Perimeter Calculator
The perimeter is the total length of a shape's edge: what you'd need in fencing, baseboard, edging or trim. Pick a shape and enter its sides.
P = 2(l + w)
- Area
- 96 ft²
- A = l × w = 12 × 8 = 96 ft²
- P = 2(12 + 8) = 40 ft
Area in other units
| m² | 8.9187 |
| cm² | 89,186.9 |
| ft² | 96 |
| in² | 13,824 |
| yd² | 10.6667 |
| acres | 0.002204 |
| hectares | 0.000892 |
Perimeter of a triangle or any polygon
Add the lengths of every side. For a triangle with sides 5, 7 and 9, the perimeter is 21. If you only know two sides and an angle, the triangle calculator finds the third side first.
Perimeter formulas
- Square: P = 4s
- Rectangle: P = 2(l + w)
- Circle (circumference): C = 2πr = πd
- Regular polygon: P = n × s
- Trapezoid: a + b + both legs
- Ellipse: no exact elementary formula; Ramanujan's approximation is used here
Practical tip: when buying trim or fencing, subtract door openings or gates, then add 10% for offcuts and corners.
Questions people ask
Is circumference the same as perimeter?
Yes. Circumference is simply the name for the perimeter of a circle.
Can two shapes have the same perimeter but different areas?
Yes. With 20 m of fence you can enclose a 5 × 5 square (25 m²) or a 9 × 1 strip (9 m²). For a fixed perimeter a circle encloses the most area.
What units does perimeter use?
Plain length units — ft, m, in — not square units.
Stuck on a word problem?
Paste a homework question or describe your project. The AI tutor names the formula and works it through step by step. Always check the result with the calculator above.