Equilateral Triangle Perimeter Calculator

P = 3 × s

Three equal sides. A 10 cm equilateral triangle has a 30 cm perimeter. An equilateral triangle with 1 ft sides has a 3 ft (36 in) perimeter.

Where equilateral triangles show up in real life:

• Warning signs. A US Yield sign is a 36-inch-equal-sided triangle. Perimeter = 108 in = 9 ft of edge trim per sign.
• Geodesic domes. A typical hobby dome uses equilateral triangles with sides around 3 to 4 ft. Each panel has 9 to 12 ft of edge — important for sealing seams.
• Triangle road signs in many countries. Standard 70-cm side = 210 cm = 2.10 m perimeter.
• Truss design. Equilateral triangles are extremely efficient for load-bearing structures because they distribute force equally on all three sides.
• Triangular tent footprints. A 6 ft equilateral base has 18 ft of ground contact.
• Geometric jewelry. Pendant designs in 1-2 inch equilateral shapes.

Worked example — geodesic dome edging:

You’re building a 12 ft diameter geodesic dome from 3 ft equilateral triangle panels. Each panel perimeter = 9 ft. If the dome has 25 triangles, total edge length = 225 ft — but every interior edge is shared between two triangles, so the actual frame length is about half of that, around 112 ft.

Bonus formulas — everything from the side s:

• Perimeter: P = 3s
• Height: h = s × √3 / 2 ≈ 0.866 × s
• Area: A = (√3 / 4) × s² ≈ 0.4330 × s²
• Inradius (inscribed circle): r = s × √3 / 6 ≈ 0.289 × s
• Circumradius (circumscribed circle): R = s × √3 / 3 ≈ 0.577 × s

Comparison to other triangles:

For a given perimeter, the equilateral triangle has the largest area of any triangle. A perimeter of 30 ft, equilateral, gives sides of 10 ft each and area ≈ 43.3 sq ft. A 3-12-15 triangle (also perimeter 30) gives area ≈ 18 sq ft — less than half.

This “max area for given perimeter” property is why bubbles in a regular packing arrange in 120° angles (matching equilateral interior angles) and why honeycomb cells, which are forced into 6-sided shapes by neighboring cells, end up with 120° angles between walls.

Quick check. The equilateral triangle is the most symmetric triangle possible. All sides equal, all angles equal (60° each). Sum of interior angles is always 180° for any triangle, so 3 × 60° = 180° works out cleanly.