Basic square footage formula
For a rectangular area, multiply the length by the width. Both dimensions should be expressed in feet.
Example: rectangular room
A room measuring 20 feet long and 12 feet wide has an area of 240 square feet.
How to measure multiple rooms
Calculate each room separately and add the individual areas together. This is usually more reliable than trying to treat an irregular floor plan as one large rectangle.
How to handle irregular spaces
- Split the space into rectangles or other simple shapes.
- Calculate each section separately.
- Add the areas together.
- Measure alcoves, extensions and offsets independently.
Square footage for walls
Wall area is calculated using wall length multiplied by wall height. If several walls have different dimensions, calculate each separately.
Should doors and windows be deducted?
For rough estimates, small openings are sometimes left in the total. For more precise paint, siding or finish estimates, the area of significant doors and windows can be subtracted.
Use actual measured dimensions instead of nominal room sizes when purchasing materials. Small differences become significant across large areas.
Worked example for a room
A rectangular room that is 14 feet long and 11 feet wide has an area of 154 square feet. If the room contains a separate 4-by-3-foot alcove that is also part of the project, calculate its 12 square feet separately and add it, giving 166 square feet in total.
How to handle irregular spaces
Most irregular rooms can be divided into several simple rectangles. Sketch the space, mark the dimensions of each section, calculate each area and add the results. This method is generally easier to check than trying to force an irregular outline into one formula.
Wall square footage is calculated differently from floor area
For a rectangular wall, multiply wall width by wall height. For an entire room, calculate the area of each wall and add them together. Doors and windows can then be deducted when the task requires net wall area, such as a detailed finish-material estimate.
Square feet, linear feet and cubic feet are different
Square footage measures two-dimensional area. Linear footage measures length only, while cubic footage measures three-dimensional volume. Choosing the correct measurement matters because flooring may be sold by square foot, trim by linear foot and fill material by cubic volume.
Measurement tips for better estimates
- Measure to consistent reference points throughout the space.
- Keep all dimensions in the same unit before multiplying.
- Calculate irregular sections separately rather than estimating their shape visually.
- Avoid rounding individual measurements too aggressively before the final calculation.