Triangle Perimeter Calculator
Compute the perimeter of any triangle from its three sides.
Validates the triangle inequality.
Multiple units supported.
P = a + b + c
Three sides, add them all. Trivial unless the side lengths aren’t actually valid for a triangle.
The triangle inequality says no side can be longer than the sum of the other two. If you try to build a triangle with sides 3, 4, and 10, you can’t — the 3 and 4 together only reach 7, so the 10-cm side can’t close the loop. This calculator flags the violation if you enter impossible side values.
Where this comes up in real work:
- Garden bed edging around an irregular triangular patch. A 4 ft × 5 ft × 6 ft bed needs 15 ft of border timbers.
- Sailmaking. A jib sail with luff (leading edge) 25 ft, foot (bottom) 12 ft, and leech (trailing edge) 22 ft has 59 ft of edge tape needed.
- Truss design. Roof trusses use triangles for rigidity — the perimeter tells you how much lumber is needed for the chord pieces (top, bottom, and webs). A 30 ft × 18 ft truss with diagonal 22.5 ft has 70.5 ft of chord.
- Triangular plot fencing. For an unusual lot, the surveyor gives you three side measurements. Add them for the fence quantity.
- Yield, warning, and triangle road signs. A 36 in side equilateral triangle needs 108 in (9 ft) of edge trim.
Worked example — sail patching:
A storm jib measures 18 ft × 8 ft × 16 ft. You need to add bolt-rope along all three edges. P = 42 ft of bolt-rope. Order 45 ft to allow for the corner reinforcement laps.
Triangle inequality in practice:
If you measure three sticks meant to form a triangle and get 10, 4, and 4, you’ve measured wrong somewhere. 4 + 4 = 8 < 10 — these can’t form a triangle.
For a degenerate case where a + b = c (exactly), the “triangle” collapses to a straight line with zero area. Real-world measurements rarely hit this exactly, but it’s the limiting case.
Quick formulas for special triangles:
- Equilateral (all sides s): P = 3s
- Isosceles (base b, legs s): P = b + 2s
- Right triangle (legs a, b): hypotenuse = √(a² + b²), then P = a + b + √(a² + b²)
- 3-4-5 right triangle: P = 12 (scale up for larger versions: 6-8-10 has P = 24)
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.