The Basic Formula for Square Feet

Square feet is length times width. Measure the length of a space in feet, measure the width in feet, and multiply them together. That number is your square footage. If a room is 10 feet long and 12 feet wide, it is 120 square feet.

The unit matters. If you measure in inches, convert to feet first by dividing by 12. A room that is 120 inches long is 10 feet long. Once both measurements are in feet, multiply them.

This works for any rectangular space: a bedroom, a kitchen, a garage, a yard, a parking lot. If the space is not a perfect rectangle, you break it into rectangles, compute each one, and add them together.

Key Takeaways

  • Square feet equals length in feet multiplied by width in feet for any rectangular space.
  • Convert inches to feet by dividing by 12 before you multiply.
  • For L-shaped or irregular rooms, divide the space into rectangles, compute each one separately, and add the results.
  • Measure from the inside edge of walls if you are computing usable floor space, or from the outside edge if you are computing the building footprint.
  • A tape measure at least 25 feet long makes measuring large spaces faster and more accurate than a short ruler.

Measuring a Rectangular Room

Start at one corner and measure along the longest wall to the opposite corner. Write down that number in feet and inches — for example, 14 feet 6 inches. Then measure the perpendicular wall the same way. You now have length and width.

Convert any inches to a decimal. Six inches is half a foot, or 0.5. Nine inches is 0.75. Three inches is 0.25. Add that decimal to the whole feet. So 14 feet 6 inches becomes 14.5 feet.

Multiply length by width. If your room is 14.5 feet by 12 feet, the square footage is 14.5 × 12 = 174 square feet. Write this down so you do not have to measure again.

Breaking an L-Shaped or Irregular Room into Rectangles

An L-shaped room is two rectangles joined together. Imagine a vertical line that divides the L into a top rectangle and a bottom rectangle. Measure the length and width of each rectangle separately.

For example, suppose the top part of the L is 10 feet wide and 8 feet deep. That is 80 square feet. The bottom part is 6 feet wide and 5 feet deep. That is 30 square feet. The total is 80 + 30 = 110 square feet.

The line you draw in your mind does not have to be vertical. Draw it wherever it makes the most sense. The goal is to turn an odd shape into two or three straightforward rectangles that you can measure and multiply.

Measuring Spaces That Are Not Rectangular

A circular room or a room with curved walls is harder. One method is to measure the diameter (the distance across the circle through the center) and use the formula for the area of a circle: π × (radius squared). The radius is half the diameter. If the diameter is 20 feet, the radius is 10 feet. The area is 3.14159 × (10 × 10) = 314.16 square feet.

For most practical purposes, you can approximate a curved space by breaking it into rectangles and a triangle or wedge. Measure the rectangles as usual. For the curved part, estimate how much of a full circle it is, measure the radius, and use the circle formula on that portion only.

If the space is very irregular or has many angles, a smartphone app that uses your camera to measure can speed up the work, though a tape measure is still more reliable for the final numbers.

Converting Square Feet to Other Units

One square yard is 9 square feet (because one yard is 3 feet, and 3 × 3 = 9). To convert square feet to square yards, divide by 9. A room that is 180 square feet is 20 square yards.

One square meter is about 10.76 square feet. To convert square feet to square meters, divide by 10.76. A room that is 180 square feet is about 16.7 square meters.

Most real estate and construction in the United States uses square feet, so you will rarely need to convert. But if you are reading a specification from another country or comparing properties across borders, these conversions let you put the numbers on the same scale.

Common Mistakes When Computing Square Feet

The most common error is forgetting to convert inches to feet before multiplying. If you measure 10 feet 6 inches and treat it as 10.6 feet, you will be off. 10 feet 6 inches is 10.5 feet. The difference is small in one measurement but compounds when you multiply.

Another mistake is measuring from the outside of the wall instead of the inside when you need usable floor space. If you are computing how much carpet you need, measure from the inside edge of the baseboard. If you are computing the building footprint for a contractor, measure from the outside.

A third error is forgetting to add the areas when you break a space into multiple rectangles. Write down each rectangle's square footage as you go, then add them all at the end. Do not try to do it in your head.

Tools That Make Measuring Easier

A tape measure at least 25 feet long is the standard tool. It is faster and more accurate than a ruler for rooms larger than a few feet. A 25-foot tape costs between $10 and $30 and lasts for years.

A laser distance measurer bounces a laser off a wall and displays the distance on a screen. It is faster than a tape measure and eliminates the need for a second person to hold the other end. Prices range from $30 to $150 depending on the range and features. For a single room, a tape measure is usually enough. For a whole house or a large property, a laser measurer saves time.

A smartphone app that uses your camera can estimate distances and areas, but it is less accurate than a tape measure or laser. Use it to double-check your math or to get a rough estimate, not as your primary measurement tool.

Frequently Asked Questions

How do I measure a room with a sloped ceiling?

Measure the length and width of the floor as usual. The slope of the ceiling does not change the floor area. If you need to know the volume of the room (for heating or cooling), that is a different calculation that involves the height, but square feet refers only to the floor.

What if I measure in centimeters instead of feet?

Convert centimeters to feet first. One foot is 30.48 centimeters. Divide your measurement in centimeters by 30.48 to get feet. Then multiply length by width as usual. Alternatively, multiply length in centimeters by width in centimeters to get square centimeters, then divide by 929 to convert to square feet.

Do I need to subtract the area of closets or bathrooms?

That depends on what you are computing. Real estate listings usually include closets and bathrooms in the total square footage. If you are ordering flooring or paint, you might subtract closets if they already have flooring. Read the context of what you are buying or selling to know whether to include or exclude these spaces.

How accurate does my measurement need to be?

For most purposes, measuring to the nearest inch is accurate enough. If you are ordering carpet or tile, being off by a few inches in a large room will not matter — you will order a little extra anyway. If you are computing rent or sale price based on square footage, accuracy to the nearest foot is important.

Can I estimate square feet without measuring?

You can pace off a rough estimate by counting your steps, but it is not reliable. A typical adult step is about 2.5 feet, so 10 steps is about 25 feet. This works for a quick guess, but always measure with a tape for any decision that costs money.