A home’s square footage is a crucial element in determining the price of a home you’re trying to buy or sell, how much you’ll pay in taxes if you live there, and what kinds of renovations are possible in your future. Plus, a home’s square footage can be surprisingly subjective. So, if you’re asking yourself, “how to calculate square feet of a house?” here’s what you need to know.

Since most people don’t have a square foot calculator in their back pocket, here’s what you need to know to ace any square footage calculations that crop up in your future.

You probably know how to calculate the square footage of a simple room without any funny shapes. Just break out your measuring tape—or a laser measure—to get its length and width. Multiply the width by the length and voila! You have the square footage. Say a room is 20 feet wide by 13 feet long, then 20 x 13 = 260 square feet.

How to calculate the square footage of an entire house

While measuring a single room is no big deal, people get kind of intimidated when it comes to calculating the square footage of an entire home. While homes can initially seem “daunting to measure, they’re just a collection of small boxes,” says Mario Mazzamuto of SF Bay Appraisal. Don’t sweat it if a room has an outcropping. Simply break that area down into a smaller box, and measure each box individually. Add up each box’s square footage to get the room’s total area. So if your living room, bedroom, bathroom, and hallway are  500, 400, 200, and 100 square feet respectively, that means the total is 500 + 400 + 200 + 100 = 1,200 square feet total.

If you have a round kitchen and a triangular rumpus room, fear not. Just check out vCalc’s handy calculator that will tell you how to calculate square feet no matter what polygon’s thrown your way. Once you choose a shape, the calculator will prompt you for the measurements needed to compute square footage.

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How many square feet is my house? Take the square foot calculation with a grain of salt

Now that you know how to calculate the square footage of a home, it’s time to bust your bubble with a big caveat: A square footage calculation is surprisingly subjective based on who’s doing the measuring. For instance, you measured the actual, livable square footage between the interior walls. But many architects use their own square foot calculation method, measuring the square footage from the exterior walls.

This explains why there are often discrepancies between your square foot calculations and those of a real estate agent, builder, or other sources. (looking for an agent? Here’s how to find a real estate agent in your area.)

“Many MLS services require a listing’s square footage to come from a specific source,” says Robin Kencel of Connecticut’s Stevens Kencel Group. So while you can make your own estimate, you may need to hire a certain professional to come up with a number that can be used on your listing; check with your Realtor or town’s building department to determine who that is.

As a general rule, “the square footage extends through the Sheetrock and framing to the exterior of the wall,” says Mazzamuto. Generally, to do the same for your measurements, add 6 inches per measurement, he says.