Everyday utility

Rectangle Calculator

Enter the length and width to get area, perimeter, and diagonal in one step. Keep both dimensions in the same unit for a correct squared-unit area result.

Last reviewed May 19, 2026 by ToolSpilo Editorial Team.

Review method: Reviewed against standard rectangle geometry formulas and Pythagorean theorem derivation.

Calculator tool

How this calculator works

Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.

What a Rectangle Is

A rectangle is a four-sided shape where every corner is a right angle (90°). Opposite sides are equal in length. A square is a special rectangle where all four sides are equal.

Formulas

Let ll be the length and ww be the width.

MeasurementFormula
AreaA=l×wA = l \times w
PerimeterP=2(l+w)P = 2(l + w)
Diagonald=l2+w2d = \sqrt{l^2 + w^2}

The diagonal formula comes directly from the Pythagorean theorem: the diagonal splits the rectangle into two right triangles.

Worked Example

For a rectangle 10 m long and 6 m wide:

A=10×6=60 m2A = 10 \times 6 = 60\text{ m}^2
P=2(10+6)=32 mP = 2(10 + 6) = 32\text{ m}
d=102+62=136≈11.66 md = \sqrt{10^2 + 6^2} = \sqrt{136} \approx 11.66\text{ m}

Unit Rules

Use the same unit for both length and width. If you enter meters, area is in square meters. If you enter centimeters, area is in square centimeters. Mixing units — for example, entering length in meters and width in centimeters — gives a wrong result.

Where Rectangles Appear

Rectangle calculations apply to rooms (floor area, skirting board length), building plots (land area), screens and canvases (display area), picture frames, doors, windows, tiles, and garden beds. The diagonal formula is useful when measuring across a space to confirm a right-angle corner.

Frequently asked questions

How do I find the area of a rectangle?

Multiply length by width: A=l×wA = l \times w. For a room 5 m long and 4 m wide, the area is 5×4=20 m25 \times 4 = 20\text{ m}^2. This tells you how many square meters of flooring or carpet you need, ignoring waste.

How is the diagonal of a rectangle calculated?

The diagonal divides the rectangle into two identical right triangles, so the Pythagorean theorem applies directly: d=l2+w2d = \sqrt{l^2 + w^2}. For a 3 m × 4 m rectangle, the diagonal is 9+16=25=5 m\sqrt{9 + 16} = \sqrt{25} = 5\text{ m}. Builders use the diagonal to check that a corner is truly square — if the measured diagonal matches the calculated one, the right angle is correct.

What is the difference between area and perimeter for a rectangle?

Area is the surface enclosed inside the shape, measured in square units such as m². Use it for flooring, paint coverage, and material quantities. Perimeter is the total distance around the shape, measured in linear units such as meters. Use it for fencing, framing, or skirting boards. A 10 m × 2 m rectangle and a 5 m × 4 m rectangle both have area 20 m², but their perimeters differ: 24 m versus 18 m.

Can I use this calculator for a square?

Yes. Enter the same value for both length and width, and the calculator gives the correct area, perimeter, and diagonal for a square. For a dedicated square tool with inscribed-circle radius and other square-specific values, use the Square Calculator.