Square Footage Calculator

What this square footage calculator does

Square footage is simply area measured in square feet, yet the number ends up everywhere. It prices a flooring job, sizes a can of paint, shows up on a real estate listing and decides whether the new sofa fits the plan. This square footage calculator takes your measurements and returns the area in square feet, square yards, square meters, square inches and acres in one step.

It handles the plain rectangle, but also circles, triangles, trapezoids, L-shaped rooms and ovals. If you are measuring a whole floor, switch to Several rooms and it adds everything up for you.

It also does the follow-up work that most tools skip. It takes off areas you will not cover, adds a waste allowance, estimates wall and ceiling area, and gives a rough cost and box count if you enter a price and box coverage. The 3D room view and the bar chart are there for a practical reason: they help you catch a typo, like 120 instead of 12, before it turns into a wrong order.

How to use the calculator

1. Pick a mode. Use One shape for a single room or area. Use Several rooms when you want a total for an apartment, a floor or a list of rooms.
2. Choose the unit. Feet is the default. If your tape reads in inches, meters or centimeters, change the unit first so the labels match.
3. Enter your measurements. Tap the small i beside any field if you are unsure what to measure. Each tip explains exactly where to place the tape.
4. Subtract what you will not cover. Kitchen islands, stair openings, a fireplace hearth or a built-in all count as areas to subtract from the floor.
5. Set the waste allowance. The default is 10 percent, which suits most flooring and tile jobs. Lower it for simple layouts and raise it for diagonal patterns.
6. Read the result. You get the net area, the order quantity with waste, a chart, a rotatable 3D view and a full list of conversions.

Everything updates as you type, so you can change a number and watch the area move.

How to calculate square footage: the basic formula

For any rectangle or square, the rule is the same: length × width = area. Measure both sides in feet, multiply them, and the answer is in square feet. A room that is 12 feet by 10 feet has 12 × 10 = 120 square feet.

Tape measures rarely land on whole feet, so convert inches to decimals first. Divide the inches by 12 and add them to the feet. A wall of 11 feet 6 inches is 11.5 feet, and a wall of 9 feet 3 inches is 9.25 feet. Multiply the two: 11.5 × 9.25 = 106.375, or about 106.4 square feet.

If you measured everything in inches, you can multiply first and convert after. A room of 150 inches by 120 inches covers 18,000 square inches. Divide by 144 and you get 125 square feet. The 144 is not a typo, and the reason is covered in the conversions section below.

How to calculate square footage of a room that is not a rectangle

Most real rooms are not clean boxes. They have alcoves, angled walls, bay windows and closets. The method never changes: break the floor into shapes you already know, find each area, then add or subtract. The table lists the formulas this calculator uses, each with a worked example.

L-shaped rooms, two ways

There are two honest ways to do an L. You can split it into two rectangles and add them, or you can measure the whole bounding rectangle and subtract the missing corner. Take a 20 by 15 foot footprint with an 8 by 6 foot corner missing. The bounding rectangle is 300 square feet, the missing corner is 48, so the floor is 252 square feet. The L-shape option in the calculator uses the subtraction method because it needs only four measurements.

Irregular floor plans

For an odd room, sketch it on paper, draw lines until you have only rectangles and right triangles, and label every side. Use the Several rooms mode and enter each rectangle as a separate row. The total is the sum of the parts. This is the same approach flooring installers use on a site visit, and it is more reliable than trying to estimate the whole shape at once.

How to measure square feet accurately

The formula is easy. Getting good measurements is where jobs go wrong. A few habits make a real difference:

• Use a rigid tool. A 25-foot locking tape measure or a laser measure is better than a cloth tape or a ruler you keep moving.
• Measure wall to wall at floor level. Walls are not always straight. If the length changes along the room, record the longest figure for materials you cannot easily trim, and note the difference.
• Check that the room is square. Measure both diagonals of a rectangular room. If they match within about half an inch, the corners are close to square. If they do not, split the room into two triangles or measure in sections.
• Include doorways. Flooring normally runs under the door, so add the width of the opening times the wall thickness to the area.
• Write it down immediately. Sketch the room and mark each measurement. Memory is not a measuring tool.
• Measure twice. If two readings differ, a third one will tell you which to trust.

Keep full precision until the final step. Rounding every wall up to the next foot before multiplying can inflate a 12-wall house plan by dozens of square feet.

Using square footage for flooring, paint, drywall and roofing

The area is only the starting line. Each trade turns it into a different quantity, and each adds its own allowance for cutting and mistakes.

Flooring and tile

Most installers add 5 to 10 percent for straight-lay flooring and 10 to 15 percent for diagonal tile or patterned layouts. A 120 square foot room with 10 percent waste means buying 132 square feet. If one box covers 20 square feet, you need 6.6 boxes, so you buy 7. Enter the box coverage in the calculator and it does that rounding for you. Our flooring calculators go further on materials.

Paint

Paint depends on wall area, not floor area. Take the room perimeter, multiply by the ceiling height, then subtract doors and windows. A 12 by 10 foot room has a perimeter of 44 feet. At 8 feet high that is 352 square feet of wall, and a door and one window (about 35 square feet together) leave roughly 317 square feet. Many paint labels quote about 350 to 400 square feet per gallon, so this room needs under one gallon per coat. Always check the can you buy. See the painting calculators for a full paint estimate.

Drywall

The common drywall sheet is 4 by 8 feet, which covers 32 square feet. Divide the wall and ceiling area by 32 and round up. The sheetrock calculator counts sheets, joint compound, tape and screws, and the drywall installation cost calculator turns the area into a budget. Browse the full drywall calculators hub for the whole job.

Roofing

Do not use floor area for a roof. A sloped roof has more surface than the footprint under it, and the steeper it is, the larger the gap. Start with the roof pitch calculator, then use the roof calculator for the true area. Roofing is sold by the square, and one roofing square covers 100 square feet. The rafter calculator and the roof replacement cost calculator cover the framing and the budget, and all of them sit in the roofing calculators hub.

Square footage conversions: sq ft, sq yd, sq m, sq in and acres

Area units are squared, which is why the conversion factors look bigger than people expect. One foot is 12 inches, but one square foot is 12 × 12 = 144 square inches. So converting square feet to square inches (ft sq to in sq) means multiplying by 144, not by 12. The same logic gives 9 square feet in a square yard, because a yard is 3 feet and 3 × 3 = 9.

Lot square footage to acres

For land, divide the square footage by 43,560 to get acres. To calculate acreage from square footage for a 10,000 square foot lot: 10,000 ÷ 43,560 = 0.2296 acre. Going the other way, the acre to square footage conversion is multiplication: a half acre is 21,780 square feet. For a sense of scale, a one-acre square plot is about 208.7 feet on each side.

For the official definitions behind these factors, see the NIST unit conversion resources, and the background pages on the square foot and the acre.

How is the square footage of a house calculated?

This is a different question from measuring a room, and the answer matters if you are buying, selling or insuring a home. Listing figures usually describe finished living area, not every floor surface. In the United States, many appraisers and agents follow the ANSI Z765 method for single-family homes, which measures from the exterior walls and counts finished, heated space above ground. Garages, unfinished basements, open porches and most attics are normally left out of that figure, although local practice and MLS rules vary.

This calculator measures the floor area of the shapes you enter, from interior wall to interior wall. That is the right number for flooring and finishing work, but it is not an appraisal and it will usually be a little smaller than a listed house size, because wall thickness is not part of it. If a listing figure matters to a price, rely on a licensed appraiser or the measurement standard your local market uses.

Common square footage mistakes

• Mixing units. Entering feet for the length and inches for the width gives a number that looks plausible and is badly wrong. Use one unit for the whole calculation.
• Converting with 12 instead of 144. Linear conversions use 12. Area conversions use 144.
• Confusing area with perimeter. Perimeter is the distance around the room and is used for baseboards and paint. Area is the floor surface.
• Forgetting waste. Buying exactly the net area leaves you short after the first awkward cut.
• Skipping what you will not cover. A big kitchen island or a stair opening can remove 30 to 60 square feet. Subtract it, or you will overbuy.
• Rounding too early. Round only the final answer, and round up when you are ordering material.

Square footage calculator FAQs

These calculators give planning estimates only. They do not replace advice from a licensed contractor or inspector. Read our Disclaimer for details, or contact us if you spot a mistake.