How to Calculate Square Feet
Square footage is area -- length times width for a rectangle, and a shape-specific formula for anything else. Add your room's shape and dimensions below for an instant total, or read the formulas and worked examples underneath.
Total area
120.0 sq ft
In square meters
11.1 sq m
Total cost
--
At the calculator's own defaults (a 12x10 ft rectangle) that's 120 sq ft, or 11.15 sq m.
How it's calculated
- Identify the shape (or shapes) that make up the space -- most rooms are a rectangle, or a small number of rectangles joined together.
- Measure each dimension in feet, converting any inches to a decimal first (inches ÷ 12).
- Apply the matching formula below to each shape.
- Add the shapes together for a total, and subtract any cutouts (a closet, an island) that aren't part of the usable floor.
| Shape | Formula | Worked example |
|---|---|---|
| Rectangle / square | length x width | 12 ft x 10 ft = 120 sq ft |
| Triangle | 0.5 x base x height | 0.5 x 8 ft x 6 ft = 24 sq ft |
| Circle | pi x radius^2 | pi x (5 ft)^2 = 78.54 sq ft |
| Trapezoid | 0.5 x (base1 + base2) x height | 0.5 x (10 + 6) ft x 4 ft = 32 sq ft |
| L-shape | rectangle A + rectangle B | (10 x 8) + (4 x 6) = 80 + 24 = 104 sq ft |
Rectangles (and squares)
The simplest and most common case: multiply length by width. A 12 ft by 10 ft room is 12 × 10 = 120 sq ft. A square is just a rectangle where both sides are equal, so a 10 ft square is 10 × 10 = 100 sq ft.
L-shaped rooms
Split the room into two rectangles along the inside corner, find each rectangle's area separately, then add them. A room made of a 10×8 ft section and a 4×6 ft section is 80 + 24 = 104 sq ft. Where you draw the split line doesn't matter, as long as the two rectangles together cover the whole floor with no overlap and no gap.
Circles
Area = pi (≈3.14159) × radius². Radius is half the diameter -- measure straight across the widest point and divide by 2. A round room or patio 10 ft across (5 ft radius) is pi × 5² = pi × 25 ≈ 78.54 sq ft.
Triangles
Area = 0.5 × base × height, where height is measured perpendicular to the base (straight up from it, not along a slanted side). An 8 ft base with a 6 ft height is 0.5 × 8 × 6 = 24 sq ft -- this comes up for an angled wall, a gable end, or a triangular lot.
Trapezoids
Area = 0.5 × (base1 + base2) × height, for a shape with two parallel sides of different lengths (a tapered room, a lot that narrows toward the back). Bases of 10 ft and 6 ft with a 4 ft height give 0.5 × (10 + 6) × 4 = 32 sq ft.
Unit conversions
Square feet to square meters: multiply by 0.09290304 (a 120 sq ft room is about 11.15 sq m). Square feet to acres: divide by 43,560 (the exact number of square feet in an acre). For the metric conversion with a full reference table, see the square feet to square meters converter; for acres and hectares, see the acreage calculator.
Once you have a room's square footage, most of the other calculators on this site take that number (or the same length/width measurements) straight in: the flooring calculator and carpet calculator for floor coverings, the paint calculator and wallpaper calculator for walls, the drywall calculator for new construction, and the deck and fence calculators for outdoor projects. Trim work (the baseboard calculator) uses the room's perimeter instead of its area, and a whole lot rather than a room is a job for the acreage calculator.
Keep going
- The multi-shape calculator on this page is the same tool as the site's main calculator -- use the full version to save multiple rooms and a running total. Square Footage Calculator
- Once you have a room's square footage, the flooring calculator turns that number into boxes or planks with the right waste allowance. Flooring Calculator
- For carpet specifically, sold by the roll rather than the box, the carpet calculator converts the same square footage into linear feet and square yards. Carpet Calculator
- Need just the metric conversion on its own, with a reference table, rather than working it out by hand? This converter does that one step directly. Square Feet to Square Meters Converter
Frequently Asked Questions
What's the fastest way to calculate square feet?
Measure length and width in feet, then multiply: length x width = square feet. That's the whole formula for a rectangle or square room, which covers most floor plans. Odd shapes just mean applying a different formula (or splitting the room into rectangles), not a harder process.
Do I need to convert inches before I calculate?
Yes -- every formula on this page expects feet, not feet-and-inches. Convert inches to a decimal first: inches ÷ 12 = decimal feet, so 10 ft 6 in becomes 10.5 ft (6 ÷ 12 = 0.5). The calculator above does this conversion for you if you enter feet and inches separately.
How do I measure a room that isn't a rectangle?
Split it into shapes you do have a formula for -- an L-shaped room is two rectangles, an octagonal nook might be a rectangle plus triangles at the corners -- find each piece's area and add them together. That's exactly what the "add another room" calculator above does: each shape you add is one piece of a larger total.
Should I measure interior walls or include the wall thickness?
For almost any practical purpose -- flooring, paint, furniture layout, most real estate listings -- measure wall-to-wall along the interior finished surfaces. That gives you usable floor area, which is what the material calculators on this site expect as input.
How accurate does my measurement need to be?
Round to the nearest inch (or half-inch for a small space) with a tape measure or laser measure; that's precise enough for flooring, paint, or furniture planning. If you're pricing material, most calculators build in a waste allowance on top of your raw measurement specifically to cover small measuring error and cutting loss.