Dimensions of a Rectangle Calculator

Enter a rectangle's length and width to get its area (A = L × W), perimeter (P = 2(L + W)), and diagonal (d = √(L² + W²)), plus an optional material cost estimate.

Quick Facts

Area formula
A = L × W
Multiply length by width; doubling either dimension doubles the area.
Perimeter formula
P = 2(L + W)
Sum of all four sides — twice the length plus twice the width.
Diagonal formula
d = √(L² + W²)
Follows from the Pythagorean theorem across length and width.

Your Results

Calculated
Area
-
A = length × width, in square units
Perimeter
-
P = 2 × (length + width)
Diagonal
-
d = √(length² + width²)
Estimated Material Cost
-
Area × cost per square unit

Ready

Enter a length, width, and unit, then press Calculate.

Formula and Method for the Dimensions of a Rectangle

A rectangle is a four-sided polygon with two pairs of equal, parallel sides and four 90° interior angles. Its two distinct dimensions — length (L) and width (W) — determine everything else about its size: area A = L × W, perimeter P = 2(L + W), and diagonal d = √(L² + W²). This calculator takes the length and width you enter and derives the area, perimeter, and diagonal, plus an optional material cost estimate.

How the calculation works

Enter the length and width and choose the unit they are measured in. The calculator multiplies the two values to get the area (in square units, such as ft² or m²), adds the length and width and doubles the sum to get the perimeter (P = 2(L + W)), and applies the Pythagorean theorem to get the diagonal (d = √(L² + W²)) — the diagonal is the hypotenuse of the right triangle formed by the length and width sides. If you enter a cost per square unit, the tool multiplies it by the area to estimate total material cost.

Common mistakes

  • Confusing dimensions with area: a rectangle that is 8 ft long and 5 ft wide has an area of 40 ft², not 13 ft (that's half the perimeter) and not 26 ft (that's the full perimeter).
  • Mixing units: keep length and width in the same unit — convert inches to feet, or centimeters to meters, before entering either value.
  • Diagonal vs. side length: the diagonal is always longer than either the length or the width; do not substitute it for a side when computing area or perimeter.

Real-world applications

  • Flooring, tiling, and carpet estimates use area directly to calculate how much material to buy (add 5–10% extra for cuts and waste).
  • Fencing, edging, and baseboard projects use perimeter to determine how much linear material is needed.
  • Framing and layout checks use the diagonal to confirm a rectangle is truly square-cornered — equal diagonals on both corner pairs confirm 90° angles.
  • Construction and landscaping budgets combine area with a cost per square unit to estimate material costs before purchasing.

Frequently Asked Questions

What is the formula for the area of a rectangle?
The area of a rectangle equals its length times its width: A = L × W. For example, a rectangle 8 ft long and 5 ft wide has an area of 8 × 5 = 40 ft².
How do I find the perimeter of a rectangle?
Add the length and width, then double the result: P = 2(L + W). A rectangle that is 8 ft by 5 ft has a perimeter of 2 × (8 + 5) = 26 ft.
How is the diagonal of a rectangle calculated?
The diagonal is the square root of the length squared plus the width squared: d = √(L² + W²), which follows from the Pythagorean theorem applied to the right triangle formed by the length, width, and diagonal. An 8 ft by 5 ft rectangle has a diagonal of √(64 + 25) = √89 ≈ 9.43 ft.
How much extra material should I buy for a tiling or flooring project?
Add roughly 5–10% to the calculated area to cover cuts, mismatched pieces, and waste, and more for diagonal layouts or intricate patterns.

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