Square footage and materials calculator (paint, tile, flooring)
Floor square footage is length times width. For paint, multiply the room perimeter by wall height, subtract doors and windows, then divide by about 350 sq ft per gallon for each coat. A 15 × 12 ft room with 8 ft walls needs about 2.5 gallons for two coats, so buy 3 gallons. Add 10% extra tile for cuts.
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📌 Key facts
- Floor square footage is length times width, in feet.
- For walls, multiply the room perimeter by the wall height, then subtract doors and windows.
- One gallon of paint covers about 350 square feet per coat; a 15 × 12 ft room with 8 ft walls needs about 2.5 gallons for two coats.
- Order about 10% extra tile for cuts, and more for diagonal layouts.
- One square meter equals about 10.76 square feet.
The formulas
\[ \begin{gathered} \text{Floor area} = \text{length} \times \text{width} \\ \text{Wall area} = 2 \times (\text{length} + \text{width}) \times \text{height} - \text{doors and windows} \\ \text{Paint (gal)} = \dfrac{\text{area} \times \text{coats}}{\text{coverage (sq ft per gallon)}} \\ \text{Tiles} = \dfrac{\text{area} \times (1 + \text{waste})}{\text{area of one tile}} \end{gathered} \]
A standard door is about 21 sq ft and a window 12 to 20 sq ft. A gallon of interior paint typically covers 350 to 400 sq ft; use 300 on rough or porous drywall that is not primed.
How much extra to order
| Installation | Waste to add |
|---|---|
| Straight tile | 5 to 10% |
| Diagonal tile | 10 to 15% |
| Straight hardwood or laminate | 5 to 7% |
| Herringbone flooring | 10 to 15% |
Buy all tile or flooring at once from the same dye lot to avoid color differences.
Handy conversions
1 square meter = 10.764 sq ft. 1 US gallon = 3.785 liters. 1 acre = 43,560 sq ft. 350 sq ft per gallon equals about 8.6 m² per liter.
Square footage calculator: a full bedroom example
Take a bedroom that measures 14 ft by 12 ft with 8 ft ceilings. First, the floor area is 14 × 12, or 168 sq ft. Then the perimeter is 2 × (14 + 12), or 52 ft. Next, the wall area is 52 × 8, or 416 sq ft. Finally, subtract a door of 20 sq ft and a window of 15 sq ft, which leaves 381 sq ft to paint.
For example, two coats cover 762 sq ft. At 350 sq ft per gallon, you need about 2.2 gallons. So buy 3 gallons, and you keep a bit for touch-ups.
Square footage calculator: paint, tile and flooring
Each material follows its own rule. Here are the numbers to know before you shop.
| Material | Area to measure | Extra to order |
|---|---|---|
| Wall paint | Walls minus openings | 5 to 10% |
| Straight tile | Floor | 10% |
| Diagonal tile | Floor | 15% |
| Flooring | Floor | 5 to 10% |
In fact, cuts around corners and doorways create waste. Also, keep a few spare tiles for later repairs, because a new batch may have a slightly different shade.
Square footage calculator: common mistakes and helpful tools
A few mistakes come up often. First, people forget the ceiling, which uses the same math as the floor. Second, they subtract openings that are too small or forget them. Third, they mix up floor area and living area. So the result is too low or too high.
To convert measures, use the unit converter. For a budget, apply percentages with the percentage calculator. Finally, for a quantity ratio, open the proportion calculator.
Square footage calculator: buying and installing tips
Before you buy, measure each room twice. Then sketch a simple plan with the lengths. That way, you spot corners and alcoves early. For an L-shaped room, split it into two rectangles and add the areas. For example, 13 × 10 ft plus 6 × 6 ft gives 166 square feet. Also, note baseboard thickness, because it trims the tiled area slightly.
Finally, compare prices per square foot, not per box. In fact, a box of flooring often covers 20 square feet and a gallon of paint covers 350. So always round up to the next full unit, or the square footage will fall short at the end of the job. A good square footage plan saves a second trip to the store.
Also, keep your room sketch. Then you can reuse the square footage numbers when you repaint or replace the floor years later.
3 real-life examples
A 15 × 12 ft room with 8 ft walls and no deduction
Floor: 180 sq ft. Walls: \( 2 \times 27 \times 8 = \) 432 sq ft. Paint, 2 coats at 350 sq ft per gallon: \( \dfrac{432 \times 2}{350} = \) 2.5 gallons, so buy 3.
Tiling 180 sq ft with 12 × 12 in tiles
180 × 1.10 (10% waste) = 198 sq ft; each tile is 1 sq ft, so 198 tiles.
Laying flooring over 200 sq ft
\( 200 \times 1.05 = 210 \, \text{sq} \) ft. With boxes of 20 sq ft, \( \dfrac{210}{20} = 10.5 \), so 11 boxes.