What Are Trigonometric Ratios In Radians?
Trigonometric ratios in radians are the values of sine, cosine, tangent, and their reciprocals when the input angle is expressed in radians. The ratio definitions are unchanged — radians are just the second standard way of naming an angle, and the one used everywhere past school.
The six ratios, with $\theta$ in radians:
$$\sin\theta, \quad \cos\theta, \quad \tan\theta = \frac{\sin\theta}{\cos\theta}$$
$$\csc\theta = \frac{1}{\sin\theta}, \quad \sec\theta = \frac{1}{\cos\theta}, \quad \cot\theta = \frac{1}{\tan\theta}$$
The mnemonic SOH-CAH-TOA still works for an acute angle in a right triangle: Sine = Opposite/Hypotenuse, Cosine = Adjacent/Hypotenuse, Tangent = Opposite/Adjacent. SOH-CAH-TOA is taught in degrees, but the right triangle doesn't know the difference — a $\frac{\pi}{6}$ angle is the same wedge as a $30°$ angle. The radian label is just the arc-length way of measuring that same opening.
The Trigonometric Ratios In Radians Table (Standard Angles)
Here is the value of every ratio at the five standard angles. This is the table to know cold — almost every radian problem at this level reduces to one of these five inputs.
Angle (deg) | Angle (rad) | $\sin\theta$ | $\cos\theta$ | $\tan\theta$ | $\csc\theta$ | $\sec\theta$ | $\cot\theta$ |
|---|---|---|---|---|---|---|---|
$0°$ | $0$ | $0$ | $1$ | $0$ | undefined | $1$ | undefined |
$30°$ | $\frac{\pi}{6}$ | $\frac{1}{2}$ | $\frac{\sqrt{3}}{2}$ | $\frac{1}{\sqrt{3}}$ | $2$ | $\frac{2}{\sqrt{3}}$ | $\sqrt{3}$ |
$45°$ | $\frac{\pi}{4}$ | $\frac{1}{\sqrt{2}}$ | $\frac{1}{\sqrt{2}}$ | $1$ | $\sqrt{2}$ | $\sqrt{2}$ | $1$ |
$60°$ | $\frac{\pi}{3}$ | $\frac{\sqrt{3}}{2}$ | $\frac{1}{2}$ | $\sqrt{3}$ | $\frac{2}{\sqrt{3}}$ | $2$ | $\frac{1}{\sqrt{3}}$ |
$90°$ | $\frac{\pi}{2}$ | $1$ | $0$ | undefined | $1$ | undefined | $0$ |
A quicker way to hold the sine row: write $\frac{\sqrt{0}}{2}, \frac{\sqrt{1}}{2}, \frac{\sqrt{2}}{2}, \frac{\sqrt{3}}{2}, \frac{\sqrt{4}}{2}$ for $0, \frac{\pi}{6}, \frac{\pi}{4}, \frac{\pi}{3}, \frac{\pi}{2}$. The cosine row is the same list reversed. For a degree-first version of the same values, see the trigonometric chart.
How do you convert degrees to radians? Multiply the degree measure by $\frac{\pi}{180}$. To go the other way, multiply radians by $\frac{180}{\pi}$. So $30° \times \frac{\pi}{180} = \frac{\pi}{6}$, and $\frac{\pi}{4} \times \frac{180}{\pi} = 45°$. (For a worked conversion in the other direction, 1 radian to degrees shows the arithmetic for a non-standard value.)
Examples Of Trigonometric Ratios In Radians
Example 1
Find $\sin\frac{\pi}{6}$.
$\frac{\pi}{6}$ is the radian name for $30°$.
From the table, $\sin 30° = \frac{1}{2}$.
Final answer: $\sin\frac{\pi}{6} = \frac{1}{2}$.
Example 2
Evaluate $\cos\frac{\pi}{3} + \sin\frac{\pi}{6}$. A common first instinct is to "add the angles first" — to compute $\cos\left(\frac{\pi}{3} + \frac{\pi}{6}\right) = \cos\frac{\pi}{2}$ and call the answer $0$.
Watch why that breaks. The expression is two separate values being added, not one cosine of a summed angle — trig functions don't distribute over addition that way. If you could merge them, you'd be claiming $\cos A + \sin B = \cos(A+B)$, which fails even a quick check: $\cos 0 + \sin 0 = 1 + 0 = 1$, but $\cos(0+0) = 1$ only by coincidence here, and $\cos\frac{\pi}{2} + \sin 0 = 0$ while $\cos\frac{\pi}{2} = 0$ too — the agreement is accidental, and it collapses for most inputs.
The correct route is to evaluate each term on its own:
$$\cos\frac{\pi}{3} = \frac{1}{2}$$
$$\sin\frac{\pi}{6} = \frac{1}{2}$$
$$\frac{1}{2} + \frac{1}{2} = 1$$
Final answer: $1$.
Example 3
Find $\tan\frac{\pi}{4}$ using the right-triangle definition, then confirm from the table.
A $45°$ angle sits in an isosceles right triangle where opposite = adjacent.
$$\tan\frac{\pi}{4} = \frac{\text{opposite}}{\text{adjacent}} = \frac{1}{1} = 1$$
The table agrees: $\tan\frac{\pi}{4} = 1$.
Final answer: $1$.
Example 4
Evaluate $\csc\frac{\pi}{3}$.
Cosecant is the reciprocal of sine.
$$\sin\frac{\pi}{3} = \frac{\sqrt{3}}{2}$$
$$\csc\frac{\pi}{3} = \frac{1}{\sin\frac{\pi}{3}} = \frac{1}{\frac{\sqrt{3}}{2}} = \frac{2}{\sqrt{3}}$$
Final answer: $\csc\frac{\pi}{3} = \frac{2}{\sqrt{3}}$ (or $\frac{2\sqrt{3}}{3}$ when rationalised).
Example 5
A wheel turns through $\frac{\pi}{4}$ radians. What are $\sin$ and $\cos$ of that turn, and what point on the unit circle does it land on?
$\frac{\pi}{4}$ is $45°$, the first-quadrant diagonal.
$$\sin\frac{\pi}{4} = \frac{1}{\sqrt{2}}, \qquad \cos\frac{\pi}{4} = \frac{1}{\sqrt{2}}$$
On the unit circle, the point is $(\cos\theta, \sin\theta) = \left(\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}}\right)$.
Final answer: both ratios equal $\frac{1}{\sqrt{2}}$; the point is $\left(\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}}\right)$.
Example 6
Simplify $\frac{\sin\frac{\pi}{3}}{\cos\frac{\pi}{6}}$.
$$\sin\frac{\pi}{3} = \frac{\sqrt{3}}{2}, \qquad \cos\frac{\pi}{6} = \frac{\sqrt{3}}{2}$$
$$\frac{\sin\frac{\pi}{3}}{\cos\frac{\pi}{6}} = \frac{\frac{\sqrt{3}}{2}}{\frac{\sqrt{3}}{2}} = 1$$
This is no accident — $\frac{\pi}{3}$ and $\frac{\pi}{6}$ are complementary (they add to $\frac{\pi}{2}$), so $\sin\frac{\pi}{3} = \cos\frac{\pi}{6}$.
Final answer: $1$.
Where Radian Ratios Earn Their Keep
Radians look like extra notation until you meet the place they were built for: the rate-of-change machinery behind every wave, orbit, and signal. The derivative of $\sin\theta$ is exactly $\cos\theta$ — but only when $\theta$ is in radians. In degrees you'd carry an ugly factor of $\frac{\pi}{180}$ through every line of calculus, so physics and engineering simply abandoned degrees.
That choice shows up wherever something oscillates or rotates:
Circular motion and orbits — an object's angular position is tracked in radians per second, so satellite and planetary models are written in radians from the first line.
Waves and signals — sound, light, and alternating current are modelled as $\sin(\omega t)$ with $\omega$ in radians per second; the trigonometric functions become functions of time.
The arc-length shortcut — arc length on a circle is simply $s = r\theta$ when $\theta$ is in radians. That clean formula is the whole reason the unit was defined this way, and it falls apart in degrees.
This is the destination: by the time a student reaches calculus, radians aren't an alternative to degrees — they're the only measure that keeps the math honest.
Tripping Points To Avoid
Mistake 1: Leaving the calculator in degree mode
Where it slips in: Any problem that gives the angle in radians — a $\sin(1.2)$ or $\cos\frac{\pi}{5}$ — typed into a calculator still set to degrees.
Don't do this: Type $\sin(1.2)$ in degree mode and read off $0.0209$.
The correct way: Switch to radian mode first; $\sin(1.2\ \text{rad}) \approx 0.932$. When the angle carries a $\pi$ or no degree symbol, it's radians — set the mode before you press a key.
Mistake 2: Reading $\frac{\pi}{6}$ as "$\pi$ divided by 6 degrees"
Where it slips in: Converting or evaluating a standard angle when the radian value still feels unfamiliar.
Don't do this: Treat $\frac{\pi}{6}$ as some fraction of degrees and look for $\sin$ of a tiny angle.
The correct way: $\frac{\pi}{6}$ radians is $30°$. The whole fraction names one angle; memorise the five standard pairs so the translation is instant.
Mistake 3: Distributing a ratio over a sum of angles
Where it slips in: Expressions like $\sin\left(\frac{\pi}{6} + \frac{\pi}{3}\right)$, where it's tempting to write $\sin\frac{\pi}{6} + \sin\frac{\pi}{3}$.
Don't do this: Split the function across the sum. $\sin\left(\frac{\pi}{2}\right) = 1$, but $\sin\frac{\pi}{6} + \sin\frac{\pi}{3} = \frac{1}{2} + \frac{\sqrt{3}}{2} \approx 1.37$ — not equal.
The correct way: Evaluate the angle inside the bracket first, or use the proper sum and difference identities. The reciprocal-versus-inverse swap and this distribute-over-sum habit are the two errors that quietly produce most wrong answers in radian work — naming them out loud is half the cure.
Key Takeaways
Trigonometric ratios in radians are the same six ratios as in degrees — only the angle's label changes, because $360° = 2\pi$ radians.
The five standard angles $0, \frac{\pi}{6}, \frac{\pi}{4}, \frac{\pi}{3}, \frac{\pi}{2}$ correspond to $0°, 30°, 45°, 60°, 90°$ — memorise the table.
Convert with $\times\frac{\pi}{180}$ (degrees to radians) and $\times\frac{180}{\pi}$ (radians to degrees).
Radians are the standard in calculus and physics because $\frac{d}{d\theta}\sin\theta = \cos\theta$ and $s = r\theta$ only hold in radians.
Always set the calculator's angle mode to match the angle in front of you.
To build fluency with radian-based ratios alongside a teacher, explore Bhanzu's trigonometry tutor sessions, work with a high school math tutor, or browse math classes online.
Read More
What Is a Radian — the radian measure explained from arc length.
Trigonometric Table — the full standard-angle reference table.
Value of Sin pi — a worked radian-value evaluation.
Radians to Degrees — the conversion rule with step-by-step examples.
Sin Cos Tan — a focused look at the three primary ratios.
Unit Circle — the standard angles as points on a circle.
Was this article helpful?
Your feedback helps us write better content