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A triangle calculator solves a triangle from the values you know (three sides, two sides and an angle, or two angles and a side) using the law of sines and law of cosines. It returns every missing side and angle plus the area and perimeter. Enter the values you have and leave the rest blank.

You need any three values that include at least one side. From that it picks the right theorem: the law of cosines recovers an angle from three sides or a third side from two sides and their included angle, while the law of sines pairs a known side with its opposite angle to find the rest. Because a triangle’s angles always sum to 180°, once two angles are known the third follows immediately. It then reports the perimeter and the area.

For a worked example, entering the three sides 3, 4 and 5 identifies a right triangle: the angle opposite the longest side is exactly 90°, the other two are about 36.87° and 53.13°, the perimeter is 12 and the area is ½·3·4 = 6. Give it two sides and the included angle instead — say 5, 7 and 40° — and it uses the law of cosines to find the third side. It is used in geometry and trigonometry homework, surveying, construction layout and any job that needs the missing measurements of a triangle.