Triangle calculator
Solve any triangle from three values
Enter any three values, including at least one side. Typing into a fourth hands the oldest back to the calculator.
Side a is opposite angle A, and so on. Use any length unit, the same for all three sides.
Result
Your answer appears here as you type.
Triangle calculator
Enter any three sides and angles to solve the rest — including the ambiguous case where two different triangles fit — with area, perimeter, heights and the inscribed and circumscribed circles.
Frequently asked questions
How do you find the missing side of a right triangle?
Use the Pythagorean theorem, a² + b² = c², where c is the hypotenuse: the longest side, opposite the right angle. To find the hypotenuse, add the squares of the other two sides and take the square root, so sides of 3 and 4 give √(9 + 16) = 5.
To find one of the shorter sides, subtract instead: b = √(c² − a²). If you know a side and an angle rather than two sides, use sine, cosine or tangent, or enter the three values above and the calculator solves the rest, as with sides 3 and 4 and a right angle.
How do you find the angles of a right triangle?
The three angles of any triangle add up to 180°, so in a right triangle the other two add up to 90°. Find one of them with inverse trigonometry and subtract it from 90° to get the other.
Use whichever two sides you know: angle = arctan(opposite ÷ adjacent), arcsin(opposite ÷ hypotenuse) or arccos(adjacent ÷ hypotenuse). In a 3-4-5 triangle, arctan(3 ÷ 4) ≈ 36.87°, so the other angle is 53.13°.
How do you find the area of a triangle?
Multiply the base by the height and halve it: area = ½ × base × height, with the height measured at right angles to the base. A triangle with a 10 ft base and a 6 ft height covers 30 square feet. Measure in feet to get square feet, or in metres to get square metres.
If you know the three sides but not the height, use Heron’s formula, or enter the sides above: the calculator gives the area, the perimeter and all three heights. Working backwards, a height is twice the area divided by its base.
Guides
About these measurements
Readings are estimates taken from rendered pixels and consumer device sensors. Do not rely on them for structural engineering, surveying, manufacturing inspection, navigation, or medical decisions. Check anything safety-critical with a calibrated instrument.
All processing happens in your browser. No image, camera frame or sensor reading is ever uploaded.
More calculators
Angle converter
Degrees, radians, gradians and more
Convert an angle between degrees, degrees-minutes-seconds, radians, gradians, turns, arcminutes, arcseconds, mils and milliradians, with its slope, related angles and trig values.
Polygon angle calculator
Interior, exterior and missing angles
Find the interior, exterior and central angles of any regular polygon, work out how many sides an angle implies, or find the missing angle of any polygon — with area and radii from a side length.
Complementary and supplementary angles
Complement, supplement and explement
Find the complement, supplement and explement of an angle, or check whether two angles are complementary, supplementary or explementary.