Perimeter Calculator
Find the perimeter of squares, rectangles, triangles, circles and parallelograms in seconds, with the semi-perimeter included.
Perimeter
20
Semi-perimeter
10
Half the perimeter, used in Heron's formula
The perimeter is the total distance around the shape. For a circle it is called the circumference, found as 2πr.
Last updated: January 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Perimeter is the measurement you reach for whenever something must go around the outside of a shape. In India, that means fencing a plot or balcony railing, quoting for compound-wall boundary work, estimating pipe or cable length for a layout, or buying paper edging for school projects. Municipal approvals and property documents often list plot dimensions, and this calculator converts those dimensions into the running-metre quantity contractors price per metre. It is equally useful for tailoring — a saree border or dupatta edging needs the perimeter of the garment piece — and for framing: the frame length around a photo or mirror equals its perimeter. Students use it constantly while solving mensuration problems, where checking the perimeter of a rectangle or triangle against a hand calculation takes seconds.
How to Use This Calculator
- Step 1: Pick the shape — square, rectangle, triangle, circle or parallelogram.
- Step 2: Enter the side lengths, or the radius for a circle, in any unit such as metres, feet or inches.
- Step 3: Read the perimeter (the circumference for a circle) in the same unit you entered.
- Step 4: Use the semi-perimeter shown below it when working with Heron's formula or inscribed-circle problems.
Worked Example
A homeowner in Bengaluru wants to fence a rectangular plot that is 12 m long and 8 m wide. Selecting the rectangle shape and entering 12 and 8 gives a perimeter of 2 × (12 + 8) = 40 m, which is the exact fencing length to buy. The same plot's semi-perimeter is 20 m. For a circular garden bed with radius 35 cm, the tool reports a circumference of about 219.91 cm — enough rope to edge it completely. Triangle plots, common in corner layouts, simply need all three side lengths added.
Tips and Common Mistakes
- •Tip 1: Keep all inputs in one unit — mixing metres and centimetres will make the result wrong by a factor of 100.
- •Tip 2: For a rectangle you only need length and width; the tool multiplies the sum by 2 for you.
- •Tip 3: When buying fencing or edging, add 5–10% extra for overlaps, corners and cutting waste.
- ✗Mistake 1: Entering the diameter where the radius is expected for a circle, which halves the circumference.
- ✗Mistake 2: Confusing perimeter with area — 40 m of fencing is a length, not the 96 m² the plot covers.
Frequently Asked Questions
What is the perimeter of a shape?
The perimeter is the total distance around the outside of a shape, found by adding up all its sides. For a rectangle with length 4 and width 7, the perimeter is 2 × (4 + 7) = 22 units.
How do I find the perimeter of a rectangle?
Add the length and width, then multiply by 2: perimeter = 2 × (length + width). A plot that is 12 ft by 8 ft has a perimeter of 2 × (12 + 8) = 40 ft, which is the fencing length you would need.
What is the difference between perimeter and circumference?
Both measure the distance around a shape. Circumference is the specific name used for a circle, calculated as 2πr, while perimeter is used for polygons. A circle with radius 7 has a circumference of about 43.98 units.
What is the semi-perimeter used for?
The semi-perimeter is exactly half the perimeter. It appears in Heron's formula for the area of a triangle and in formulas for inscribed circles, so many students and builders use it alongside the perimeter.
Can the perimeter be smaller than the area?
Yes — perimeter and area are different units (length versus squared length), so the numbers cannot be compared directly. A small square of side 0.5 has perimeter 2 but area only 0.25, while a large square behaves the opposite way.
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