Geometry desk
Rectangle Area & Perimeter Calculator
Calculate a rectangle's area and perimeter from length and width.
Rectangle Area & Perimeter Calculator: Compare the surface covered by a rectangle with the total distance around its four sides. Enter length and width to get both measurements, so you can distinguish the amount of sheet material from the amount of edging a rectangular piece needs. Runs 100% locally in your browser with zero server file uploads.
- Category
- Calculators
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Area
96 cm²
- Perimeter
- 40 cm
Area = length × width; Perimeter = 2 × (length + width)- Dimensions are measured in the same unit.
- Values must be finite and greater than zero.
Formula and worked example
Area = length × width; Perimeter = 2 × (length + width)
A worked example with sample values:
| Inputs | |
|---|---|
| Length | 12 cm |
| Width | 8 cm |
| Results | |
|---|---|
| Area | 96 cm² |
| Perimeter | 40 cm |
Counting squares and adding edges
Let L mean length and W mean width. Area A = L × W counts rows of unit squares across the rectangle. Perimeter P = 2 × (L + W) adds two copies of each side. The first formula measures the interior; the second follows the outside boundary and never multiplies side lengths together.
For L = 12 cm and W = 8 cm, A = 12 × 8 = 96 cm². The edge lengths add to 12 + 8 + 12 + 8 = 40 cm, equivalently P = 2 × (12 + 8) = 40 cm. The calculator therefore shows Area = 96 cm² and Perimeter = 40 cm.
OpenStax's rectangle treatment (https://openstax.org/books/prealgebra-2e/pages/9-4-use-properties-of-rectangles-triangles-and-trapezoids) uses these separate measures. A square is included as the case L = W.
Different conversion factors for the two results
Convert mixed inputs before calculating: a length of 1.2 m and width of 80 cm become either 1.2 m by 0.8 m or 120 cm by 80 cm. The area is 0.96 m², equivalent to 9,600 cm², while the perimeter is 4 m, equivalent to 400 cm. Thus area conversions square the length factor, but perimeter conversions use it once. The form assumes four right angles; adjacent lengths alone do not establish the area of a slanted quadrilateral.
How to use it
- Enter positive dimensions in the same unit.
- Choose the unit used for every dimension.
- Calculate, then review the result and formula.
Privacy & limitations
Every value and calculation stays in your browser. No dimensions are uploaded or sent to an external service.
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Frequently asked questions
Does swapping a rectangle's length and width change the answer?
No. The two dimensions play interchangeable roles in both formulas. Entering 12 and 8 gives the same area and perimeter as entering 8 and 12; the longer side need not be entered first.
Can rectangles have equal areas but different perimeters?
Yes. A 12 by 8 rectangle has area 96 and perimeter 40. A 24 by 4 rectangle still has area 96, but its perimeter is 56. Coverage alone does not determine how much edging is required.
How can I find a missing width from area or perimeter?
When length L is known, use width W = area ÷ L or W = perimeter ÷ 2 − L. For area 96 cm² and length 12 cm, width is 8 cm. A non-positive recovered width cannot describe a usable rectangle.
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