Rectangle Calculator (area, perimeter)
From length and width, get a rectangle's area, perimeter and diagonal.
Result
Area
40.0000
Perimeter
28.0000
Diagonal
10.7703
How it works
Three numbers describe a rectangle once you know its length and width. The area L×W measures the surface inside; the perimeter 2(L+W) measures the border around; and the diagonal √(L²+W²) — straight from the Pythagorean theorem — measures corner to corner. A 5×3 rectangle has area 15, perimeter 16 and a diagonal of about 5.83. Area and perimeter answer different questions and are easy to mix up: paint, tiles or turf depend on the area; fencing, skirting boards or picture frames depend on the perimeter. A subtle fact: the same perimeter can enclose very different areas. With 16 metres of fence, a 4×4 square encloses 16 m² but a 7×1 strip only 7 m² — among all rectangles of equal perimeter, the square always encloses the most.
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Frequently asked questions
How do I find a rectangle's area?
Multiply length by width: area = L × W.
How is the diagonal found?
By the Pythagorean theorem: diagonal = √(length² + width²).
Can two rectangles have the same perimeter but different areas?
Yes. A 4×4 and a 7×1 rectangle both have a perimeter of 16, but areas of 16 and 7. For a fixed perimeter, the closer to a square, the larger the area.
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