Perimeter Calculator
Calculate the perimeter (or circumference) of rectangles, squares, circles, triangles, trapezoids and parallelograms. Optional cost-per-unit for fencing, trim and framing estimates.
Leave at 0 to skip — useful for fencing, framing, trim
Perimeter
36 units
Shape
Rectangle
Perimeter
36
Formula
Perimeter = 2 × (width + height)
The standard equation
The calculation
Perimeter = 2 × (width + height)
2 × (10 + 8) = 36
Perimeter is the distance around the outside of a shape — useful for fencing, framing, edging, and trim. For a circle, this is its circumference. For area, use the Area Calculator.
How to use this calculator
- Pick a shape from the toggle — rectangle, square, circle, triangle, trapezoid or parallelogram.
- Enter the measurements the shape needs.
- Read the perimeter (or circumference, for a circle) in the same units you measured in.
- Optional: enter a cost per unit to estimate fencing, framing or trim costs — total cost = perimeter × cost.
How it works
Perimeter is just the sum of all the outside edges:
- Rectangle —
2 × (width + height) - Square —
4 × side - Circle —
2 × π × r(called circumference for circles specifically) - Triangle —
a + b + c - Trapezoid —
a + b + c + d(sum of all four sides) - Parallelogram —
2 × (base + slanted side)
For triangles, the calculator checks the triangle inequality — every side must be shorter than the sum of the other two, or the shape can’t physically exist. For just the area of common shapes, use the Area Calculator.
Frequently Asked Questions
What is perimeter? ▾
Perimeter is the total distance around the outside of a 2D shape. If you walked along the edge once, the perimeter is how far you'd walk. It's measured in the same units as the sides (feet, metres, inches) — not in square units, which is for area.
What is the difference between perimeter and circumference? ▾
They mean the same thing, but circumference is used specifically for circles and perimeter is used for everything else. The formula for a circle's circumference is 2 × π × r — that's a special case of perimeter where the 'side' is curved.
Can any three numbers be the sides of a triangle? ▾
No. To form a real triangle, every side must be strictly shorter than the sum of the other two — the triangle inequality. For example, sides 1, 2 and 10 can't form a triangle: 1 + 2 = 3, which is less than 10. The calculator flags this and won't return a perimeter for impossible sides.
Is the slanted side of a parallelogram the same as its perpendicular height? ▾
No — and this trips people up for area, not perimeter. For perimeter the slanted side IS what you want (it's the actual edge you'd walk along). For area, you need the perpendicular height instead. The Area Calculator uses height; this one uses the slanted side.
When would I need to calculate perimeter? ▾
Fencing a yard, framing a picture, adding edging or trim, ordering a baseboard or border, calculating wallpaper trim. Anywhere the cost or amount scales with distance around an edge.
Can the calculator estimate the cost of fencing or trim? ▾
Yes — enter a cost per unit in the optional field below the inputs (e.g. $8 per linear foot for fence panels). The result panel shows the total cost as perimeter × cost. Leave the field at 0 to skip the cost estimate.