Outer faces and curved surfaces
Triangular Prism Surface Area Calculator
Enter the three sides of the triangular end and the prism length. For a right triangle, you can use its two perpendicular legs instead.
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Triangular prism surface area formula
A right triangular prism has three rectangular side faces and two triangular ends. Use A = (a + b + c)L + 2T, where T is the area of one triangular end and L is the prism length.
The lateral area is (a + b + c)L. In “Three sides” mode, the tool calculates T with Heron’s formula: T = √[p(p − a)(p − b)(p − c)], where p = (a + b + c) ÷ 2.
Right-triangle mode
Enter two perpendicular legs a and b. The third side is √(a² + b²), and the triangle area is ab ÷ 2. The tool calculates both automatically. A sloping side cannot be used in place of a perpendicular leg.
Example: sides 3, 4 and 5 ft; prism length 10 ft
The triangle area is 6 ft² and its perimeter is 12 ft. Lateral area = 12 × 10 = 120 ft². Two ends add 12 ft², giving 132 ft², or approximately 12.26 m².
Triangle sides must form a real triangle
The sum of any two side lengths must exceed the third. Values such as 2, 3 and 6 do not form a triangle and are rejected. Three valid sides determine the triangle area without needing an additional height measurement.
This calculator assumes a right prism: its long edges are perpendicular to the triangular end faces. An oblique prism needs different measurements for its side faces.
For capacity instead of exterior area, use the triangular prism volume calculator.
Enter every length in the same unit. Results are rounded for display; calculations use unrounded values and the full precision of π.