Inverse Tangent (Arctan): Domain, Range & Graph

The Function That Turns Coordinates Into Direction

A drone reports how far east and how far north it has drifted, but the autopilot needs a single heading angle. Converting those two distances into one direction is the inverse tangent function at work. Because it accepts any ratio, from the tiniest to the enormous, arctan is the function that graphics engines, radar systems, and navigation code reach for whenever a pair of coordinates has to become an angle.

Its sibling arctan-of-a-single-ratio has a quirk the others do not share: its input can be any real number at all, yet its output stays trapped inside a narrow band it never quite reaches.

What Is The Inverse Tangent Function?

The inverse tangent function undoes the tangent function. If $\tan\theta = x$, then $\arctan x = \theta$, with a restriction fixed below. It is written $\tan^{-1}x$ or $\arctan x$, both read as "the angle whose tangent is $x$."

The $-1$ is not an exponent. $\tan^{-1}x$ does not mean $\dfrac{1}{\tan x}$; that reciprocal is cotangent. Inverse tangent belongs to the family of inverse trigonometric functions, which reverse the ordinary trigonometric functions so you can move from a ratio back to an angle.

Why Does Tangent Need A Principal Branch?

Tangent is a periodic function with period $\pi$, so it repeats the same output infinitely often. The angles $\dfrac{\pi}{4}$, $\dfrac{5\pi}{4}$, $\dfrac{9\pi}{4}$, and countless others all have a tangent of $1$. "The angle whose tangent is $1$" therefore has no single answer until we choose one branch.

Mathematicians restrict tangent to the open interval $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$, the largest stretch around zero on which tangent is one-to-one and continuous. This is the principal branch, and the single value arctan returns from it is the principal value. The endpoints are excluded because tangent blows up to infinity there, so no finite angle sits exactly at $\pm\dfrac{\pi}{2}$.

What Are The Domain And Range Of Arctan?

Because inverting a function swaps inputs and outputs, arctan's domain is tangent's range and vice versa.

• Domain: all real numbers, $(-\infty, \infty)$. Tangent produces every real value, so arctan accepts every real input. There is no size limit here, unlike arcsin.

• Range: the open interval $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$. Every output lands strictly between $-90^\circ$ and $90^\circ$; the boundaries are never hit.

• Horizontal asymptotes: as $x \to \infty$, $\arctan x \to \dfrac{\pi}{2}$; as $x \to -\infty$, $\arctan x \to -\dfrac{\pi}{2}$. The curve flattens toward these lines forever.

• Odd function: $\arctan(-x) = -\arctan x$, so the graph is symmetric through the origin.

• Always increasing: the output rises steadily across the whole real line, with no flat sections.

The contrast with inverse sine is worth holding: arcsin has a bounded domain $[-1, 1]$ and a closed range, while arctan has an unbounded domain and an open range. Same idea, opposite shape.

Examples Of The Inverse Tangent Function

[IMAGE PROMPT: A right triangle with the opposite side 1 and adjacent side 1, the base angle θ marked, and the label tan θ = 1/1 = 1, so θ = arctan(1) = π/4 = 45°. Beside it, a coordinate-plane panel shows the point (1, 1) with a ray from the origin making a 45° angle, illustrating arctan turning the y-over-x ratio into a direction. Caption: "Arctan converts an opposite-over-adjacent ratio, or a y-over-x coordinate ratio, into an angle." Alt text: Right triangle and coordinate point both showing arctan of one equals forty-five degrees or pi over four.]

Example 1

Evaluate $\arctan 1$.

Which angle in $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$ has a tangent of $1$?

$$\tan\left(\dfrac{\pi}{4}\right) = 1, \quad \text{and } \dfrac{\pi}{4} \text{ is inside the range.}$$

Final answer: $\arctan 1 = \dfrac{\pi}{4}$.

Example 2

Evaluate $\arctan(-\sqrt{3})$. First instinct, then the correct route.

The tempting move is to find the reference angle $\dfrac{\pi}{3}$ and report $\dfrac{2\pi}{3}$, since that angle also has a tangent related to $\sqrt{3}$ and it feels like the natural "second-quadrant" partner.

Check it against the range. $\dfrac{2\pi}{3}$ is $120^\circ$, outside $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$, and its tangent is actually $-\sqrt{3}$ only because it sits in the wrong branch. Arctan is not allowed to return it.

The rescue: arctan is odd, and a negative input gives a negative angle inside the range.

$$\arctan(-\sqrt{3}) = -\arctan(\sqrt{3}) = -\dfrac{\pi}{3}$$

Final answer: $\arctan(-\sqrt{3}) = -\dfrac{\pi}{3}$.

Example 3

A ramp rises $3$ metres over a horizontal run of $10$ metres. Find its angle of incline.

The tangent of the angle is $\dfrac{\text{rise}}{\text{run}} = \dfrac{3}{10}$.

$$\theta = \arctan\left(\dfrac{3}{10}\right) \approx 16.7^\circ$$

Final answer: the incline is about $16.7^\circ$.

Example 4

As $x$ grows without bound, what does $\arctan x$ approach?

Tangent shoots to infinity as the angle nears $\dfrac{\pi}{2}$. Running that backward, ever-larger inputs push arctan toward, but never onto, $\dfrac{\pi}{2}$.

$$\lim_{x \to \infty} \arctan x = \dfrac{\pi}{2}$$

Final answer: it approaches $\dfrac{\pi}{2}$ as a horizontal asymptote.

Example 5

Evaluate $\tan(\arctan 12)$.

Tangent and arctan undo each other, and arctan accepts every real input, so no domain worry here.

$$\tan(\arctan 12) = 12$$

Final answer: $12$.

Example 6

Find $\sin(\arctan x)$ as an algebraic expression.

Let $\theta = \arctan x$, so $\tan\theta = \dfrac{x}{1}$. Build a right triangle with opposite $x$ and adjacent $1$; the hypotenuse is $\sqrt{x^2 + 1}$. Since $\theta$ is in $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$, the sign follows $x$.

$$\sin(\arctan x) = \dfrac{x}{\sqrt{x^2 + 1}}$$

Final answer: $\dfrac{x}{\sqrt{x^2 + 1}}$. Substitutions like this are the reason arctan appears throughout integral calculus.

Where Is The Inverse Tangent Function Used?

Inverse tangent exists to convert a ratio of two lengths into the angle between them, and its unbounded domain is exactly why it, rather than arcsin, is the coordinate-to-angle workhorse.

• Computer graphics and games. The extended version atan2(y, x) reads a point's coordinates and returns its direction, which is how a character turns to face the cursor or a camera tracks a target.

• Navigation and robotics. Heading from velocity components, bearing from a position offset, and the tilt of a sensor are all recovered with arctan.

• Signal processing. The phase angle of a complex number is $\arctan\left(\dfrac{\text{imaginary part}}{\text{real part}}\right)$, central to Fourier analysis and audio work.

What single-ratio arctan cannot do is tell which half of the plane a point is in, because $\dfrac{1}{1}$ and $\dfrac{-1}{-1}$ both give $1$. That is precisely the gap atan2 fills by reading the signs of both coordinates separately. The Wolfram MathWorld entry on the inverse tangent sets out the full definition and its series expansion.

Common Mistakes With The Inverse Tangent Function

Mistake 1: Reading $\tan^{-1}x$ As $\dfrac{1}{\tan x}$

Where it slips in: Whenever the $-1$ superscript is treated as a power.

Don't do this: Writing $\tan^{-1}(1) = \dfrac{1}{\tan 1}$.

The correct way: $\tan^{-1}$ is the inverse function returning the angle $\dfrac{\pi}{4}$. The reciprocal $\dfrac{1}{\tan x}$ is cotangent, a different function. Reading the symbol aloud as "arctan" every single time keeps the two apart.

Mistake 2: Forcing The Endpoints Into The Range

Where it slips in: Evaluating arctan of a very large number and rounding the answer up to exactly $\dfrac{\pi}{2}$.

Don't do this: Writing $\arctan(1000) = \dfrac{\pi}{2}$.

The correct way: The range is open. $\arctan(1000)$ is extremely close to $\dfrac{\pi}{2}$ but strictly less than it. The second-guesser who "tidies" the answer to the round value $\dfrac{\pi}{2}$ introduces an error that the asymptote definition specifically forbids.

Mistake 3: Trusting Arctan For The Quadrant

Where it slips in: Recovering a direction from coordinates when both are negative.

Don't do this: Taking a point at $(-1, -1)$, computing $\arctan\left(\dfrac{-1}{-1}\right) = \arctan 1 = \dfrac{\pi}{4}$, and reporting a first-quadrant heading.

The correct way: The true direction is $\dfrac{5\pi}{4}$, deep in the third quadrant. Plain arctan lost the sign information the moment the two negatives cancelled. This exact quadrant collapse has bitten real navigation code: the atan2 function exists precisely because using single-ratio arctan for headings sends objects the wrong way. Use both coordinates, not their quotient, when direction matters.

Key Takeaways

• The inverse tangent function $\arctan x$ returns the angle in $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$ whose tangent is $x$.

• Its domain is all real numbers and its range is the open interval $\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$.

• The graph has horizontal asymptotes at $y = \pm\dfrac{\pi}{2}$, approached but never reached.

• $\tan^{-1}x$ is the inverse function, never the reciprocal $\dfrac{1}{\tan x}$.

• For a full direction from coordinates, use atan2, not single-ratio arctan.

To take inverse tangent further with a teacher, explore Bhanzu's trigonometry tutor sessions, work with a high school math tutor on graphing asymptotes, or join live math tutoring with peers from 20+ countries.

A Practical Next Step

Practice these to solidify your understanding: evaluate $\arctan\left(\dfrac{1}{\sqrt{3}}\right)$, then simplify $\cos(\arctan x)$ with a right triangle. If you get stuck on the open range, return to the asymptote section above. Want a live Bhanzu trainer to graph these with you? Book a free demo class.

Read More

• Arctan — the arctan formula, identities, and graph as a reference lookup.

• Arctan 0 — the value $\arctan 0$ worked through step by step.

• Arctan 1 — evaluating $\arctan 1$ and where it appears.

• Arctan 2 — a non-special-angle arctan value and its decimal.

• Inverse Trigonometric Ratios — all six inverse ratios side by side.