Square footage is just area measured in feet. For a plain rectangular room it takes about five seconds. Where people get tripped up is on rooms that aren't rectangles, on mixed measurements like "11 feet 7 inches," and on figuring out how much material to actually buy once they have the number.
This covers all three.
The basic formula
Example: A bedroom measuring 12 feet by 10 feet.
12 × 10 = 120 square feet
That's the whole thing. The complications come from everything else.
When your measurements include inches
Convert the inches to a decimal before multiplying. Divide the inches by 12:
| Inches | Decimal feet |
|---|---|
| 3 in | 0.25 |
| 4 in | 0.33 |
| 6 in | 0.5 |
| 8 in | 0.67 |
| 9 in | 0.75 |
So a room measuring 12 ft 6 in by 10 ft 3 in becomes 12.5 × 10.25 = 128.1 square feet.
The alternative is to measure everything in inches, multiply, then divide by 144. Both work. What doesn't work is dividing by 12 at the end — that's the single most common square footage mistake, and it leaves you with a number twelve times too large.
Rooms that aren't rectangles
L-shaped rooms
Split the L into two rectangles, calculate each, add them.
Draw the room on paper and extend one wall line across until you've cut the shape into two clean boxes. Measure each box separately.
Example: The main section is 12 × 10, and the extension is 6 × 5.
120 + 30 = 150 square feet
This approach scales to any irregular shape — a bay window alcove, a room with a chimney breast, an open-plan space. Cut it into rectangles, do each one, add them up.
Triangular areas
Height here means the perpendicular distance from the base to the opposite point — not the length of a slanted side. For a triangular section with a 9-foot base and a 6-foot perpendicular height: (9 × 6) ÷ 2 = 27 square feet.
Circular areas
The radius is half the width across the centre. A round patio 14 feet across has a 7-foot radius: 3.14159 × 49 = 153.9 square feet.
Subtracting a cutout
For a kitchen island, a built-in cabinet, or a stairwell opening, calculate the full room, calculate the cutout, and subtract. A 200 sq ft kitchen with a 3 × 6 island has 200 − 18 = 182 square feet of floor to cover.
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Open Square Footage Calculator →How much material to actually buy
Your square footage is the minimum. Nobody buys exactly that much, because cutting produces waste.
Flooring
| Installation type | Add |
|---|---|
| Standard straight lay | 10% |
| Diagonal layout | 15% |
| Herringbone / chevron | 20% |
| Room with many angles or cutouts | 15% |
For 150 square feet with a standard install: 150 × 1.10 = 165 square feet to order.
There's a second reason to round up beyond waste. Flooring is made in dye lots, and products get discontinued. If a plank is damaged in three years, leftover material from the original batch is the only thing guaranteed to match.
Paint
Paint is about wall area, not floor area — a different measurement entirely.
For a 12 × 10 room with 8-foot ceilings: the perimeter is (12 + 10) × 2 = 44 feet, so 44 × 8 = 352 square feet of wall.
A gallon of paint covers roughly 350–400 square feet in one coat. Most jobs need two coats, so that room needs about two gallons. Subtract openings only if they're substantial — a standard door is about 21 sq ft and an average window about 15 sq ft. With one door and one window you'd knock off 36 sq ft, which doesn't change the gallon count here. On a large room with lots of glazing it can.
Measuring mistakes that cost money
- Measuring at the baseboard instead of the wall. Trim can eat an inch on each side. Across a room that's a couple of square feet — usually irrelevant for paint, occasionally relevant for tile where you're counting whole pieces.
- Assuming the room is square. Older houses frequently aren't. Measure both the length and the width at each end of the room; if they differ, use the larger number so you don't come up short.
- Mixing units mid-calculation. Feet in one dimension and inches in the other produces a meaningless number. Convert first, then multiply.
- Forgetting closets and alcoves. If flooring is going in there, it counts. A pair of closets can easily be 20 square feet.
- Rounding down to save money. Running 5% short mid-project means a second delivery, a possible dye lot mismatch, and a stalled installation. The waste allowance is cheaper than the delay.
Square footage of a whole house
Listed house square footage usually refers to gross living area — finished, heated space measured from the exterior walls. It typically excludes garages, unfinished basements, attics with low headroom, and open porches, which is why a listing's number rarely matches the sum of the rooms you measure yourself.
For your own project estimates, measure the actual rooms you're working on. The listing figure is for comparing properties, not for ordering materials.
Related: Percentage Calculations You'll Actually Use — useful for the waste-allowance math above.