ $S n (r) = 2 π r V n -1 (r)$	$V n (r) = r / n S n -1 (r)$ $S n (r) = τ r V n -1 (r)$	$V 0 (r) = 1$ $S 0 (r) = 2$
Cauchy's integral formula
Standard normal distribution
Stirling's approximation
Euler's identity	0 $e i π = -1$ $e i π + 1 = 0$	0 $e i τ = 1$ $e i τ - 1 = 0$	For any integer $k$ , $e ikτ = 1$
$n$ th roots of unity
Planck constant			$ħ$ is the reduced Planck constant.
Angular frequency

Examples of use

As an angular unit, the turn is particularly useful in many applications, such as in connection with electromagnetic coils (e.g., transformers) and rotating objects. See also Winding number.
Pie charts illustrate proportions of a whole as fractions of a turn. Each one percent is shown as an angle of one centiturn.

In the ISQ/SI

A concept related to the angular unit "turn" is the physical quantity rotation (symbol N) defined as number of revolutions:

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:

N= φ/2π rad

where φ denotes the measure of rotational displacement.

The above definition is part of the International System of Quantities (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 angles. 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:

N=φ/tr={φ}_tr

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, n=dN/dt, with SI base unit of reciprocal seconds (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.