GeomBench

Rectangle Area Calculator

Width × height — with perimeter and diagonal thrown in for free.

Width

Height

Area

A = w × h

Perimeter: 26
Diagonal: 9.434

What you entered

Width: 8Height: 5

1
Area
8 × 5= 40
2
Perimeter
2 × (8 + 5)= 26
3
Diagonal
√(8² + 5²)= 9.434

Result

Area: 40

A 8 × 5 rectangle has an area of 40, a perimeter of 26, and a diagonal of 9.434.

Rectangle area, perimeter and diagonal

Area is width times height, perimeter is twice their sum, and the diagonal follows from Pythagoras.

Width x Height	Area	Perimeter	Diagonal
3 x 4	12.00	14.00	5.000
5 x 8	40.00	26.00	9.434
6 x 10	60.00	32.00	11.662
8 x 12	96.00	40.00	14.422
10 x 15	150.00	50.00	18.028
12 x 20	240.00	64.00	23.324
15 x 24	360.00	78.00	28.302

The 3 x 4 row gives a diagonal of exactly 5 because those are the sides of the best-known Pythagorean triple. Every other row lands on an irrational number, which is why the diagonal column carries three decimals while area and perimeter carry two. The diagonal is the figure to check when working out whether furniture fits through a doorway or a sheet of material will lie flat in a van - the width and length alone will not tell you.

Area, perimeter, and diagonal all from two numbers

Width and height alone determine everything else about a rectangle: area (w×h) for covering it, perimeter (2(w+h)) for framing it, and the diagonal (via the Pythagorean theorem) for confirming it's a true rectangle and not a skewed parallelogram.

Why it's one of the most-used calculators

Flooring, paint coverage, screen and paper sizes, room layouts, and shipping box dimensions are almost always rectangular — this is the single most common area calculation in everyday life.

Frequently asked questions

How do I calculate the area of a rectangle?

Area = length × width. A room 12 feet by 15 feet has area 180 square feet. Always use the same units for both dimensions — if one is in inches and the other in feet, convert first.

How do I calculate how much flooring I need?

Measure the room's length and width in feet, multiply for square footage. Add 10% for cuts and waste. A 12×15 room = 180 sq ft + 10% waste = 198 sq ft of material. For irregularly shaped rooms, divide into rectangles and add the areas.

How do I find the diagonal of a rectangle?

Diagonal = √(length² + width²). A 12×15 rectangle has diagonal √(144 + 225) = √369 = 19.21. This is the Pythagorean theorem — the diagonal splits the rectangle into two right triangles.