Isosceles Triangles Calculator

Isosceles Triangle
Given lengths of sides a and b;
Find c, P, s, K, ha, hb and hc
a =
b =
c =
P =
s =
K =
ha =
hb =
hc =

Isosceles Triangle Shape

Isosceles Triangle Diagram with Angles A, B and C and sides opposite those angles a, b and c respectively
A = angle A
a = side a
B = angle B
b = side b
C = angle C
c = side c
K = area
P = perimeter

See Diagram Below:
ha = altitude of a
hb = altitude of b
hc = altitude of c

Isosceles Triangle Diagram with Angles A, B and C and altitudes ha, hb and hc respectively

Notes and Formulas for Isosceles Triangle Calculations:

An isosceles triangle is a special case of a triangle where 2 sides, a and c, are equal and 2 angles, A and C, are equal.

In our calculations for a right triangle we only consider 2 known sides to calculate the other 7 unknowns. For example, if we know a and b we know c since c = a.. Once we know sides a, b, and c we can calculate the perimeter = P, the semiperimeter = s, the area = K, and the altitudes: ha, hb, and hc. Let us know if you have any other suggestions!

Formulas and Calculations for an isosceles triangle:

  • Sides of Isosceles Triangle: a = c
  • Angles of Isosceles Triangle: A = C
  • Altitudes of Isosceles Triangle: ha = hc
  • Perimeter of Isosceles Triangle: P = a + b + c = 2a + b
  • Semiperimeter of Isosceles Triangle: s = (a + b + c) / 2 = a + (b/2)
  • Area of Isosceles Triangle: K = (b/4) * √(4a2 - b2)
  • Altitude a of Isosceles Triangle: ha = (b/2a) * √(4a2 - b2)
  • Altitude b of Isosceles Triangle: hb = (1/2) * √(4a2 - b2)
  • Altitude c of Isosceles Triangle: hc = (b/2a) * √(4a2 - b2)

Given sides a and b find side c and the perimeter, semiperimeter, area and altitudes

  • a and b are known; find c, P, s, K, ha, hb, and hc
  • c = a
  • P = 2a + b
  • s = a + (b/2)
  • K = (b/4) * √(4a2 - b2)
  • ha = (b/2a) * √(4a2 - b2)
  • hb = (1/2) * √(4a2 - b2)
  • hc = (b/2a) * √(4a2 - b2)

For more information on right triangles see:

Weisstein, Eric W. "Isosceles Triangle." From MathWorld--A Wolfram Web Resource.


Cite this content, page or calculator as:

Furey, Edward "Isosceles Triangles Calculator" From - Online Calculator Resource.