Three formulas, and only three
Rectangle: length × width. That is the floor inside the baseboard, not the outside of the siding, unless you are pricing siding.
Triangle: base × perpendicular height ÷ 2. The height is the line that meets the base at a right angle. A gable end that is 24 feet wide and 8 feet tall at the ridge is 96 square feet of wall, not 24 × the rafter length.
Circle: π × radius², and the radius is half the diameter you can tape. An 8-foot round patio is π × 16, about 50.3 square feet. The same circle at 4 inches of concrete is a different calculator. Take the area with you; do not retype it from memory.
The sheet adds the pieces. Overlapping shapes are your problem to avoid: this page will happily count the same floor twice if you hand it two rectangles that cover the same corner.
Worked example: an L that looks like one room
A living room is 20 feet by 12, and a dining bump is 8 feet by 6. Those are 240 and 48 square feet. Together they are 288 square feet. The outer rectangle of 20 × 18 would have been 360, which invents 72 square feet of house. That is the mistake the add-up is for.
The allowance on this page starts at 0% because a square foot is not a yield. Tile setters often buy extra for cuts, and that extra belongs on the material order, with the product’s own waste, not smeared into the room size. If you do raise the percent, the sheet labels it as extra so the measured area is still visible.
What people tape wrong
- Inside dimensions for flooring, outside dimensions for paint on the exterior. Mixing them is a whole wall of error on a small house.
- Using the sloping rafter as the height of a triangle.
- Forgetting that a round bed’s “10 foot” was a radius someone guessed from the center.
- Adding a porch that is not part of the heated floor, then using the total for paint.
- Carrying the number to gravel without the depth. Area is not volume. A 288 square foot bed at 3 inches is 72 cubic feet, which is the mulch and topsoil pages’ job.
FAQ
How do I measure an L-shaped room?
Split it into two rectangles and add them. A 20 × 12 room with an 8 × 6 notch filled in is 240 + 48 = 288 square feet. Measuring the outer box and forgetting to subtract the missing corner counts floor you do not have.
Which height does a triangle use?
The perpendicular height, the shortest line from the opposite corner to the base. The slanted side is longer and will overstate the area. Area = base × height ÷ 2.
Is a circle based on diameter or radius?
This page asks for diameter, because that is the number you tape across a round bed. Area = π × (diameter ÷ 2)². Entering the diameter in the radius’s place multiplies the area by four.
Why is the allowance zero?
Square feet are a measurement, not a bag yield. Zero means the tape. Add a percent only when this total is about to become tile, sod, or flooring, and use the material calculator for anything that has its own published waste.
Sources
- NIST Special Publication 330, The International System of Units — The foot used here is the international foot, 0.3048 meters exactly, which is the conversion behind the metric toggle.
- NIST Handbook 44, Appendix C, units of length — A square foot is a foot on each side. A square yard is 9 square feet. This page stays in square feet (or square meters) and does not call the total a material order.
- CalcWright, What America Calculates Before It Builds — Their study lists “square footage calculator” at about 201,000 US searches a month, second among the home-project terms they tracked. Read as their estimate, not a Keyword Planner export.