Area of a Rectangle Formula

Geometry Formulas: Area, Volume, Perimeter
Formula:

What is the formula for the area of a rectangle?

Answer

Area of a rectangle
A = a · b

Here a is the side (or base), b is the second side. The result comes out in square units.

Where the formula comes from

A = a · b

Rule the rectangle into unit squares: b rows of a squares each. Counting them is multiplication, and that is the whole derivation — every other area formula on this site is eventually reduced to this one.

Worked examples

Let a = 8, b = 5. Then the area of a rectangle = 40 square units.

Let a = 12.5, b = 4. Then the area of a rectangle = 50 square units.

The common mistake

Mixing units — multiplying metres by centimetres. The product is not meaningless, it is in m·cm, which is nobody's idea of an area: convert both sides to the same unit first and the answer lands in m² or cm².

Other quantities: Rectangle

Related Calculations

See Also

Geometry Formulas: Area, Volume, Perimeter

These are the formulas for the area, perimeter, circumference, volume and surface area of the standard shapes — 31 of them. Each page shows not only the formula but where it comes from, what every letter stands for, two worked examples and the mistake that this particular shape invites. For example, it can help you find out what is the formula for the area of a rectangle? (The answer is: A = a · b). Pick the pair — "Area of a circle", for instance — and hit the "Find out" button.
Geometry Formulas: Area, Volume, Perimeter
Formula:

Values from the Formula (b = 5)

If a =Area
15 square units
210 square units
315 square units
420 square units
525 square units
1050 square units

FAQ

What is the formula for the area of a rectangle?

Area of a rectangle A = a · b

Where does this formula come from?

Rule the rectangle into unit squares: b rows of a squares each. Counting them is multiplication, and that is the whole derivation — every other area formula on this site is eventually reduced to this one.

What is the most common mistake here?

Mixing units — multiplying metres by centimetres. The product is not meaningless, it is in m·cm, which is nobody's idea of an area: convert both sides to the same unit first and the answer lands in m² or cm².

How do I use this formula?

Put your own numbers in place of the letters. With a = 12.5, b = 4, for instance, the formula A = a · b gives = 50 square units.

See Also