Room Size Calculator

Square feet come from length times width

Square footage is the number every other estimate starts from. Measure the inside of the room from wall to wall at floor level, do the same in the other direction, and multiply the two numbers. A room that is 12 feet long and 10 feet wide is 12 times 10, or 120 square feet. That is the whole calculation for a plain rectangular room.

Two habits keep the number honest. Measure at the baseboard rather than at eye level, because older walls are rarely plumb and a room can be three or four inches wider at the floor. Then take the measurement in two or three places, use the smallest figure for flooring and the largest for paint. Flooring has to fit the tightest spot, while paint has to cover the widest one.

Inches need one extra step. Divide the inches by 12 before you multiply. A room that measures 11 feet 6 inches by 9 feet 3 inches becomes 11.5 by 9.25, which is 106.4 square feet. Rounding 6 inches to half a foot is fine for paint, but for carpet or tile keep the decimal, because half a foot across a 12 foot wall is 6 square feet of material.

Closets, alcoves and doorways are part of the floor. Include the closet floor when you buy flooring for it, and leave it out when you paint, since the inside of a closet is a separate job. Write the numbers down as you go. Most measuring mistakes are not bad tape work, they are a number that got lost between the room and the store.

A worked example: the 12 by 10 bedroom

Take a bedroom that measures 12 feet by 10 feet with an 8 foot ceiling and walk the numbers through the calculator.

• Floor area: 12 x 10 = 120 square feet
• Floor area in square yards: 120 divided by 9 = 13.3 square yards
• Perimeter: 2 x (12 + 10) = 44 feet
• Wall area before openings: 44 x 8 = 352 square feet
• Wall area after a 10% allowance for the door and window: 316.8 square feet
• Two coats of paint: 316.8 x 2 = 633.6 square feet of coverage
• Paint at 350 square feet per gallon: 633.6 divided by 350 = 1.81 gallons
• Ceiling, one coat: 120 divided by 350 = 0.34 gallons
• Cooling at 20 BTU per square foot: 120 x 20 = 2,400 BTU

The paint figure is the one people get wrong at the register. A gallon and four fifths is not a product you can buy, so the answer is two gallons for the walls and a quart for the ceiling. The quart is cheaper than a third gallon and safer than stretching the second one thin, because a thin coat on a repaint shows every roller line.

The 10% deduction is a planning allowance, not a measurement. A bedroom with one door and one window loses closer to 8% of its wall area, while a room with a sliding glass door and three windows can lose 18%. The calculator uses 10% as a middle figure. If your room is mostly glass, add a quarter gallon for every extra window and you will not run short on the last wall.

L-shaped and irregular rooms: split, then add

An L-shaped room, a room with an alcove, or a room that steps out under a bay window is still just rectangles. Sketch the floor plan on paper, cut it into rectangles that do not overlap, measure each one, and add the areas. Nothing else about the method changes.

That 216 square foot room needs more paint than the total suggests, because the wall length grows with the shape. The outside edge measures 2 x (14 + 12) plus 2 x (8 + 6), or 80 feet. At an 8 foot ceiling that is 640 square feet of wall before openings, 576 square feet after the 10% allowance, and 1,152 square feet of coverage for two coats. At 350 square feet per gallon the job takes 3.29 gallons, so you buy four gallons. Cooling the same room at 20 BTU per square foot calls for 4,320 BTU.

The perimeter needs the same care as the area. Add the outside edges of both rectangles and do not count the wall the two rectangles share twice. A quick check is to walk the plan with your finger and count only the walls you would touch standing inside the room. If your finger passes a wall twice, the perimeter is too long and the paint estimate is too high.

Round rooms and angled walls are the exception. For a circle, measure across the middle and use 3.14 times the radius squared. For a triangle, multiply the base by the height and divide by two. For a room with a 45 degree corner, split it into a rectangle and a triangle and treat each one on its own. Three rectangles and a triangle will handle almost any floor plan in a house.

Square feet, square yards and what flooring is sold in

Flooring is sold by the square foot, by the square yard, or by the box, and the box rarely covers a round number. Convert once, at the start, and write the converted figure on the same sheet as the measurements.

Waste is the part of the order that is not floor area. Carpet needs about 10% extra for seams and pattern matching, and a patterned carpet with a large repeat can need 15%. Tile needs 10% for cuts and 15% if you are laying it on the diagonal. Vinyl plank and laminate need 5% to 10%, mostly for the cuts at the end of each row.

A 120 square foot bedroom therefore takes 132 square feet of carpet, or 14.7 square yards. The same room in 12 inch tile takes 132 square feet of tile, which is 132 tiles plus the ones you break. Buy the extra box and keep the receipt. An unopened box goes back to the store, while a floor that runs one box short waits a week for a dye lot that may not match.

Ceiling height moves the paint and the cooling load

Ceiling height does not change the floor area. A 12 by 10 room is 120 square feet whether the ceiling is 8 feet or 12 feet. It does change two things that cost money: the wall area you paint, and the volume of air you cool.

Going from an 8 foot to a 10 foot ceiling adds 79 square feet of wall and 600 BTU on the same floor plan, which is a quarter more paint and a quarter more cooling for a room that has not gained a single square foot of floor. In a house with 9 or 10 foot ceilings, budget paint from the wall area and never from the floor area.

Volume matters more than area for air. A room with a high ceiling holds more warm air above the furniture, and that air has to be cooled before the room feels comfortable. Ceiling fans help here, because they move the stratified air without adding cooling capacity. Run a fan counterclockwise in summer and you can raise the thermostat two degrees without noticing.

Paint math: coats, openings and the 350 square foot gallon

A gallon of interior latex covers 350 to 400 square feet on a smooth, primed wall. New drywall drinks more, and a deep color over a light wall often needs three coats, so the coverage figure to use for a budget is 350. That is the number the calculator uses.

Count the coats before you count the gallons. One coat over the same color is a touch-up job. Two coats is the standard for a color change. Three coats is normal when going from a dark wall to white, or from white to a saturated red or navy. Each extra coat is another full pass over the wall area, not another pass over the floor area.

Openings and trim change the total by more than people expect. A standard door is about 21 square feet, a standard window about 15, and a sliding glass door about 42. The calculator applies a flat 10% deduction, which matches a room with one door and one window. For a room with four openings, count them and subtract the real area instead. Trim, baseboard and crown molding are usually painted with a separate quart and should not be rolled into the wall figure.

Primer is a separate line on the estimate. Bare drywall, patched walls and any surface going from dark to light need a primer coat, which covers about the same area per gallon as paint but costs less. Skipping primer to save a step usually costs a third coat of paint, and the finish looks uneven under a window where the light rakes across the wall.

Sizing a room air conditioner from the floor area

The common rule for a room air conditioner is 20 BTU of cooling for every square foot of floor, assuming an 8 foot ceiling and average insulation. A 120 square foot bedroom works out to 2,400 BTU, and a 300 square foot living room to 6,000 BTU.

Units do not start at 2,400 BTU. The smallest window units sold in the United States are 5,000 BTU and cover up to about 150 square feet. That means a small bedroom gets a unit that is larger than the rule calls for, which is fine as long as it is not wildly oversized. A unit that is too large cools the air fast, shuts off before it removes much humidity, and leaves the room feeling cold and clammy.

Adjust the rule for the things the floor area cannot see. Add 10% for a room that faces south or west with unshaded glass, add 600 BTU for each person beyond two who spends the evening in the room, and subtract 10% for a basement room with two walls underground. A kitchen needs another 4,000 BTU because of the stove and the refrigerator. The Department of Energy's guide to room air conditioners explains sizing and efficiency ratings in plain terms, and its advice lines up with the 20 BTU rule for ordinary rooms.

Measuring mistakes that cost money

The expensive errors are rarely arithmetic. They come from the measurement itself, and five of them show up again and again.

• Measuring one spot. Rooms are not square. A 12 by 10 room can be 11 feet 10 inches at one end, and the flooring has to fit the narrow end.
• Measuring outside the baseboard. Baseboard is usually three quarters of an inch thick on each side, which is 1.5 inches off the room in each direction and about 2.5 square feet in a bedroom.
• Forgetting the door swing. A door that opens into the room takes floor space that no furniture can use, but it is still floor area for flooring and still wall area for paint. Do not subtract it from either.
• Treating a sloped ceiling as full height. For living area, count floor under a ceiling that is at least 5 feet high, and count only the part where the ceiling is 7 feet or higher as full area. An attic room measured to the kneewall is usually 20% to 30% smaller than it looks.
• Mixing feet and inches. A tape that reads 137 inches is 11 feet 5 inches, not 13.7 feet. Convert before you multiply, every time.

A laser measure removes most of the tape error but not the judgment. Point it at the wall, not at the curtains, and check the reading against a tape on one wall. If the two disagree by more than an inch, trust the tape and find out why. The Census Bureau's American Housing Survey publishes square footage for millions of homes, and its tables show how widely the same floor plan can be reported, which is a useful reminder that a number on a listing is not a measurement.