The perimeter is the total distance around the outside of a two-dimensional shape. It's the boundary length, measured in linear units (cm, ft, m). For a square: 4 × side. For a rectangle: 2 × (length + width). For a circle: circumference = 2πr.

How do you find a circle's perimeter?

A circle's perimeter is called circumference: C = 2 × π × r (using radius) or C = π × d (using diameter). For r = 5: C = 2π × 5 ≈ 31.42. For d = 10: C = π × 10 ≈ 31.42. Both formulas give same answer.

How is perimeter different from area?

Perimeter: distance around the boundary (linear units like cm, ft, m). Area: amount of space inside (squared units like cm², ft², m²). They measure different things. Double all dimensions: perimeter doubles, area quadruples.

How do I find the perimeter of an irregular shape?

Sum all side lengths. For L-shaped, T-shaped, or other composite shapes: identify every outer edge, measure each, add them up. Skip interior shared edges (only outer perimeter counts). CAD or GIS software computes automatically for drawn shapes.

Which shape has the smallest perimeter for a given area?

A circle. For any given area, a circle requires less boundary than any other shape. This is the isoperimetric inequality. Why bubbles are spherical (minimize surface tension energy), why bees use hexagons (best tile-able polygon), why round containers are efficient.

How much fencing do I need for my yard?

For rectangular yard L × W: P = 2(L+W). Add 5-10% for waste and corners. For 100' × 50' yard: P = 300 ft fence + ~30 ft extra = order 330 ft. Account for gates (subtract from total) and post placement (typically every 6-8 ft).

How is perimeter used in real estate?

Property line length determines fencing needs, easements, and sometimes value. Larger perimeter relative to area suggests irregular or elongated lot. Square lots have minimum perimeter for given area (efficient). Long narrow lots have higher perimeter (more fence).

Can I calculate perimeter from area?

Not uniquely — many shapes can have same area with different perimeters. For circle (only one possible shape for given area): C = 2√(πA). For other shapes: depends on shape. A 100 m² area could be 10×10 square (P = 40 m) or 50×2 rectangle (P = 104 m) — same area, very different perimeters.

Perimeter Calculator

Select a shape and enter its dimensions to calculate the perimeter. Supports rectangles, triangles, circles (circumference), and regular polygons.

Last updated: May 31, 2026

About this calculator

Perimeter is the total distance around the outside of a two-dimensional shape — the boundary length. For a rectangular garden, perimeter tells you how much fence to buy. For a circular pool, it's how much trim to order. For an irregular plot, it's the total walked distance around the border. Calculating perimeter is fundamental in construction, landscaping, sewing, real estate, and any field involving 2D boundaries.

Each shape has its own perimeter formula: - **Rectangle**: 2 × (length + width). - **Square**: 4 × side. - **Triangle**: sum of three sides (or 3 × side for equilateral). - **Circle**: 2 × π × radius (called "circumference"). - **Regular polygon**: n × side length. - **Irregular shapes**: sum of all sides.

Perimeter differs from area: perimeter is measured in linear units (cm, ft), while area is measured in square units (cm², ft²). They scale differently: doubling all dimensions doubles perimeter but quadruples area.

Among shapes with the same area, the circle has the smallest perimeter. This is the isoperimetric inequality. It's why bubbles are spherical (minimize surface for given volume), why bees build hexagonal cells (best polygon for tiling), and why rectangular plots use more fence than circular for same area.

Common applications: fencing, framing, edging, trim, materials lists, garden borders, sewing hems, building outlines, and any project involving the boundary length of a shape.

Formula

**Perimeter formulas:** | Shape | Formula | |---|---| | Square | 4 × s | | Rectangle | 2 × (length + width) | | Triangle | a + b + c | | Equilateral triangle | 3 × s | | Isosceles triangle | 2 × equal_side + base | | Parallelogram | 2 × (a + b) | | Rhombus | 4 × s | | Trapezoid | a + b + c + d | | Regular polygon (n sides) | n × s | | Circle (circumference) | 2πr = πd | | Ellipse (approx) | π × [3(a+b) - √((3a+b)(a+3b))] | **Worked examples:** **Rectangle** 10 × 5: P = 2 × (10 + 5) = 30 units. **Square** with side 7: P = 4 × 7 = 28 units. **Triangle** with sides 3, 4, 5: P = 3 + 4 + 5 = 12 units (classic 3-4-5 right triangle). **Circle** with radius 6: P = 2π × 6 ≈ 37.70 units. **Regular hexagon** with side 5: P = 6 × 5 = 30 units. **Perimeter vs area:** | Shape | Perimeter | Area | |---|---|---| | Square (s = 10) | 40 | 100 | | Rectangle (10 × 5) | 30 | 50 | | Circle (r = 10) | 62.83 | 314.16 | | Equilateral triangle (s = 10) | 30 | 43.3 | Units: perimeter in length, area in length². **Isoperimetric inequality:** Among all shapes with the same perimeter P, the circle has the largest area. Equivalently: among all shapes with same area, circle has smallest perimeter. Used in nature (bubbles, cells) and engineering (efficient containers). **Comparison: same perimeter shapes:** For P = 40: - Square (s = 10): area = 100. - Rectangle (15 × 5): area = 75. - Circle (C = 40, r ≈ 6.37): area ≈ 127.3. - Regular hexagon (s ≈ 6.67): area ≈ 115.5. Circle wins for area; rectangles are progressively worse as they become elongated. **Composite shapes:** For irregular shapes: 1. Identify all sides. 2. Sum all side lengths. For L-shaped lot: P = sum of 6 sides (since L has 6 corners, 6 sides). **Pythagorean theorem application:** For right triangle: if you know two legs, third side (hypotenuse) = √(a² + b²). For sides 3 and 4: hypotenuse = √(9+16) = 5. Perimeter = 3 + 4 + 5 = 12. **Common irregular shapes:** L-shape with arm widths a, b, c, d: P = sum of 6 sides depending on configuration. T-shape: P = sum of 8 sides. **Perimeter in different units:** | From | To | Factor | |---|---|---| | cm | m | × 0.01 | | m | ft | × 3.281 | | ft | yd | × 0.333 | | in | mm | × 25.4 | | miles | km | × 1.609 | **Practical applications:** **Fencing**: linear footage of fence to buy. For 50' × 30' yard: P = 160 ft of fence. **Trim molding**: baseboard or crown molding. For 12' × 14' room: P = 52 ft of molding (less doorway widths). **Picture framing**: total frame length. For 8" × 10" photo: P = 36" of frame moulding. **Garden edging**: border length. For 20' × 15' garden: 70' of edging. **Walking distance**: Walking around a 0.25 mile track once = perimeter ≈ 0.25 mile. **Sports fields:** Standard track (400 m): perimeter = 400 m (designed for exact metric distance). Soccer field: typically 100-110 m × 64-75 m, so perimeter 328-370 m. Football field: 100 yd × 53.33 yd, perimeter ≈ 306.67 yd. **Buildings:** Linear footage of exterior walls determines siding, foundation, roofing edge. **Real estate:** Property line length affects fencing and easements. **Software:** - **CAD**: built-in perimeter calculation for any drawn shape. - **GIS**: perimeter for any geographic polygon. - **Excel**: simple formulas for known shapes. - **Surveying**: total station + GIS combination. **Pitfalls:** - **Confusing perimeter with area**: linear vs squared units. - **Forgetting to sum all sides**: irregular shapes can have many sides. - **For circle: using diameter without considering** (C = πd works with diameter). - **For ellipse: only approximation exists** for perimeter. - **Imperial vs metric**: ensure consistent units. **Areas that share perimeter:** Many shapes with same perimeter, very different areas: - 100 ft fence → square: area = 625 ft². - 100 ft fence → 30×20 rect: area = 600 ft². - 100 ft fence → 40×10 rect: area = 400 ft². - 100 ft fence → circle: area ≈ 795 ft². Long thin rectangles are very inefficient. **Common applications:** - **Construction**: fence length, perimeter walls. - **Landscaping**: garden border, lawn edging. - **Sewing**: hem length, trim. - **Picture framing**: frame moulding. - **Track and field**: lap distance. - **Surveying**: property boundaries. - **Manufacturing**: rim/edge calculations. - **Real estate**: property line measurements. - **Container design**: rim or edge length.

How to use this calculator

Choose shape: rectangle, triangle, circle, or regular polygon.
Enter dimensions appropriate to shape.
Calculator returns perimeter in your chosen units.
For irregular shapes: sum all side lengths manually.
For triangles: ensure triangle inequality (each side < sum of others).
For circles: result is circumference (C = 2πr).

Worked examples

Garden fencing

**Scenario:** Rectangular vegetable garden 12 m × 8 m. Need fence. **Calculation:** P = 2 × (12 + 8) = 40 m. **Result:** Need 40 m of fencing. Add ~10% for waste/corners: order ~44 m. Include cost of posts (typically every 2-3 m): ~15-20 posts. Linear-foot pricing for materials.

Picture frame moulding

**Scenario:** Frame for 11" × 14" photo using 2" wide moulding. **Calculation:** Inner perimeter: 2 × (11 + 14) = 50 in. Outer perimeter (after moulding): 2 × (15 + 18) = 66 in. Order moulding: 66 in + waste = ~72 in. **Result:** Order ~6 ft of moulding. Mitre cuts at 45° at each corner. Calculate carefully because frame moulding has specific corner-cutting requirements.

Running track

**Scenario:** Standard 400 m Olympic track. Compute total distance for one lap and three laps. **Calculation:** Inner lane: 400 m exactly (designed). 3 laps: 1200 m. **Result:** 3 laps = 1200 m = 0.75 mile. 4 laps = 1 mile (rough; actually 1.609 km = 4.02 laps). Standardized track distances allow direct comparison globally.

When to use this calculator

**Use perimeter calculations for:**

- **Fencing**: amount needed for boundaries. - **Framing**: picture frames, doors, windows. - **Edging**: garden borders, lawn edging. - **Sewing**: hems, trim, bias tape. - **Construction**: linear footage of walls. - **Trim work**: baseboard, crown molding. - **Running**: track lap distances. - **Real estate**: property line measurements.

**Add waste factor:**

Real materials need extra for cuts, corners, mistakes: - **Fencing**: 5-10% extra. - **Wood trim**: 10-15%. - **Bias tape**: 10-15%. - **Curved cuts**: 15-25%.

**Composite/irregular shapes:**

Sum all side lengths. For complex shapes: 1. Divide into rectangles, triangles, sectors. 2. Identify all outer edges. 3. Sum lengths. 4. Skip interior shared edges (only count outer perimeter).

**Linear vs squared:**

- Perimeter: linear units (cm, ft, m). One dimension. - Area: squared units (cm², ft², m²). Two dimensions. - Volume: cubed units. Three dimensions.

Don't confuse measurement types.

**Scaling laws:**

Double all linear dimensions: - Perimeter doubles (×2). - Area quadruples (×4). - Volume octuples (×8).

This is why small things have high surface-to-volume ratio (good for heat exchange, bad for heat retention).

**Common applications:**

- **DIY projects**: fence, trim, edging materials. - **Real estate**: lot boundaries. - **Athletic events**: track lap counts. - **Sewing**: hem lengths, ribbon trim. - **Cooking**: pie crust edge, baking pan rim. - **Architecture**: building exterior dimensions. - **Manufacturing**: edge work, finishing.

**Surveying methods:**

- **Tape measure**: small properties. - **GPS**: outdoor surveying. - **Total station**: professional precision. - **Drone photogrammetry**: large or complex sites. - **Online mapping**: rough estimates.

**Software:**

- **CAD**: AutoCAD, SolidWorks compute perimeter automatically. - **GIS**: ArcGIS, QGIS for property boundaries. - **Sketch apps**: many DIY apps for rooms. - **Spreadsheets**: known-shape calculations.

**Pitfalls:**

- **Confusing perimeter with area**: linear vs squared units. - **Forgetting all sides**: irregular shapes can have many. - **Imperial vs metric**: mixing causes errors. - **For circles: confusing radius and diameter**: C = 2πr or πd. - **For ellipse: no exact perimeter formula**: use approximation. - **Forgetting waste factor**: order short. - **Composite shape complexity**: easy to miscount sides.

**Practical tips:**

- **Always sketch the shape** with all dimensions. - **Double-check measurements**: walk the perimeter physically if possible. - **Round up** for material orders. - **Account for** doorways, gates, openings (subtract from total). - **Buy extra**: spare material for repairs/mistakes.

**For circular shapes:**

C = 2πr = πd

Pi ≈ 3.14159; for everyday work 3.14 is sufficient.

For elliptical: no exact formula; use Ramanujan approximation: P ≈ π × [3(a+b) − √((3a+b)(a+3b))]

**For polygons with curved sides** (rare): combine line segments + arc lengths.

**For complex shapes:**

Many CAD/GIS tools have automatic perimeter calculation. Manual calculation only needed for hand-sketched or theoretical shapes.

**Pitfalls (continued):**

- **Mixing in vs out**: perimeter is outer boundary; not inner cavities. - **Holes/cutouts**: separately compute hole perimeters if needed. - **For 3D shapes**: confuse perimeter (2D) with edge length (3D). - **Curved boundaries**: use appropriate formula or numerical estimation.

Common mistakes to avoid

Confusing perimeter (linear) with area (squared units).
Forgetting to include all sides of irregular shapes.
For circles: confusing radius (in 2πr) with diameter (in πd).
Mixing units (some sides in cm, others in m).
Forgetting waste factor when ordering materials.
Computing perimeter of composite shapes incorrectly.
For triangles: not verifying triangle inequality.
Confusing inner and outer perimeter.

Frequently Asked Questions

Sources & further reading

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About this calculator

Common applications: fencing, framing, edging, trim, materials lists, garden borders, sewing hems, building outlines, and any project involving the boundary length of a shape.

Formula

How to use this calculator

Choose shape: rectangle, triangle, circle, or regular polygon.

Enter dimensions appropriate to shape.

Calculator returns perimeter in your chosen units.

For irregular shapes: sum all side lengths manually.

For triangles: ensure triangle inequality (each side < sum of others).

For circles: result is circumference (C = 2πr).

Worked examples

Garden fencing

Picture frame moulding

Running track

When to use this calculator

**Use perimeter calculations for:**

**Add waste factor:**

Real materials need extra for cuts, corners, mistakes: - **Fencing**: 5-10% extra. - **Wood trim**: 10-15%. - **Bias tape**: 10-15%. - **Curved cuts**: 15-25%.

**Composite/irregular shapes:**

Sum all side lengths. For complex shapes: 1. Divide into rectangles, triangles, sectors. 2. Identify all outer edges. 3. Sum lengths. 4. Skip interior shared edges (only count outer perimeter).

**Linear vs squared:**

- Perimeter: linear units (cm, ft, m). One dimension. - Area: squared units (cm², ft², m²). Two dimensions. - Volume: cubed units. Three dimensions.

Don't confuse measurement types.

**Scaling laws:**

Double all linear dimensions: - Perimeter doubles (×2). - Area quadruples (×4). - Volume octuples (×8).

This is why small things have high surface-to-volume ratio (good for heat exchange, bad for heat retention).

**Common applications:**

**Surveying methods:**

**Software:**

**Pitfalls:**

**Practical tips:**

**For circular shapes:**

C = 2πr = πd

Pi ≈ 3.14159; for everyday work 3.14 is sufficient.

For elliptical: no exact formula; use Ramanujan approximation: P ≈ π × [3(a+b) − √((3a+b)(a+3b))]

**For polygons with curved sides** (rare): combine line segments + arc lengths.

**For complex shapes:**

Many CAD/GIS tools have automatic perimeter calculation. Manual calculation only needed for hand-sketched or theoretical shapes.

**Pitfalls (continued):**

Common mistakes to avoid

Confusing perimeter (linear) with area (squared units).

Forgetting to include all sides of irregular shapes.

For circles: confusing radius (in 2πr) with diameter (in πd).

Mixing units (some sides in cm, others in m).

Forgetting waste factor when ordering materials.

Computing perimeter of composite shapes incorrectly.

For triangles: not verifying triangle inequality.

Confusing inner and outer perimeter.

Frequently Asked Questions

About this calculator

Formula

How to use this calculator

Worked examples

Garden fencing

Picture frame moulding

Running track

When to use this calculator

Common mistakes to avoid

Frequently Asked Questions

What is a perimeter?

How do you find a circle's perimeter?

How is perimeter different from area?

How do I find the perimeter of an irregular shape?

Which shape has the smallest perimeter for a given area?

How much fencing do I need for my yard?

How is perimeter used in real estate?

Can I calculate perimeter from area?

Sources & further reading

Related Calculators

Area Calculator

Triangle Calculator

Circle Calculator

Hexagon Calculator

Cylinder Volume Calculator

Cone Volume Calculator

Inputs

Results

About this calculator

Formula

How to use this calculator

Worked examples

Garden fencing

Picture frame moulding

Running track

When to use this calculator

Common mistakes to avoid

Frequently Asked Questions

What is a perimeter?

How do you find a circle's perimeter?

How is perimeter different from area?

How do I find the perimeter of an irregular shape?

Which shape has the smallest perimeter for a given area?

How much fencing do I need for my yard?

How is perimeter used in real estate?

Can I calculate perimeter from area?

Sources & further reading

Related Calculators

Area Calculator

Triangle Calculator

Circle Calculator

Hexagon Calculator

Cylinder Volume Calculator

Cone Volume Calculator

Inputs

Results