This rectangle area calculator computes the area, perimeter, and diagonal of any rectangle from length and width using A = L×W, P = 2(L+W), and d = √(L²+W²). It is unit-agnostic, with practical applications in room measurement, real estate, flooring, screen sizing, and garden planning.
Rectangle Area Calculator
Calculate area, perimeter, and diagonal of a rectangle from length and width.
How the Rectangle Area Calculator Works
The Rectangle Area Calculator turns two simple measurements, length and width, into three practical numbers: area, perimeter, and diagonal length. It works with any unit you choose (inches, feet, meters, pixels) since the math is unit-agnostic — just be consistent between the two fields.
Formula: Area = Length × Width. Perimeter = 2 × (Length + Width). Diagonal = √(Length² + Width²).
- Length (L) — the longer or reference side of the rectangle, entered in the Length field.
- Width (W) — the shorter or adjacent side of the rectangle, entered in the Width field.
- Area — the enclosed surface, L × W, shown in square units.
- Perimeter — the total distance around the rectangle, 2(L + W).
- Diagonal — the straight-line distance corner to corner, √(L² + W²), derived from the Pythagorean theorem.
Example Scenarios
| Length | Width | Area |
|---|---|---|
| 4 | 3 | 12 |
| 10 | 5 | 50 |
| 7.5 | 2 | 15 |
| 12 | 12 | 144 |
| 20 | 8 | 160 |
Measuring Real Spaces for a Rectangle Area Calculator
Rooms, plots and panels are rarely perfect, so a rectangle area calculator is only as good as the tape readings behind it.
- Measure each side twice, at both ends. If opposite sides differ, the shape is not a true rectangle; use the average for a rough figure.
- Check squareness with the diagonal: a rectangle area calculator gives d = √(L²+W²), and if both measured diagonals match that value, the corners are square.
- Split L-shaped rooms into two rectangles, run the rectangle area calculator for each, then add the results.
- Keep units consistent. Mixing feet and inches is the most common slip, so convert everything first.
Area units are squared: 1 square metre is about 10.76 square feet, and 1 square yard is 9 square feet. For flooring, tile or paint, add the waste allowance your supplier recommends on top of the rectangle area calculator result.
The rectangle article on Wikipedia covers the geometry behind each formula the rectangle area calculator uses.
Rectangle Area Calculator FAQ
How do I find the area of a rectangle if I only know the perimeter and one side?
Solve the perimeter formula for the missing side first: Width = (Perimeter ÷ 2) − Length. Once you have both Length and Width, enter them into the calculator to get the area, or multiply them directly using A = L × W.What’s the difference between area and perimeter of a rectangle?
Area measures the two-dimensional surface enclosed by the rectangle, expressed in square units like square feet. Perimeter measures the one-dimensional distance around the outside edge, expressed in linear units like feet. They answer different questions: how much surface versus how much boundary.How is the diagonal of a rectangle calculated from length and width?
The diagonal splits the rectangle into two right triangles, so the Pythagorean theorem applies: diagonal = √(Length² + Width²). This calculator computes it automatically alongside area and perimeter whenever you enter both dimensions.Can I use this calculator for square footage in real estate listings?
Yes. Enter a room’s length and width in feet and the Area result gives you square footage directly. For a whole floor plan, calculate each rectangular room separately and add the areas together, since most homes aren’t a single perfect rectangle.Does it matter what units I use for length and width?
No specific unit is required, but both fields must use the same unit for the result to be meaningful. If you enter length in meters and width in feet, the area, perimeter, and diagonal will all be mathematically wrong even though the calculator won’t flag an error.How do I calculate the area of an L-shaped or irregular room using rectangle math?
Split the irregular shape into two or more non-overlapping rectangles, calculate the area of each one separately with this tool, then add the individual areas together for the total floor or surface area.Related Calculators
Working with more complex shapes or projects? Try our general-purpose area calculator for multiple shapes, use the triangle calculator for angled or non-rectangular spaces, or head to the roof area calculator for pitched roofing estimates if you’re planning a home improvement project.