Enter the length and width of a room or area to get the square footage and acreage. Add as many rooms as you need and the calculator keeps a running total.
Square footage is simply length multiplied by width, where both measurements are in feet, and it is one of the few math facts homeowners look up constantly. Real estate listings quote it to compare homes. Flooring and carpet are priced per square foot, so a wrong measurement means ordering too little material or paying for waste. Paint coverage, tile counts, and even HVAC sizing all start from the same square footage number before anything gets adjusted for ceiling height or room shape.
Every room you add is measured the same way and then summed, which is why the calculator above lets you stack multiple rooms into one running total. One acre equals 43,560 square feet, so any total in square feet converts to acres by dividing by that constant, and to square meters by dividing by roughly 10.764.
A bedroom measures 12 feet long and 10 feet wide. What is its square footage?
Mixed units cause most of the errors here: if a room is measured in inches, divide by 12 before multiplying, since squaring an unconverted inch measurement gives an area 144 times too large. Irregular rooms with an alcove or bump-out are not one rectangle; split them into two or more rectangles, calculate each separately using the multi-room fields above, and add the parts together rather than measuring one outer boundary that does not match the actual floor shape.
Working on a project that also involves 3D space? The volume calculator covers boxes, cylinders, and cones. For electrical work on the same project, try the Ohm's law calculator.
Once the floor area is set, volume and electrical load are the next two numbers most projects need.
Multiply the length by the width, both in feet. For a room that is 12 feet long and 10 feet wide, square footage = 12 x 10 = 120 sq ft. If you measured in inches, divide each measurement by 12 first to convert to feet.
A 12x12 room is 144 square feet. The formula is length x width: 12 x 12 = 144 sq ft.
A 20x20 area is 400 square feet. Multiply 20 x 20 = 400 sq ft.
A 40x60 area is 2,400 square feet. Multiply 40 x 60 = 2,400 sq ft. That equals about 0.055 acres.
Marcus Vance spent a decade building data pipelines and testing calculation logic before he started writing about the math behind the tools. On MathCalcTools he checks every formula against the source math, then writes out the steps so a reader can follow the same path a calculator takes.