How to Calculate Square Footage
Square footage is just length times width — until the room has a bay window, a sloping wall or a chimney breast. This guide covers the simple case and every awkward one.
To calculate square footage, measure the length and width of the space in feet and multiply them together. A room 20 ft by 12 ft is 20 × 12 = 240 square feet. If a space is not rectangular, divide it into rectangles, triangles and circles, calculate each piece separately, and add the results together.
What square footage actually measures
Square footage is a measure of area — the amount of flat surface a space covers. One square foot is a square measuring one foot on each side.
That definition matters because it explains the single most common mistake in the whole subject. When you convert a length from inches to feet you divide by 12. When you convert an area from square inches to square feet you divide by 144, because both dimensions are being converted. More on that in the inches guide.
Step 1 — Measure in feet
Run a tape measure along each wall at floor level, wall to wall. A few things that improve accuracy:
- Measure at the floor, not at waist height. Baseboards and skirting taper outward at the bottom, so a measurement taken higher up reads slightly short.
- Take the longest and widest points. Rooms are rarely perfectly square. If two opposite walls differ, use the larger figure when ordering materials.
- Write down feet and inches, then convert. 12 ft 7 in becomes 12 ÷ 1 + 7 ÷ 12 = 12.583 ft.
Step 2 — Apply the formula for the shape
Most rooms are rectangles, so multiplying length by width is all you need.
| Shape | Formula | Example | Result |
|---|---|---|---|
| Square or rectangle | L × W | 20 ft × 12 ft | 240 ft² |
| Circle | π × d² ÷ 4 | 10 ft diameter | 78.54 ft² |
| Triangle | (b × h) ÷ 2 | 20 ft × 9 ft | 90 ft² |
| Trapezoid | ((a + b) ÷ 2) × h | 8 and 12 ft, height 5 ft | 50 ft² |
| Oval | π × a × b ÷ 4 | 16 ft × 10 ft | 125.66 ft² |
A fuller reference including hexagons and parallelograms is in square footage formulas by shape.
Step 3 — Add the parts together
This is the step that turns a hard problem into an easy one. Almost no real room is a clean rectangle, but almost every room is a combination of clean rectangles.
An L-shaped living room, 20 ft × 15 ft with an 8 ft × 6 ft notch:
- Draw a line across the inside corner, splitting it into two rectangles.
- Rectangle A: 20 ft × 9 ft = 180 ft²
- Rectangle B: 12 ft × 6 ft = 72 ft²
- Total: 180 + 72 = 252 square feet
Always cross-check with the subtraction method: the full rectangle is 20 × 15 = 300 ft², minus the 8 × 6 = 48 ft² notch, which also gives 252 ft². If your two methods disagree, something has been mis-measured.
Handling the awkward bits
Bay windows are a rectangle plus a trapezoid. Measure the straight section and the angled section separately.
Chimney breasts and columns are subtracted. Measure the room as if they were not there, then subtract the footprint of the obstruction.
Alcoves and closets are added as small rectangles. For flooring you almost always include them, since you will be laying material inside.
Sloping ceilings do not change the floor area at all — square footage is measured on the flat. They matter for a property listing, though, where space under a 7 ft ceiling is usually excluded from the finished living area.
Stairs count on each floor they serve, using the footprint the staircase occupies.
Converting to other units
Once you have square feet, every other area unit is a single multiplication.
| To convert | Do this | Example |
|---|---|---|
| Sq ft → sq inches | × 144 | 10 ft² = 1,440 in² |
| Sq ft → sq yards | ÷ 9 | 90 ft² = 10 yd² |
| Sq ft → sq meters | ÷ 10.7639 | 500 ft² = 46.45 m² |
| Sq ft → acres | ÷ 43,560 | 10,000 ft² = 0.2296 acres |
Dedicated tools: sqft to sqm, sqft to acres, sqft to square yards and sqft to linear feet.
Common mistakes
- Dividing by 12 instead of 144 when converting from inches. This gives an answer twelve times too large.
- Using a sloping side as the height of a triangle. The height must be perpendicular to the base.
- Forgetting the waste allowance. For flooring, add 10% for a straight lay and 15% for diagonal patterns — see the flooring guide.
- Measuring only one wall of each pair and assuming the opposite wall matches. In older buildings it frequently does not.
- Mixing units mid-calculation. Convert everything to feet before you multiply anything.
Which calculator to use
| If you are measuring | Use |
|---|---|
| A single room or whole house | Room sqft calculator |
| Something round | Circle sqft calculator |
| A triangular section | Triangle sqft calculator |
| Measurements in inches | Inches to sqft calculator |
| A lot or parcel of land | Sqft to acres calculator |
| Cost of a project | Price per sqft calculator |
Frequently asked questions
What is the formula for square footage?
For a rectangle it is length × width. For a circle it is π × diameter² ÷ 4. For a triangle it is (base × height) ÷ 2. All measurements must be in feet for the result to be in square feet.
How do I calculate square footage from feet and inches?
Convert the inches to a decimal by dividing by 12, then add it to the feet. 12 ft 6 in becomes 12.5 ft. Multiply the converted dimensions together as normal.
How many square feet is a 10x10 room?
A 10 by 10 foot room is 100 square feet.
How do I calculate square footage of an irregular shape?
Split it into regular shapes — rectangles, triangles and parts of circles. Calculate the area of each piece, then add them together. Every straight-sided shape can be divided into triangles if nothing else works.
Do I measure the inside or outside of the walls?
For flooring, paint and materials, measure the inside. For a property listing or appraisal, the convention is to measure to the outside of the exterior walls.