Isosceles Triangle Calculator

An isosceles triangle is one with two sides the same length, the familiar symmetric triangle you see in roof gables, gable ends, warning signs, coat hangers and countless designs, and this calculator works out all of its measurements from just two inputs: the length of the two equal sides and the length of the base. Enter those and it returns the height, the area, the perimeter, the apex angle at the top and the two equal base angles, all updating as you type. The key to an isosceles triangle is its line of symmetry, which runs from the apex straight down to the middle of the base, splitting the shape into two identical right triangles. That single idea unlocks everything: the height is found with Pythagoras from the equal side and half the base, the area is then simply half the base times that height, and the angles follow from basic trigonometry. The two base angles are always equal to each other, sitting opposite the two equal sides, and together with the apex angle they add to 180 degrees, so knowing one tells you the others. The calculator also checks that your sides can actually form a triangle, since the two equal sides must be long enough to reach across the base. That makes the tool genuinely useful for students learning geometry and trigonometry and checking their working, for builders and roofers setting out gables and trusses where the pitch and span matter, for crafters and sign-makers, and for anyone needing the height, area or angles of a symmetric triangle. Because the results update live, you can adjust the base or the equal sides and watch the triangle grow taller or flatter and the angles shift in response. The formulas and a worked example are explained clearly below.

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