Perimeter Calculator
Calculate, compare, and apply perimeter formulas accurately for homework, construction, architecture, and land measurement. Pick a shape, enter your dimensions, and get instant results in your chosen unit.
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Perimeter Calculator
Choose a shape, enter dimensions in one unit, then calculate. Results are linear units (not squared).
Perimeter
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units (same unit as your inputs)
Choose a shape, enter dimensions, then calculate.
Using this calculator
- Select the shape that matches your diagram.
- Type each dimension using the same unit throughout.
- Press Calculate perimeter to see the distance around the boundary.
- Switch shapes anytime; input fields update automatically.
Example calculations
- Rectangle: length 10, width 6 → P = 2(10 + 6) = 32 units
- Triangle: sides 5, 6, 7 → P = 18 units
- Circle: radius 4 → P = 2π(4) ≈ 25.13 units
- Square: side 8 → P = 4(8) = 32 units (use Rectangle with equal length and width)
What Is Perimeter?
Definition, meaning, boundary length, units, and real-world uses.
Perimeter is the total distance around the outside edge of a flat shape. It answers one question: how far would you travel if you walked once along every side of the boundary?
That measurement is a boundary length. It is not the space inside the figure. Students, builders, and surveyors all rely on the same idea, even when the shape looks complicated on paper.
- Definition: the sum of all outer side lengths of a closed 2D figure
- Meaning: how far you go around the outline, not across the interior
- Units of measurement: meters, feet, inches, centimeters, and other length units (never squared)
- Real-world applications: fencing, trim, picture frames, race tracks, property lines, and geometry classwork
Perimeter Formula
Core formulas for common shapes. The rectangle rule is the foundation for many jobs.
The core perimeter formula for a rectangle is P = 2(l + w), where l is length and w is width. A square with side s uses P = 4s, which is the same idea with equal sides.
Triangles add three sides: P = a + b + c. Circles use P = 2πr, often called circumference. Pick the formula that matches your diagram before you type numbers into the calculator above.
Match the formula to your shape before you substitute numbers for homework or site work.
Rectangle perimeter formula
P = 2(l + w) when opposite sides are equal and corners are right angles.
Square perimeter formula
P = 4s where s is one side length.
Triangle perimeter formula
P = a + b + c for any triangle when you know all three sides.
Circle circumference relationship
P = 2πr or P = πd when diameter d is given. Circumference is the perimeter of a full circle.
Rectangle
P = 2(l + w)
Square
P = 4s
Triangle
P = a + b + c
Circle
P = 2πr
How to Calculate Perimeter
A reliable step-by-step method for class, construction, and everyday checks.
Start with a clear sketch. Label every side you will include in the total. For composite layouts, split the figure into rectangles and triangles before you add anything.
Choose one unit system and stick with it. Convert mixed feet and inches to a single unit before you add, or your total will be wrong even when each formula is correct.
- Step 1: measure or read each side length on the boundary
- Step 2: choose the correct formula for that shape
- Step 3: add boundary lengths (or apply the formula)
- Step 4: attach the correct length unit to your answer
- Unit conversion: convert all inputs to one unit first, then calculate
Perimeter Examples
Worked examples you can mirror in the calculator above.
These samples use one unit system throughout. Open the tool, pick the matching shape, and enter the same values to confirm your hand work.
Real-world examples
Fence quote for a rectangular lot 40 ft by 30 ft: perimeter 140 ft of railing along the boundary (not the area of the yard).
Picture frame with inner opening 20 cm by 15 cm needs molding length based on the outer perimeter you actually install.
Rectangle example
A garden bed 12 m by 7 m: P = 2(12 + 7) = 38 m.
Square example
A tile outline with side 5 ft: P = 4(5) = 20 ft.
Triangle example
Sides 9 in, 10 in, 11 in: P = 30 in.
Polygon example
Regular hexagon, side 4 cm: P = 6 × 4 = 24 cm.
Rectangle Perimeter Calculator
Length and width in, perimeter out. Built for construction layouts and math class.
Select Rectangle in the calculator above, enter length and width, then press Calculate perimeter. The result uses the same unit you typed.
P = 2(l + w) is the standard rule. Crews use it for foundation outlines, garden borders, and room trim plans.
- Formula: P = 2(l + w)
- Construction: fencing, framing, and slab edges
- Education: label diagrams with l and w before you substitute
Triangle Perimeter Calculator
Scalene, isosceles, and equilateral triangles all add three sides.
Choose Triangle above and enter sides a, b, and c. The tool checks that the three lengths can form a real triangle before it totals them.
Equilateral triangles have three equal sides, so P = 3s. Isosceles triangles repeat two equal sides plus a base. Scalene triangles use three different lengths.
- Scalene: P = a + b + c with all sides different
- Isosceles: two equal sides plus a base
- Equilateral: P = 3s when every side matches
Square Perimeter Calculator
One side length defines the whole boundary.
A square is a rectangle with equal length and width. Enter the same value in both fields, or use side length logic: P = 4s.
Playgrounds, tiles, and chessboards are common square outlines in word problems.
- Formula: P = 4s
- Property: all four sides equal, four right angles
- Practical use: borders, grids, and symmetric plots
Perimeter vs Area
Two different questions: around the edge versus inside the surface.
Perimeter measures distance around a shape in linear units (ft, m, in). Area measures coverage inside the boundary in square units (ft², m²).
Fence posts follow perimeter. Sod or paint for a flat lawn follow area. Mixing the two is one of the most common mistakes on tests and on job sites.
Perimeter
P = 2(l + w) for rectangles; add sides or use 2πr for circles.
Area
A = l × w for rectangles; different tools and units.
When to use each
Trim and fencing → perimeter. Flooring and turf → area.
Common Perimeter Calculation Mistakes
Catch these errors before they reach a grade sheet or a materials list.
Students often multiply when they should add, or they report area units on a perimeter answer. Always ask: did the problem want distance around or space inside?
Another trap is mixing units, such as adding feet and inches without converting. Another is using diameter in 2πr without halving to radius first.
For circles, do not confuse circumference (perimeter) with πr² (area).
- Using area formulas for perimeter problems
- Forgetting to include every outer edge on composite figures
- Leaving off the straight diameter on a semicircle boundary
- Skipping unit conversion when measurements use mixed systems
- Rounding π too early on precision homework
Perimeter of Polygons
Regular and irregular polygons: add sides or use n × s when every side matches.
Any polygon perimeter is the sum of its side lengths. Walk the outline once and add each edge.
Regular polygons (equal sides and angles) simplify to P = n × s, where n is the number of sides and s is side length. The calculator above includes pentagon, hexagon, octagon, and a custom regular polygon option.
Irregular quadrilaterals need four separate sides. Split complex plots into rectangles and triangles when a single formula does not fit.
- Regular polygon: P = n × s
- Irregular polygon: add every side you measure
- Composite figures: sum perimeters of parts, excluding shared internal edges
FAQs About Perimeter
- What is perimeter in simple terms?
- Perimeter is the total distance around the outside of a flat shape, measured in length units such as meters or feet.
- What is the perimeter formula for a rectangle?
- Use P = 2(l + w), where l is length and w is width. Example: 10 m by 6 m gives P = 32 m.
- What is the difference between perimeter and area?
- Perimeter is distance around the boundary (linear units). Area is surface inside the shape (square units). See our perimeter vs area section.
- How do I find the perimeter of a triangle?
- Add all three side lengths: P = a + b + c. Each side must be positive and satisfy the triangle inequality.
- Is circumference the same as perimeter?
- For a full circle, yes. Circumference is the perimeter of a circular boundary, calculated with P = 2πr.
- Does this calculator convert units?
- Enter one unit consistently for every input. The result uses that same unit. Convert measurements yourself before calculating if the problem mixes inches and feet.
- Does the semicircle include the straight edge?
- Yes. This tool adds the curved arc (πr) and the diameter (2r) for the full semicircle boundary.
- Is my data sent to a server?
- No. All math runs in your browser. Nothing you type is uploaded.