Determining the length of an irregular arc segment is also called rectification of a curve. Historically, many methods were used for specific curves. The advent of infinitesimal calculus led to a general formula that provides closed-form solutions in some cases.
General approach 
A curve in the plane can be approximated by connecting a finite number of points on the curve using line segments to create a polygonal path. Since it is straightforward to calculate the length of each linear segment (using the Pythagorean theorem in Euclidean space, for example), the total length of the approximation can be found by summing the lengths of each linear segment.
Polygonal approximations are linearly dependent on the curve in a few select cases. One of these cases is when the curve is simply a point function as is its polygonal approximation. Another case where the polygonal approximation is linearly dependent on the curve is when the curve is linear. This would mean the approximation is also linear and the curve and its approximation overlap. Both of these two circumstances result in an eigenvalue equal to one. There are also a set of circumstances where the polygonal approximation is still linearly dependent but the eigenvalue is equal to zero. This case is a function with petals where all points for the polygonal approximation are at the origin.
If the curve is not already a polygonal path, better approximations to the curve can be obtained by following the shape of the curve increasingly more closely. The approach is to use an increasingly larger number of segments of smaller lengths. The lengths of the successive approximations do not decrease and will eventually keep increasing—possibly indefinitely, but for smooth curves this will tend to a limit as the lengths of the segments get arbitrarily small.
For some curves there is a smallest number L that is an upper bound on the length of any polygonal approximation. If such a number exists, then the curve is said to be rectifiable and the curve is defined to have arc length L.
From a partition a = t0 < t1 < ... < tn−1 < tn = b of the interval [a, b] we obtain a finite collection of points f(t0), f(t1), ..., f(tn−1), f(tn) on the curve C. Denote the distance from f(ti) to f(ti+1) by d(f(ti), f(ti+1)), which is the length of the line segment connecting the two points.
The arc length L of C is then defined to be
where the supremum is taken over all possible partitions of [a, b] and n is unbounded.
The arc length L is either finite or infinite. If L < ∞ then we say that C is rectifiable, and is non-rectifiable otherwise. This definition of arc length does not require that C be defined by a differentiable function. In fact in general, the notion of differentiability is not defined on a metric space.
A curve may be parametrized in many ways. Suppose C also has the parametrization g : [c, d] → X. Provided that f and g are injective, there is a continuous monotone function S from [a, b] to [c, d] so that g(S(t)) = f(t) and an inverse function S−1 from [c, d] to [a, b]. It is clear that any sum of the form can be made equal to a sum of the form by taking , and similarly a sum involving g can be made equal to a sum involving f. So the arc length is an intrinsic property of the curve, meaning that it does not depend on the choice of parametrization.
The definition of arc length for the curve is analogous to the definition of the total variation of a real-valued function.
Finding arc lengths by integrating 
Consider a real function f(x) such that f(x) and (its derivative with respect to x) are continuous on [a, b]. The length s of the part of the graph of f between x = a and x = b can be found as follows:
Consider an infinitesimal part of the curve ds (or consider this as a limit in which the change in s approaches ds). According to Pythagoras' theorem , from which:
If a curve is defined parametrically by x = X(t) and y = Y(t), then its arc length between t = a and t = b is
This is more clearly a consequence of the distance formula where instead of a and , we take the limit. A useful mnemonic is
If a function is defined as a function of x by then it is simply a special case of a parametric equation where and , and the arc length is given by:
If a function is defined in polar coordinates by then the arc length is given by
Curves with closed-form solution for arc length include the catenary, circle, cycloid, logarithmic spiral, parabola, semicubical parabola and (mathematically, a curve) straight line. The lack of closed form solution for the arc length of an elliptic arc led to the development of the elliptic integrals.
In order to approximate the arc length of the curve, it is split into many linear segments. To make the value exact, and not an approximation, infinitely many linear elements are needed. This means that each element is infinitely small. This fact manifests itself later on when an integral is used.
Begin by looking at a representative linear segment (see image) and observe that its length (element of the arc length) will be the differential ds. We will call the horizontal element of this distance dx, and the vertical element dy.
The Pythagorean theorem tells us that
Since the function is defined in time, segments (ds) are added up across infinitesimally small intervals of time (dt) yielding the integral
If y is a function of x, so that we could take t = x, then we have:
which is the arc length from x = a to x = b of the graph of the function ƒ.
For example, the curve in this figure is defined by
Subsequently, the arc length integral for values of t from -1 to 1 is
Using computational approximations, we can obtain a very accurate (but still approximate) arc length of 2.905.
Another way to obtain the integral formula 
Suppose that there exists a rectifiable curve given by a function f(x). To approximate the arc length S along f between two points a and b in that curve, construct a series of right triangles whose concatenated hypotenuses "cover" the arc of curve chosen as shown in the figure. For convenience, the bases of all those triangles can be set equal to , so that for each one an associated exists. The length of any given hypotenuse is given by the Pythagorean Theorem:
The summation of the lengths of the n hypotenuses approximates S:
Multiplying the radicand by produces:
Then, our previous result becomes:
As the length of these segments decreases, the approximation improves. The limit of the approximation, as goes to zero, is equal to :
Another proof 
We know that the formula for a line integral is . If we set the surface f(x,y) to 1, we will get arc length multiplied by 1, or . If x = t, and y = f(t), then y = f(x), from when x is a to when x is b. If we set these equations into our formula we get: (Note: If x = t then dt = dx). This is the arc length formula.
Simple cases 
Arcs of circles 
Arc lengths are denoted by s, since arcs "subtend" an angle.
In the following lines, represents the radius of a circle, is its diameter, is its circumference, is the length of an arc of the circle, and is the angle which the arc subtends at the centre of the circle. The distances and are expressed in the same units.
- which is the same as (This equation is a definition of (pi).)
- If the arc is a semicircle, then
- If is in radians then (This is a definition of the radian.)
- If is in degrees, then which is the same as
- If is in grads (100 grads, or grades, or gradians are one right-angle), then which is the same as
- If is in turns (one turn is a complete rotation, or 360°, or 400 grads, or radians), then
Arcs of great circles on the Earth 
Two units of length, the nautical mile and the metre (or kilometre), were originally defined so the lengths of arcs of great circles on the Earth's surface would be simply numerically related to the angles they subtend at its centre. The simple equation applies in the following circumstances:
- if is in nautical miles, and is in arcminutes (1⁄60 degree), or
- if is in kilometres, and is in centigrades (1⁄100 grad).
The lengths of the distance units were chosen to make the circumference of the Earth equal 40,000 kilometres, or 21,600 nautical miles. These are the numbers of the corresponding angle units in one complete turn.
These definitions of the metre and nautical mile have been superseded by more precise ones, but the original definitions are still accurate enough for conceptual purposes, and for some calculations. For example, they imply that one kilometre is exactly 0.54 nautical miles. Using modern definitions, the ratio is 0.53995680.
Length of an arc of a parabola 
If a point X is located on a parabola which has focal length and if is the perpendicular distance from X to the axis of symmetry of the parabola, then the lengths of arcs of the parabola which terminate at X can be calculated from and as follows, assuming they are all expressed in the same units.
This quantity, , is the length of the arc between X and the vertex of the parabola.
The length of the arc between X and the symmetrically opposite point on the other side of the parabola is
The perpendicular distance, , can be given a positive or negative sign to indicate on which side of the axis of symmetry X is situated. Reversing the sign of reverses the signs of and without changing their absolute values. If these quantities are signed, the length of the arc between any two points on the parabola is always shown by the difference between their values of The calculation can be simplified by using the properties of logarithms:
This calculation can be used for a parabola in any orientation. It is not restricted to the situation where the axis of symmetry is parallel to the y-axis.
Historical methods 
For much of the history of mathematics, even the greatest thinkers considered it impossible to compute the length of an irregular arc. Although Archimedes had pioneered a way of finding the area beneath a curve with his method of exhaustion, few believed it was even possible for curves to have definite lengths, as do straight lines. The first ground was broken in this field, as it often has been in calculus, by approximation. People began to inscribe polygons within the curves and compute the length of the sides for a somewhat accurate measurement of the length. By using more segments, and by decreasing the length of each segment, they were able to obtain a more and more accurate approximation. In particular, by inscribing a polygon of many sides in a circle, they were able to find approximate values of π.
In the 17th century, the method of exhaustion led to the rectification by geometrical methods of several transcendental curves: the logarithmic spiral by Evangelista Torricelli in 1645 (some sources say John Wallis in the 1650s), the cycloid by Christopher Wren in 1658, and the catenary by Gottfried Leibniz in 1691.
Integral form 
In 1659 van Heuraet published a construction showing that the problem of determining arc length could be transformed into the problem of determining the area under a curve (i.e., an integral). As an example of his method, he determined the arc length of a semicubical parabola, which required finding the area under a parabola. In 1660, Fermat published a more general theory containing the same result in his De linearum curvarum cum lineis rectis comparatione dissertatio geometrica (Geometric dissertation on curved lines in comparison with straight lines).
Building on his previous work with tangents, Fermat used the curve
so the tangent line would have the equation
Next, he increased a by a small amount to a + ε, making segment AC a relatively good approximation for the length of the curve from A to D. To find the length of the segment AC, he used the Pythagorean theorem:
which, when solved, yields
In order to approximate the length, Fermat would sum up a sequence of short segments.
Curves with infinite length 
As mentioned above, some curves are non-rectifiable, that is, there is no upper bound on the lengths of polygonal approximations; the length can be made arbitrarily large. Informally, such curves are said to have infinite length. There are continuous curves on which every arc (other than a single-point arc) has infinite length. An example of such a curve is the Koch curve. Another example of a curve with infinite length is the graph of the function defined by f(x) = x sin(1/x) for any open set with 0 as one of its delimiters and f(0) = 0. Sometimes the Hausdorff dimension and Hausdorff measure are used to "measure" the size of such curves.
Generalization to (pseudo-)Riemannian manifolds 
Let M be a (pseudo-)Riemannian manifold, γ : [0, 1] → M a curve in M and g the (pseudo-) metric tensor.
The length of γ is defined to be
where γ'(t) ∈ Tγ(t)M is the tangent vector of γ at t. The sign in the square root is chosen once for a given curve, to ensure that the square root is a real number. The positive sign is chosen for spacelike curves; in a pseudo-Riemannian manifold, the negative sign may be chosen for timelike curves.
See also 
- CRC Handbook of Chemistry and Physics, page F-254
- John Wallis, Tractatus Duo. Prior, De Cycloide et de Corporibus inde Genitis. … (Oxford, England: University Press, 1659), pages 91-96; the accompanying figures appear on page 145. On page 91, William Neile is mentioned as "Gulielmus Nelius".
- Henricus van Heuraet, "Epistola de transmutatione curvarum linearum in rectas" (Letter on the transformation of curved lines into right ones [i.e., Letter on the rectification of curves]), Renati Des-Cartes Geometria, 2nd ed. (Amsterdam ["Amstelædami"], (Netherlands): Louis & Daniel Elzevir, 1659), pages 517-520.
- "M.P.E.A.S." (pseudonym of Fermat) De Linearum Curvarum cum Lineis Rectis Comparatione Dissertatio Geometrica (Toulouse [Tolosæ], France: Arnaud Colomer, 1660).
- Farouki, Rida T. (1999). Curves from motion, motion from curves. In P-J. Laurent, P. Sablonniere, and L. L. Schumaker (Eds.), Curve and Surface Design: Saint-Malo 1999, pp. 63–90, Vanderbilt Univ. Press. ISBN 0-8265-1356-5.
- Hazewinkel, Michiel, ed. (2001), "Rectifiable curve", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Math Before Calculus
- The History of Curvature
- Weisstein, Eric W., "Arc Length", MathWorld.
- Arc Length by Ed Pegg, Jr., The Wolfram Demonstrations Project, 2007.
- Calculus Study Guide – Arc Length (Rectification)
- Famous Curves Index The MacTutor History of Mathematics archive
- Arc Length Approximation by Chad Pierson, Josh Fritz, and Angela Sharp, The Wolfram Demonstrations Project.
- Length of a Curve Experiment Illustrates numerical solution of finding length of a curve.