Perimeter is the total distance around the edge of a flat shape. For polygons you just add the sides — 2(l + w) for a rectangle, 4s for a square, a + b + c for a triangle. For a circle, the same idea is the circumference, 2πr. This calculator finds the perimeter (or circumference) for each shape and the area as well, showing the formula it used.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: standard perimeter and area formulas, recomputed in code.
Perimeter formulas by shape
Every polygon's perimeter is just the sum of its sides; the rectangle and square formulas are shortcuts because their sides repeat. A circle has no straight sides, so its "perimeter" is the circumference, 2π times the radius. Perimeter always comes out in linear units — the same unit you measured the sides in — while the area the calculator also gives is in square units.
Worked example — rectangle 8 × 5
Scenario: a rectangle 8 long and 5 wide.
The rectangle's perimeter is 26 and its area is 40 square units — you'd need 26 units of fencing but 40 units of turf. A square of side 6 has perimeter 24 and area 36, and a 3-4-5 triangle has perimeter 12. For a circle of radius 7 the circumference is 43.98 and the area 153.94. Perimeter and area answer different questions: distance around versus space inside.
Frequently Asked Questions
The total distance around a 2D shape — the sum of its sides. For a circle it's the circumference. Measured in linear units.
P = 2(l + w). An 8 by 5 rectangle: 2 × 13 = 26. Its area is l × w = 40.
The circumference: C = 2πr = πd. Radius 7 → ≈ 43.98; area πr² ≈ 153.94.
Perimeter is the distance around (a length); area is the space inside (square units). 8×5: perimeter 26, area 40.
Add the three sides: P = a + b + c. A 3-4-5 triangle has perimeter 12. No angles needed.