Square footage is the number almost every home project starts with. Flooring, tile, paint, and sod are all sold based on it, so getting it right saves you from buying too much or coming up short. The good news is that the math is simple once you know how to handle the shape you are measuring.
The basic formula for a rectangle
Most rooms are rectangles, and for a rectangle you simply multiply length by width. Both measurements need to be in the same unit, usually feet.
A room that is 12 feet long and 15 feet wide is 12 times 15, which is 180 square feet. That is all there is to it. If your measurements are in inches, divide each one by 12 first to convert to feet, then multiply.
How to handle an L-shaped room
Real homes are full of rooms that are not neat rectangles. The trick for any irregular room is to break it into rectangles you can measure separately, then add the pieces together.
For an L-shaped room, draw a line that splits the L into two rectangles. Measure each rectangle on its own, calculate its square footage, and add the two numbers. A room made of a 12 by 10 section and a 6 by 8 section is 120 plus 48, or 168 square feet. The same approach works for a T-shape or any layout with jogs and bump-outs.
Circles and triangles
Some spaces are not made of straight lines at all.
For a circle, such as a round patio, measure the distance from the center to the edge (the radius) and use 3.14159 times the radius squared. A patio with a 7 foot radius is 3.14159 times 49, which is about 154 square feet. If you only know the full width across (the diameter), the radius is just half of that.
For a triangle, multiply the base by the height and take half. A triangular deck section with a 10 foot base and 8 foot height is one half times 10 times 8, or 40 square feet.
Converting square footage to materials
Square footage is only the first step. Once you have it, you can work out almost any material:
- Flooring and tile are sold by coverage per box. Divide your square footage (plus a waste factor) by the coverage on the box.
- Paint covers about 350 square feet per gallon for one coat, or plan on 175 for two coats.
- Sod and turf are sold by the square foot or by the pallet, which usually covers around 450 square feet.
Always add a waste factor before you buy. Use 10 percent for simple layouts and 15 percent for diagonal patterns or rooms with lots of corners. That margin covers cuts, offcuts, and the occasional damaged piece.
A few measuring tips
- Measure at floor level, not at a countertop or windowsill, since those can stick out.
- Round each measurement to the nearest inch. Guessing to the nearest foot can throw off a large room by a full box of flooring.
- Measure into closets and doorways if you are flooring them, and subtract permanent fixtures like a kitchen island if you are not.
Let the calculator do the shapes
The formulas are easy, but juggling an L-shape or a circle by hand invites mistakes. Our square footage calculator handles rectangles, circles, triangles, and combinations, and it converts the result into square yards and square meters too. Once you have your number, feed it into the flooring calculator or the tile calculator to get an exact box count with waste included.
Frequently Asked Questions
Does square footage include closets?
For flooring, yes. Include every area that gets flooring, which means closets, pantries, and any recessed spaces. For home listing purposes, “livable square footage” typically includes closets but not garages or unfinished basements. When measuring for flooring, always measure the actual space you are covering, not what is on the listing.
How do I measure the square footage of an L-shaped room?
Split the room into two rectangles. Measure the length and width of each rectangle separately, multiply each pair, and add the two results. For example: a 10 by 12 section plus a 6 by 8 addition is 120 plus 48 equals 168 square feet. Any irregular room can be broken down into rectangles this way.
What is the difference between gross and net square footage?
Gross square footage measures from the outside of exterior walls and includes wall thickness. Net square footage measures from the inside faces of walls and represents the usable floor area. For flooring and tile, you want net square footage since that is the area you are actually covering. The difference is usually 5 to 10 percent in a typical room.
Do I measure square footage from the inside or outside of walls?
From the inside, wall to wall, for flooring and tile projects. Measure the actual floor area, not the architectural footprint. In most rooms, measuring to the baseboard is close enough since the flooring goes up to the baseboard. If you are replacing baseboard as part of the project, measure to the drywall face.
How do I measure a room with a bay window?
Include the bay. Walk into the bay, measure its depth and width, and add that area to the main room rectangle. A typical bay window alcove is 2 to 4 feet deep and 4 to 6 feet wide, which adds 8 to 24 square feet to the room total. Do not skip it even if the bay seems small.
How many square feet is a 12 by 12 room?
Exactly 144 square feet. This is a useful reference point: a 12 by 12 room is also exactly one square (a unit used in roofing) and comfortably fits a king bed with room to walk around it. For flooring, add the waste factor: 144 times 1.10 equals about 160 square feet, which means you need just over 5 boxes if each box covers 30 square feet.