Perimeter Calculator
Pick a shape, enter the dimensions, and get the perimeter in metres and feet — for fencing, framing, landscaping, and geometry homework.
Calculate
Calculate
Measurements convert when you switch units
Result
The formula
Rectangle perimeter is 2(L + W). Square is 4 × side. Circle circumference is 2πr or πd. Ellipse uses Ramanujan's approximation. Triangle is a + b + c. Parallelogram is 2(a + b). Trapezoid is a + b + c + d. Results convert with 1 m ≈ 3.28084 ft.
Worked example
- Rectangle 5 m × 4 m → 2(5+4) = 18 m
- Circle r = 2 m → 2π×2 ≈ 12.57 m
- Triangle 3 m, 4 m, 5 m → 3+4+5 = 12 m
Result: Shape formulas with automatic m/ft display
How multi-shape perimeter is calculated
Each mode applies the standard perimeter formula for that shape, then converts between metres and feet.
Rectangles, squares, and circles
A rectangle perimeter is 2 times (length plus width). A square is 4 times the side length since all sides are equal. A circle uses circumference: 2πr where r is the radius, or πd where d is the diameter. An ellipse uses Ramanujan's first approximation for more accuracy than simpler formulas.
Triangles, parallelograms, and trapezoids
Triangle perimeter is simply the sum of all three sides (a + b + c). A parallelogram has opposite sides equal, so perimeter is 2(a + b). A trapezoid has four sides with two parallel; perimeter is the sum of all four sides (a + b + c + d).
Units
Metric mode expects metres and reports m first. Imperial mode expects feet and reports ft first. Mixed units are not converted automatically — convert sides first if needed.
What we do not model
No border overlap, corner radius smoothing, or GIS survey feet. For legal property perimeter, use a surveyor's plan.
Interesting facts
π is irrational
Circle and ellipse perimeter use Math.PI (≈ 3.141592653589793). Display rounds; engineering tolerances may need more digits.
Ellipse circumference is complex
No simple closed formula exists; this calculator uses Ramanujan's first approximation: π(a+b)[1 + 3h/(10+√(4−3h))] where h = (a−b)²/(a+b)².
Triangle inequality matters
Three sides can only form a triangle if the sum of any two sides exceeds the third. Sides 1, 2, 5 cannot close.
m ↔ ft
1 m ≈ 3.28084 ft using the international foot (0.3048 m exactly).
Composite perimeter
For L-shaped or irregular boundaries, break them into segments, sum each segment, and add shared edges as needed.
Frequently asked questions
Choose Circle, enter the radius (or diameter ÷ 2), and calculate. Circumference = 2πr or πd. If you know diameter, divide by 2 first for radius.
Perimeter = 2 × (length + width). For a 5 m × 4 m rectangle, P = 2×(5+4) = 18 m.
Add all three sides: P = a + b + c. For sides 3 m, 4 m, 5 m, perimeter = 12 m.
No — pick metric or imperial so all inputs share a unit. Convert lengths first with the unit converter if needed.
Choose Ellipse and enter the semi-major axis (a) and semi-minor axis (b). The calculator uses Ramanujan's approximation since no simple formula exists.
Radius is half the diameter. If diameter is 10 m, radius is 5 m. Circumference = 2πr = πd.
Break it into segments (rectangles, lines), calculate each perimeter segment, and add them. Subtract any internal edges if they don't form part of the outer boundary.
Perimeter = 4 × side = 4 × 5 = 20 m. A square is a special rectangle with all sides equal.
References
- SI units — metre SI length basis for perimeter units.
- International foot International foot = 0.3048 m exactly.
- Ramanujan's ellipse approximation Accurate approximation for ellipse circumference when simpler formulas don't suffice.