Driehoekcalculator
The detailed guide below is currently available in English.
Three ways to define a triangle
A triangle is fixed by three suitable facts. SSS gives all three sides; SAS gives two sides and the angle squeezed between them; ASA gives two angles and the side between them. Side letters match the opposite angles: side a faces angle A, and so on. In SAS the angle must sit between the two given sides — an angle at either end instead belongs to the SSA case, which this calculator does not cover because it can have two different answers.
Angle units and pi shortcuts
Angles default to degrees, but you can switch the Angle Unit to radians. In radian mode each angle box accepts plain decimals as well as short pi expressions such as pi/2, pi/4, or 2pi/3 — pi is the familiar circle constant, so pi/2 means a right angle. Whatever unit you enter angles in, the solved angles come back in that same unit, while sides, area, and perimeter are plain lengths and are unaffected by the choice.
The formulas behind the answers
For SSS, each angle comes from the law of cosines, A = cos⁻¹((b² + c² − a²) ÷ 2bc), rearranged for the other angles in turn. For SAS, the third side comes from the same law written as c² = a² + b² − 2ab·cos C, then the angles follow. For ASA, the missing angle is A = 180° − B − C, and the two unknown sides come from the law of sines, a ÷ sin A = b ÷ sin B = c ÷ sin C. Area uses Heron's formula from the three sides, or ½ab·sin C when the sides around an angle are known. The inradius divides the area by the semiperimeter, the circumradius is abc ÷ 4 × area, and each median is half the square root of twice the sum of the other two sides squared minus its own side squared.
Classifying the result
The results list names each triangle two ways. By sides: equilateral (all three equal), isosceles (exactly two equal), or scalene (no equal sides). By angles: acute (all angles under 90°), right (exactly one 90° angle), or obtuse (one angle over 90°). A right triangle always satisfies the Pythagorean theorem a² + b² = c² with c the longest side, which makes it a handy way to sanity-check the calculator's output.
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Why won’t my three sides solve?
Not every triple of positive numbers forms a triangle: the two shorter sides together must be strictly longer than the longest side. Sides 1, 2 and 3 fail because 1 + 2 equals 3, producing a straight line rather than a triangle, and anything smaller fails outright.
What does “included angle” mean in SAS mode?
It is the angle formed by the two sides you entered, sitting between them. If you know sides a and b, the included angle is C, opposite side c. Enter any other arrangement and the side lengths the solver produces will belong to a different triangle than the one you measured.
How do I type angles in radians?
Switch the Angle Unit to radians, then enter either a decimal (1.5708) or a pi expression (pi/2). The boxes understand plus, minus, multiply, divide, parentheses, and multiples of pi, so 2pi/3 is a valid entry. Results come back in radians while that unit is selected.