How the square footage calculator works
Square footage is area measured in square feet: the number of one foot by one foot squares that would cover the surface. For a rectangle it is simply length times width. Every other shape here reduces to a standard area formula, and any measurement unit is converted to feet before squaring, so you can measure in inches, yards, or meters and still read the answer in square feet, square yards, and square meters.
| Shape | Formula | Example |
|---|---|---|
| Rectangle | length x width | 12 x 15 = 180 sq ft |
| Circle | pi x diameter^2 / 4 | 10 ft diameter = 78.5 sq ft |
| Triangle, three sides | Heron: sqrt(s(s - a)(s - b)(s - c)), s = (a + b + c) / 2 | 10, 10, 12: s = 16, sqrt(16 x 6 x 6 x 4) = 48 sq ft |
| Triangle, base and height | base x height / 2 | 10 x 8 / 2 = 40 sq ft |
| Trapezoid | (top + bottom) / 2 x height | (8 + 12) / 2 x 5 = 50 sq ft |
| L shape | rectangle 1 + rectangle 2 | 20 x 12 + 8 x 6 = 288 sq ft |
Worked example
The default is a 12 by 15 foot room with flooring at $3.50 per square foot and a 10% waste allowance.
area = 12 ft x 15 ft = 180 sq ft square yards = 180 / 9 = 20 sq yd square meters = 180 x 0.09290304 = 16.72 sq m waste = 180 x 10% = 18 sq ft, so buy 198 sq ft cost = 198 x $3.50 = $693.00
Add more rooms in the extra rows and the areas are summed before the conversion and cost steps, which is how you get a whole-house total for flooring or paint. Rows left blank are ignored.
Unit conversions
| Unit | Feet per unit | Square feet per square unit |
|---|---|---|
| Inch | 1/12 (0.08333) | 1/144 (0.006944) |
| Foot | 1 | 1 |
| Yard | 3 | 9 |
| Meter | 3.28084 | 10.7639 |
The square conversion is the length conversion squared, which is the step people most often miss. A meter is 3.28 feet, so a square meter is 3.28 x 3.28 = 10.76 square feet, not 3.28. Going from square feet to square meters, multiply by 0.09290304, which is exactly 0.3048 squared because the international foot is defined as 0.3048 meters.
Measuring an irregular room
Almost every real room is a set of rectangles. Sketch the floor plan, draw lines that split it into rectangles (an L shape is two, a room with a closet bump-out is two or three), measure each rectangle's length and width, and enter each one as a room row. For a bay window or a rounded alcove, use the circle shape for the curved part and take half or a quarter of it. Measure along the walls at floor level, since walls are rarely perfectly straight, and round each measurement up to the next inch to keep the estimate on the safe side.
Waste and cost
Flooring, tile, and turf are sold by area, but cutting pieces to fit always leaves offcuts. The waste percent covers that: 10% is the usual allowance for plank flooring laid straight, 15% for diagonal layouts or patterned tile, and 20% for small rooms with many corners. Paint works differently: coverage is quoted per gallon (about 350 to 400 square feet per coat), so divide wall area by coverage and multiply by the number of coats instead of using the cost fields. Carpet is often priced per square yard; the square yard line gives you that figure directly.