Enter the length and width of your rectangle to calculate its area, perimeter, and diagonal length. The Rectangle Calculator gives you all three results at once, using the standard geometry formulas — no manual math required. Works for any rectangle, whether you're measuring a room, a screen, a pool, or a parcel. Also try the Crescent Area Calculator.
Area (A)
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Perimeter (P)
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Diagonal (d)
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Circumradius (R)
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Aspect Ratio
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A rectangle is a quadrilateral (four-sided polygon) with four right angles (90°). Opposite sides are equal in length and parallel to each other. A square is a special case of a rectangle where all four sides are equal. See also our Volume Calculator.
The area of a rectangle is calculated by multiplying its length (a) by its width (b): A = a × b. For example, a rectangle with a length of 10 m and a width of 5 m has an area of 50 m².
The perimeter is the total distance around the outside of the rectangle. Use the formula P = 2a + 2b, or equivalently P = 2(a + b). For a 10 m × 5 m rectangle, the perimeter is 30 m.
The diagonal of a rectangle can be found using the Pythagorean theorem: d = √(a² + b²). Both diagonals of a rectangle are equal in length. For a 10 m × 5 m rectangle, the diagonal is approximately 11.18 m. You might also find our Ellipse Circumference Calculator useful.
Yes, a square is a special case of a rectangle. A rectangle requires four right angles and opposite sides to be equal — a square satisfies all of these conditions, with the additional constraint that all four sides are equal in length.
The circumradius (R) is the radius of the circle that passes through all four corners of the rectangle. It equals half the diagonal: R = d / 2 = √(a² + b²) / 2. Every rectangle can be inscribed in a circle, unlike general parallelograms.
The calculator supports millimeters (mm), centimeters (cm), meters (m), kilometers (km), inches (in), feet (ft), and miles (mi). The area result is expressed in the squared version of the chosen unit (e.g., m²), and the perimeter and diagonal use the same linear unit.
Rectangle calculations are used in everyday situations like finding the floor area of a room, determining how much fencing is needed around a garden, sizing a TV screen or monitor, estimating material for a rectangular pool or tablecloth, and many construction or design tasks.