Rectangle Calculator
From a length and a width, get the area, the perimeter, and the diagonal — the three numbers that describe any rectangle.
Two sides, three answers
Enter the length and width and the calculator returns the area (L×W), the perimeter (2(L+W)), and the diagonal (√(L²+W²)) at once.
Keep units consistent
The sides are unit-agnostic — your answers come back in the same unit (squared for the area), so don't mix centimetres with inches.
What is a rectangle calculator?
Length and width in, full rectangle out
A rectangle calculator turns two measurements — the length and the width — into the three numbers that describe the whole shape: its area, its perimeter, and its diagonal. Each one is fixed once you know the two sides, so a single pair of inputs answers everything at once. That makes it the tool you reach for when sizing rooms, screens, plots of land, and sheets of material — anywhere a four-cornered shape with right angles shows up. A square is just the special case where the length and width are equal.
Enter the length and width in any length unit to get the area, perimeter, and diagonal instantly.
Three short formulas, all built from the length (L) and the width (W).
area = L × WThe perimeter — the distance all the way around the border — is 2 × (L + W), because there are two lengths and two widths. The diagonal, the straight line between opposite corners, comes from the Pythagorean theorem: √(L² + W²), since the diagonal is the hypotenuse of a right triangle whose legs are the two sides. Because the diagonal squares both sides, it is always longer than either one on its own.
Suppose you have a rectangle that is 4 long and 3 wide.
Area
4 × 3 = 12 square units — the surface inside.
Perimeter
2 × (4 + 3) = 14 — the distance once around the border.
Diagonal
√(4² + 3²) = √(16 + 9) = √25 = 5 — the classic 3-4-5 right triangle.
The three outputs answer three different everyday questions. The area (12 for a 4-by-3 rectangle) is the surface inside — the flooring you lay, the paint you roll on, the fabric you cut. The perimeter (14) is the border length — the fencing you buy for a plot, the framing for a picture, the trim around a tabletop. The diagonal (5) connects opposite corners — the measurement a TV is sold by, the brace that keeps a frame square, the longest straight cut that fits inside the shape. The key insight is that these three numbers move differently as the sides change: stretch a rectangle longer and thinner while keeping the same perimeter and the area shrinks, while a square — equal sides — packs the most area into a given perimeter. Because the diagonal uses the Pythagorean theorem, a perfect 3-4-5 rectangle always gives a whole-number diagonal of 5, which is why those proportions show up so often in construction and design.
The formulas are exact, but a couple of practical points are worth keeping in mind.
Right angles and consistent units
These formulas describe a true rectangle — four straight sides meeting at right angles. A shape that is slightly out of square, a wall that bows, or a plot with uneven edges will differ a little from the computed value. The sides are also unit-agnostic, so the answers are only meaningful if you keep one unit throughout: lengths in centimetres give a perimeter in centimetres and an area in square centimetres, never a mix.