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.