The one formula, and how to measure
Square footage is length × width, nothing more: a 12 ft × 10 ft room is 120 square feet. The mistakes happen in the measuring, not the math. Measure at floor level, wall to wall, at the room's widest points, and write each room down before moving on. Feet and inches trip people up — the calculator wants decimal feet, so convert the inches by dividing by 12: 10 feet 6 inches is 10.5 feet, and 9 feet 3 inches is 9.25 feet. If a room has an alcove, a bay window or an L-shape, treat each piece as its own rectangle, calculate the pieces, and add them — the add-a-room list above does exactly that, and it doubles as your whole-house measure-up sheet.
What the number is used for
- Flooring and carpet. Material is sold by the square foot or square yard (1 sq yd = 9 sq ft), and installers quote from the same figure. Add 5–10% for cuts.
- Paint. For walls, measure the perimeter × ceiling height, subtract roughly 20 sq ft per door and 15 per window; a gallon covers about 350 sq ft per coat.
- Tile and backsplashes. Small areas, exact counts — grout lines do not change the area you need to cover.
- Real estate. A home's advertised square footage follows appraisal rules (finished, above-ground, heated space) and may differ from your raw room-by-room total.
- Garden and yard. Mulch, sod and seed are all sold by coverage: one cubic yard of mulch spread 3 inches deep covers about 108 sq ft.
The waste allowance, honestly explained
Straight rooms with straight-lay flooring waste about 5% to cuts at the walls. Diagonal layouts, herringbone patterns, stairs and rooms with many corners waste 10–15%. Buying exactly the calculated area is the classic false economy: one miscut board or a box from a different dye lot can stall a project for days. Order the allowance, keep the receipt, and return unopened boxes — most stores take them back. The cost estimate above uses the exact area, so add your allowance before treating it as a shopping budget.
Circular and triangular areas
For a round rug, patio or garden bed, the area is π × radius² — the radius is half the diameter, so a circle 10 feet across has an area of about 78.5 sq ft. For a triangle, area is half the base × the height: a triangular corner 8 feet by 6 feet is 24 sq ft. Split odd-shaped rooms into rectangles plus one of these and the add-up method still works.
Frequently asked questions
How do I calculate square footage?
Multiply the length of the room by its width, both measured in feet. A room 12 feet long and 10 feet wide is 12 × 10 = 120 square feet. Measure wall to wall at the widest points, and for an irregular room, split it into rectangles and add the pieces.
How much extra flooring should I buy?
Add 5–10% to the calculated area for cuts and mistakes, and up to 15% for diagonal or herringbone layouts and rooms with many angles. On a 120 sq ft room that means ordering about 130 sq ft of material.
How do I convert inches or meters to feet?
Divide inches by 12 (30 inches = 2.5 feet) and multiply meters by 3.281 (4 meters ≈ 13.12 feet). Enter the measurements as decimal feet in this calculator — for example, 10 feet 6 inches is 10.5 feet.
Does the calculator include closets and alcoves?
Only if you measure them. For flooring, add closets and alcoves as their own small rectangles using the add-a-room list. For a home's official living area, closets count but garages, unfinished basements and porches usually do not, under standard appraisal rules.
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