How to calculate square footage
For a rectangular room, square footage is simply length × width measured in feet: a 12 × 15 ft bedroom is 180 sq ft. Measure to the nearest inch, enter the numbers in whatever unit your tape reads — inches, meters and centimeters are converted automatically — and the calculator reports the area in square feet, square yards (÷ 9), square meters (× 0.0929) and square inches (× 144).
Other shapes. A circle is π × radius²; a triangle is ½ × base × height; a trapezoid is ½ × (top + bottom) × height. For an L-shaped room the calculator treats it as two rectangles and adds them — enter the full outer length and width of the first block and the length and width of the second. For anything more complicated, split the floor plan into rectangles, calculate each, and use Add this area to total to sum them.
Buying materials. Flooring, tile, carpet, paint and sod are all sold by area, and every job produces off-cuts. The usual allowance is 5–10% waste for straight rectangular layouts and 15% for diagonal patterns, herringbone, or rooms with many angles. Enter a waste percentage and a price per square foot to get the quantity to order and its cost. For flooring sold by the box, divide the with-waste figure by the box coverage and round up.
Home listings measure differently. "Gross living area" in real estate counts finished, heated, above-grade space measured from the exterior walls, so it is larger than the sum of interior room measurements. Garages, unfinished basements and porches are excluded. This calculator is for interior measurement and material estimating rather than appraisal.