Short answer: for a rectangle, multiply the length by the width, both in feet. A 12 × 15 ft room is 12 × 15 = 180 square feet. For any other shape, break it into rectangles, triangles and circles, find each area and add them up.

For your own measurements, use the square footage calculator. Read on to learn the formulas, handle cut-outs and convert units.

The formulas

Shape Area Measurements you need
Rectangle or square length × width two sides
Triangle base × height ÷ 2 the base and the height measured at a right angle to it
Circle π × radius × radius the diameter, halved to get the radius

Measure everything in feet before you multiply. If you measure in inches, divide by 12 first: 30 in is 2.5 ft.

Worked example 1: a 12 × 15 ft room

12 ft × 15 ft = 180 square feet.

Worked example 2: an L-shaped area with a cut-out

Take a 20 × 10 ft area with an 8 × 6 ft extension, and a 4 × 3 ft section inside that will not be counted.

Step Calculation Result
Main rectangle 20 ft × 10 ft 200 ft²
Extension 8 ft × 6 ft 48 ft²
Cut-out 4 ft × 3 ft 12 ft²
Total 200 + 48 − 12 236 ft²

Add the parts that are included and subtract the parts that are not. Check that the rectangles do not overlap, or you will count that part twice.

Worked example 3: a round bed and a triangular corner

A round bed 10 ft across has a radius of 5 ft, so its area is π × 5 × 5 = 78.5 square feet. A triangular corner with a 10 ft base and a 6 ft height is 10 × 6 ÷ 2 = 30 square feet. Together they are 108.5 square feet.

Converting units

  • Square yards to square feet: multiply by 9. A yard is 3 feet and 3 × 3 = 9.
  • Square inches to square feet: divide by 144, because 12 × 12 = 144.
  • Square feet to square meters: multiply by 0.0929. This follows from a foot being exactly 0.3048 m.

The exact length relationships are in the NIST conversion factors listed under Sources.

Common mistakes

  • Mixing units. Multiplying 12 ft by 90 in gives a meaningless number. Convert first.
  • Using the slanted side of a triangle. The height is measured at a right angle to the base.
  • Using the diameter as the radius. Halve the diameter before squaring.
  • Counting overlaps twice. When you split an L-shape, make sure the rectangles only touch.
  • Forgetting the hole. Subtract any part you will not cover, such as a pool, a tree bed or a stairwell.

What this guide cannot tell you

  • How a listing, an appraisal or a tax record measures a home. Those follow their own rules, which this guide does not cover.
  • How much material to order. Square footage is the first step. Add waste and convert to the unit your supplier sells, for example with the cubic yard calculator when depth matters.

The calculator gives planning estimates. Our assumptions are listed on the methodology page.

Frequently asked questions

How do you calculate square footage of a room?

Measure the length and the width in feet and multiply them. A 12 × 15 ft room is 180 square feet.

How do you calculate the square footage of an L-shaped area?

Split it into two rectangles, find the area of each and add them. Measure every part in feet, or convert inches first.

How many square feet are in a square yard?

Nine, because a yard is 3 feet and 3 × 3 = 9.

How do you find square footage when the measurements are in inches?

Multiply the two measurements in inches and divide by 144, or divide each one by 12 first to get feet.

Sources