How to Calculate Square Feet

Multiply a space’s length by its width, both measured in feet. A room that’s 12 feet long and 10 feet wide covers 120 square feet: 12 × 10 = 120.

That’s the whole formula for a rectangular space. Things get more involved with irregular shapes, or when your measurements aren’t in feet to start with.

The basic formula

square feet = length (ft) × width (ft)

If your measurements are in inches

Divide by 12 first to convert to feet, then multiply. A room measuring 144 by 120 inches becomes 12 ft × 10 ft — 120 sq ft.

Don’t multiply the inch numbers directly and divide by 12 afterward. That gives you a different, wrong unit entirely.

If you’re mixing feet and inches — a wall that’s 8 feet 6 inches high, say — convert the inches to a decimal first. Six inches is 0.5 ft, so the wall measures 8.5 ft, not 8.6 ft.

Calculating an irregular room

Split the room into rectangles you can measure separately. Find each section’s square footage, then add them together.

Example — an L-shaped room:
– Section A: 10 ft × 8 ft = 80 sq ft
– Section B: 6 ft × 4 ft = 24 sq ft
– Total: 80 + 24 = 104 sq ft

Calculating square feet of a triangular space

A triangular section — a bay window nook, an angled wall, a gable end — uses a different formula: ½ × base × height. Base and height meet at a right angle, not along the slanted side.

A triangular yard section with a 20-foot base and 15-foot height works out to 0.5 × 20 × 15 = 150 sq ft.

If the triangle doesn’t have a clean right angle, split it into two right triangles first. Calculate each one, then add them — the same approach used for irregular rooms.

Calculating cubic feet (volume, not area)

Cubic feet adds a third dimension: height. Multiply square footage by the ceiling height.

cubic feet = length × width × height

A 120 sq ft room with an 8-foot ceiling holds 960 cubic feet. This matters for HVAC sizing, moving-truck estimates, and shipping — square footage alone doesn’t say how much air, or how many boxes, a space actually holds.

Converting cubic feet to cubic yards

A cubic yard measures 3 ft × 3 ft × 3 ft, or 27 cubic feet. Divide any cubic-foot total by 27 to get cubic yards.

The 960-cubic-foot room above converts to 960 ÷ 27 = 35.6 cubic yards. This conversion matters most for materials sold by the yard — mulch, gravel, concrete, topsoil — where a supplier needs volume in yards to quote a delivery.

Calculating linear feet

Linear feet is a straight-line length measurement. No multiplication involved. Twenty linear feet of baseboard is simply a wall’s length, regardless of the wall’s height.

People confuse linear feet with square feet when ordering flooring or trim. They’re not interchangeable — using square footage to order baseboard or fencing gets you the wrong quantity.

To find linear feet for a room’s whole perimeter, add up all four wall lengths instead. A 12 ft × 10 ft room has a perimeter of 2(12 + 10) = 44 linear feet — a completely different number from its 120 square feet.

Estimating roof square footage

Roof area always runs larger than the house’s footprint, because a roof slopes instead of lying flat. The steeper the pitch, the bigger that gap gets.

A rough estimate multiplies the building’s footprint by a pitch multiplier.

Roof pitch (rise per 12″ run)	Multiplier
2:12 (low slope)	1.014
4:12	1.054
6:12 (moderate)	1.118
8:12	1.202
10:12	1.302
12:12 (steep)	1.414

A 1,500 sq ft footprint with a 6:12 pitch roof works out to roughly 1,500 × 1.118 = 1,677 sq ft of actual roof surface.

Roofing materials sell by the “square” — 100 square feet. That roof needs about 17 squares: 1,677 ÷ 100, rounded up. A roofer measuring for a real quote will also account for overhangs, valleys, and waste that this formula can’t capture.

Tools that make this easier

A laser measure speeds this up considerably over a tape measure, especially in rooms with awkward corners or high ceilings. Point it at one wall, then the other — it reads the distance instantly.

Most laser models include a built-in area mode. It multiplies length by width automatically once you’ve taken both readings.

For irregular rooms, you still need to measure and calculate each section on its own. The tool speeds up individual measurements, not the work of splitting a room into rectangles and triangles.

A plain tape measure still works fine for smaller, single-rectangle rooms, where accuracy to the nearest inch is good enough.

A quick sanity check on your numbers

Before trusting a square footage number, ask whether it matches your gut sense of the space. Most bedrooms fall somewhere between 100 and 250 sq ft.

A bedroom that calculates out to 500 sq ft, or 60 sq ft, almost always means a measurement or unit error. That kind of gut check catches the two most common mistakes: forgetting to convert inches to feet, and transposing a digit while reading a tape measure.

It won’t catch small errors. But it reliably flags the big ones before they carry through into a flooring order or a paint estimate.

Frequently Asked Questions

How do I calculate square feet of a wall for paint?
Multiply the wall’s width by its height — not length by width, since walls are vertical. Subtract door and window areas for a more accurate estimate, though many painters just buy a bit extra instead.

How do I convert square feet to square yards?
Divide by 9. A square yard equals 9 square feet — 3 ft × 3 ft.

How do I calculate square footage of a circular area, like a round patio?
Use π × radius². For a 10-foot-diameter circle, the radius is 5 ft: 3.14159 × 5² = 78.5 sq ft.

How many square feet are in a roofing square?
100 square feet, by definition. It’s the standard unit roofing materials — shingles, underlayment — get priced and sold in, no matter the roof’s pitch or shape.

How do I calculate square feet of a wall for wallpaper if it has windows?
Calculate the full wall area first — width times height. Then subtract each window and door’s own area, leaving the surface that actually needs covering.

How do I convert cubic yards back to square feet?
You can’t directly. Cubic yards measure volume, and square feet measure area — converting between them requires knowing the depth or height too. Divide cubic yards by depth in yards to get square yards, then multiply by 9 for square feet.