You need the square footage of a room because you are buying flooring, tile, carpet or paint, or because a contractor asked for it and you want to check his number. Knowing how to calculate square footage of a room takes one formula and about five minutes with a tape measure. The formula is length times width. The five minutes are for the closets, the L-shape and the inches that most people skip, which is why the number is usually wrong by a box or two. This guide walks through the method, works a real example, and ends with a free calculator that does the arithmetic for you.
How to calculate square footage of a room in four steps
- Measure the length and the width in feet. Run the tape wall to wall at floor level, not along the baseboard. Write both numbers down with the inches, for example 14 ft 6 in by 11 ft 3 in.
- Turn inches into a decimal. Divide the inches by 12. Six inches is 0.5 ft, three inches is 0.25 ft, nine inches is 0.75 ft. So 14 ft 6 in becomes 14.5 and 11 ft 3 in becomes 11.25.
- Multiply. 14.5 × 11.25 = 163.1 square feet. That is the floor area of a plain rectangular room.
- Add anything that is not in the rectangle. The closet, the doorway alcove, the bay window. Each one is its own small rectangle. Measure it, multiply it, and add it to the total.
That is the whole method for a simple room. The rest of this article is about the rooms that are not simple, which is most of them.
L-shaped and odd rooms
An L-shaped room is two rectangles. A room with a nook is a rectangle plus a small rectangle. A room with a corner cut off is a rectangle minus a triangle. Sketch the outline on paper first, then draw lines that split it into pieces you can measure, number each piece, and add the pieces up.
A worked example
Take a living room shaped like an L. The main part is 16 ft 0 in by 12 ft 6 in. The leg of the L is 8 ft 0 in by 6 ft 4 in. There is a coat closet off the leg that is 4 ft 0 in by 2 ft 6 in.
| Section | Length | Width | Square feet |
|---|---|---|---|
| Main room | 16.0 ft | 12.5 ft | 200.0 |
| Leg of the L | 8.0 ft | 6.33 ft | 50.7 |
| Closet | 4.0 ft | 2.5 ft | 10.0 |
| Total | 260.7 |
Someone who measured the main room only and called it “16 by 12” would have written down 192 square feet and come up almost 70 square feet short. That is three boxes of plank flooring, and the third box comes from a different dye lot.
Converting to square yards and square meters
Carpet is sold by the square yard. Divide square feet by 9. The room above is 260.7 ÷ 9 = 29 square yards. Some tile and most imported flooring is labelled in square meters. Divide square feet by 10.764. The same room is 24.2 square meters. Keep the square footage as your working number and convert at the end, after you have added the waste.
How much extra to buy
Every material has cuts, breakage and pieces that end up too short to use. Buy the bare area and you will be back at the store. The extra is called the waste allowance, and it depends on the material and the pattern.
- Plank flooring (vinyl, laminate, hardwood) laid straight: add 10 percent.
- Plank flooring laid on a diagonal or in a herringbone pattern: add 15 to 20 percent.
- Tile: add 15 percent, more for large tiles in a small room, because every cut piece at the wall is wasted.
- Carpet: add 10 percent, and let the installer confirm, since roll width decides where the seams fall.
- Paint: the number you need is wall area, so measure the perimeter of the room, multiply by ceiling height, and subtract doors and windows.
For the L-shaped room with straight-laid plank, 260.7 × 1.10 = 286.8 square feet. A box of luxury vinyl plank usually covers about 24 square feet, so 286.8 ÷ 24 = 11.95, which means 12 boxes. Always round up to the next whole box. The National Wood Flooring Association publishes installation guidelines that cover waste and acclimation for wood floors in more detail if you are laying hardwood.
The three mistakes that throw the number off: measuring along the baseboard instead of the wall, forgetting closets and alcoves, and rounding the inches away. Fix those and your total will be within a box of what the installer measures.
A free calculator that does the math
If you would rather not do this by hand, the calculator below does it. You enter each section of the room in feet and inches, up to eight sections for L-shapes and closets, and it draws the room as you type. Pick the material, set the waste allowance, and it gives you the total in square feet, square yards and square meters, plus the number of boxes to buy. It is the same tool contractors put on their own websites, and it is free.
The full version, with your own material list and your own waste figures, is on the square footage calculator widget page, and it lives with the other free tools on the free construction website tools page.
For flooring, tile and painting contractors
If you price by the square foot, this question is the first thing every customer asks, and the answer they come up with on their own is usually wrong. Put the calculator on your website and they measure once, properly, with your material list and your waste figures. They see the total and the box count. You get their name, email and every measurement they entered, as a lead, before the first call. It goes into any page with three lines of code and you can change the materials yourself when your supplier changes a box size.
We have covered why a calculator out-pulls a plain contact form in our article on cost calculators for contractor websites, and what to do with the lead once it arrives in the one on why contractor websites don’t generate leads. The whole set of free tools, with the remodel and roof estimators and the before-and-after slider, is on the free construction website tools page. If you would rather have someone build the page around it, see our services.
