Working Out Square Footage
Square footage is area — length multiplied by width for a rectangle, with variations for other shapes. The formulas are simple; the errors come from units and from irregular rooms.
| Shape | Formula |
|---|---|
| Rectangle or square | Length × Width |
| Triangle | ½ × Base × Height |
| Circle | π × (Diameter ÷ 2)2 |
| Trapezoid | ½ × (Base1 + Base2) × Height |
| L-shape | Split into two rectangles and add |
Measuring in Feet and Inches
The most common mistake is treating 12 feet 6 inches as 12.6 feet. Inches convert as a fraction of twelve, not ten:
| Inches | Decimal feet | Inches | Decimal feet |
|---|---|---|---|
| 1 | 0.083 | 7 | 0.583 |
| 2 | 0.167 | 8 | 0.667 |
| 3 | 0.25 | 9 | 0.75 |
| 4 | 0.333 | 10 | 0.833 |
| 5 | 0.417 | 11 | 0.917 |
| 6 | 0.5 | 12 | 1.0 |
So 12 feet 6 inches is 12.5 feet. Getting this wrong on both dimensions of a room compounds the error.
Area Unit Conversions
| Unit | Square feet |
|---|---|
| 1 square inch | 0.00694 |
| 1 square foot | 1 |
| 1 square yard | 9 |
| 1 square metre | 10.764 |
| 1 square (roofing) | 100 |
| 1 acre | 43,560 |
| 1 hectare | 107,639 |
Note the roofing "square" — 100 square feet — which is how roofing material is sold and quoted in North America.
Irregular Rooms
Break the space into rectangles, triangles and part-circles, work out each area, and add them. For an L-shaped room, the simplest approach is to extend one wall mentally to complete the rectangle, calculate that, then subtract the missing corner. Both methods give the same answer; the split-and-add version is usually less error-prone on site.
Bay windows, alcoves and closets are counted or excluded depending on the purpose. For flooring, count anything you will cover. For a property listing, follow the local measurement standard.
Typical Room Sizes
| Space | Typical size | Square feet |
|---|---|---|
| Small bedroom | 10 × 10 ft | 100 |
| Master bedroom | 14 × 16 ft | 224 |
| Living room | 16 × 20 ft | 320 |
| Kitchen | 12 × 14 ft | 168 |
| Full bathroom | 5 × 8 ft | 40 |
| Two-car garage | 20 × 20 ft | 400 |
| U.S. median new home | — | ~2,300 |
How Much Material to Order
| Material | Typical coverage | Suggested waste |
|---|---|---|
| Interior paint | 350–400 sq ft per gallon per coat | 10% |
| Hardwood or laminate flooring | Varies by box | 10% straight lay, 15% diagonal |
| Tile | Varies by box | 10–15%, more for complex patterns |
| Carpet | Sold by the linear foot on 12 ft rolls | 10–20% depending on room shape |
| Sod or turf | Varies by roll | 5–10% |
Waste allowance is not optional. Cuts at walls, pattern matching and damaged pieces all consume material, and running short mid-job means a second delivery and possibly a different dye lot.
Measuring for Property Listings
Different standards count different spaces, which is why the same house can be advertised at two different sizes.
- Gross living area (GLA) counts finished, heated space above grade. Basements are excluded even when finished.
- Measurements are taken from the exterior of the walls in most U.S. appraisal practice, so the figure exceeds the sum of interior room areas.
- Garages, unfinished attics and open porches are excluded from GLA and listed separately.
- ANSI Z765 is the standard most U.S. appraisers follow; the UK uses RICS guidance, which measures differently again.
Frequently Asked Questions
How do I convert square feet to square metres?
Divide by 10.764, or multiply by 0.0929. A 1,000 sq ft flat is about 93 m².
Does square footage include the walls?
For appraisal it usually does, since measurements are taken outside the exterior walls. For flooring it should not — measure the floor you are covering.
How much paint do I need for a room?
Calculate wall area, not floor area: perimeter × ceiling height, minus doors and windows. Then divide by 350 sq ft per gallon and multiply by the number of coats.
How many square feet in an acre?
43,560 — historically the area a yoke of oxen could plough in a day, being one chain by one furlong.