What square footage measures
Square footage is the two-dimensional area of a floor, wall, lot, or other surface, measured in square feet. It is the standard unit for U.S. real estate, flooring, paint, and landscape estimates. For most shapes the formula is length × width; for irregular shapes you break the area into rectangles or use formulas for triangles and circles.
square feet = length(ft) × width(ft)1 ft² ≈ 0.0929 m² (or 1 m² ≈ 10.764 ft²)Common shape formulas
| Shape | Formula | Example |
|---|---|---|
| Rectangle | length × width | 12 × 14 = 168 ft² |
| Square | side² | 10² = 100 ft² |
| Triangle | (base × height) / 2 | (8 × 12) / 2 = 48 ft² |
| Circle | π × radius² | π × 5² ≈ 78.5 ft² |
| Trapezoid | ((a + b) / 2) × height | ((10 + 14) / 2) × 8 = 96 ft² |
| Ellipse | π × a × b | π × 6 × 4 ≈ 75.4 ft² |
For any L-shape, split it into two rectangles and add. For curves, approximate with a circle, ellipse, or sector. Real-world rooms with bay windows, alcoves, or sloped ceilings need to be measured in pieces.
Worked example — flooring a living area
An L-shaped living/dining room: a 14 × 18 main rectangle plus a 10 × 8 dining alcove.
- Main:
14 × 18 = 252 ft² - Alcove:
10 × 8 = 80 ft² - Total:
332 ft² - With 10% waste for cuts and pattern matching:
≈ 365 ft²to order
Material-specific waste factors
| Material | Recommended overage |
|---|---|
| Carpet | 5–10% |
| Vinyl plank, laminate | 7–10% |
| Hardwood — straight install | 7–10% |
| Hardwood — diagonal install | 15% |
| Tile — square pattern | 10% |
| Tile — diagonal or herringbone | 15–20% |
| Wallpaper | 10–15% (more for large patterns) |
| Paint | 10% (one extra coat allowance) |
Walls vs floors
Paint and wallpaper estimates require wall square footage, not floor. Calculate each wall as height × width and subtract openings (doors are typically 21 ft², windows 12–15 ft² each).
A 12 × 14 room with 8 ft ceilings, one door, two windows:
- Two long walls:
2 × (14 × 8) = 224 ft² - Two short walls:
2 × (12 × 8) = 192 ft² - Subtotal:
416 ft² - Subtract openings:
416 − 21 − 30 = 365 ft²of paintable wall - One gallon paint covers ~350 ft² → about 1.1 gallons per coat; order 2 gallons for two coats.
Real estate gotchas
- Gross vs net living area. Most real estate listings exclude unfinished basements, garages, and exterior walls. Some include the wall thickness; others don't.
- Heated vs unheated. Many MLS systems require "heated square footage" — a finished, climate-controlled space.
- Stairs are counted twice. Once on each floor.
- Ceilings below 7 ft — often excluded from official square footage under ANSI Z765 standards (the most-used U.S. measurement standard).
Lot and outdoor areas
- 1 acre = 43,560 ft² (a square 209 ft on a side).
- 1 hectare = 10,000 m² = ~107,639 ft² (about 2.47 acres).
- Tennis court ≈ 2,800 ft².
- American football field ≈ 57,600 ft² (1.32 acres including end zones).
Estimating roof square footage
A roof's footprint is not its surface area — pitch increases the actual area. The conversion factor depends on the roof slope (rise over run):
- 4/12 pitch — multiplier ≈ 1.054
- 6/12 pitch — multiplier ≈ 1.118
- 8/12 pitch — multiplier ≈ 1.202
- 10/12 pitch — multiplier ≈ 1.302
- 12/12 pitch (45°) — multiplier ≈ 1.414
Multiply the building's footprint by the slope multiplier to get roof area. Roofers also use "squares" — 1 square = 100 ft² — so a 2,400 ft² roof is 24 squares.