Rectangle Perimeter Calculator

P = 2(l + w). Length plus width, doubled.

Inputs

Result

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General calculation reads

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Jordan Ellenberg - math thinking
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Scientific calculator (Casio fx-991EX)
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How to use this calculator

  • Enter length and width.
  • Read perimeter, area, and diagonal.

About this calculator

A rectangle's perimeter is the sum of all four sides: P = 2(l + w). Useful for fencing, picture-framing, room baseboards, or anything that traces the boundary of a rectangle. The formula is symmetric — swap length and width without effect. Area scales differently (l × w), so a square (l = w) gives the maximum area for a given perimeter.

Frequently asked

Square is a special rectangle?+
Yes — when l = w. Perimeter formula reduces to P = 4s.
How is this different from area?+
Perimeter = boundary length (linear). Area = enclosed surface (square units). Don't confuse.
Picture-framing use?+
Frame outside dimensions: outer perimeter. Inside (mat) cutout: inner perimeter. Add both for total moulding length needed.
Why is diagonal useful?+
For laying out, knowing diagonals helps verify a rectangle is square (corners 90°): equal diagonals = right angles.
Doors and windows?+
Subtract the perimeter portion that's a door/window if you're computing trim length, not the actual rectangle perimeter.

Related calculators

Square Perimeter Calculator
P = 4s. Four equal sides.
Area of a Rectangle Calculator
A = length × width.
Rectangular Prism Volume
V = l × w × h. Box / cuboid volume.
Triangle Perimeter Calculator
P = a + b + c. Sum of all three sides.
Circle Circumference Calculator
C = 2πr (or πd). Area = πr². The fundamental circle formulas.
Trapezoid Perimeter Calculator
P = a + b + c + d. Sum of two parallel sides + two non-parallel.

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