Quick answer: To calculate square footage, measure the length and width of a space in feet and multiply them. Length × Width = Square Feet. A room that is 12 ft by 15 ft is 12 × 15 = 180 sq ft. For irregular spaces, split the area into simple shapes, calculate each one, and add them together.
Square footage is the area of a flat surface measured in square feet (sq ft or ft²). One square foot is a square 1 ft long on each side. You need it to buy flooring, price a paint job, order concrete, compare rent, or check a home listing. This guide covers the formula for every common shape, how to handle inches and meters, and the mistakes that throw estimates off.
Want the answer without the math? Use our free square footage calculator. It handles rectangles, triangles, circles, trapezoids and L-shaped rooms, and gives the result in square feet, square yards, square meters and acres.
What you need
- A tape measure or laser measure. A laser is faster and more accurate in large rooms.
- Paper or a phone to sketch the space and write down each measurement.
- A calculator (or the Areo app, which does all of the steps below for you).
Step 1: Measure the length and width
Measure wall to wall at floor level, along the longest straight lines in the space. Write each measurement next to the matching side on your sketch. For flooring, measure into doorways and closets you intend to cover.
Step 2: Convert everything to feet
Formulas only work when every measurement uses the same unit. Most tape readings come in feet and inches, so convert the inches to a decimal fraction of a foot by dividing by 12:
| Inches | Decimal feet |
|---|---|
| 1 in | 0.083 ft |
| 3 in | 0.25 ft |
| 4 in | 0.333 ft |
| 6 in | 0.5 ft |
| 8 in | 0.667 ft |
| 9 in | 0.75 ft |
So 14 ft 6 in = 14.5 ft and 11 ft 3 in = 11.25 ft. A very common mistake is writing 10 ft 6 in as 10.6. It’s actually 10.5.
Step 3: Apply the right formula
Square and rectangle
Area = Length × Width
Example: a bedroom measuring 14 ft 6 in × 11 ft 3 in → 14.5 × 11.25 = 163.13 sq ft.
Triangle
If you know the base and the perpendicular height:
Area = ½ × Base × Height
Example: base 12 ft, height 9 ft → 0.5 × 12 × 9 = 54 sq ft.
In a real room it’s usually easier to measure the three sides than the height. In that case use Heron’s formula:
- Find the semi-perimeter: s = (a + b + c) ÷ 2
- Area = √( s × (s − a) × (s − b) × (s − c) )
Example: sides of 13, 14 and 15 ft → s = 21 → √(21 × 8 × 7 × 6) = √7,056 = 84 sq ft. The triangle option in our calculator uses this method.
Circle
Area = π × Radius² (radius = diameter ÷ 2, π ≈ 3.1416)
Example: a round patio 10 ft across → radius 5 ft → 3.1416 × 25 = 78.54 sq ft.
A half circle is simply half of that: a 10 ft semicircle is 39.27 sq ft.
Trapezoid
A trapezoid has two parallel sides of different lengths, which is common for bay windows, angled garden beds and some lots.
Area = (a + b) ÷ 2 × h, where a and b are the parallel sides and h is the distance between them.
Example: parallel sides of 10 ft and 16 ft, 7 ft apart → (10 + 16) ÷ 2 × 7 = 91 sq ft.
L-shaped and other composite shapes
Break the space into rectangles, calculate each, and add them. Or measure the overall rectangle and subtract the missing corner:
(20 × 15) − (8 × 6) = 300 − 48 = 252 sq ft
We cover L-, T- and U-shaped rooms, angled walls and curves in detail in how to find the square footage of an irregular room.
Measured in inches or meters?
Inches: multiply length × width to get square inches, then divide by 144 (12 in × 12 in). 150 in × 120 in = 18,000 sq in ÷ 144 = 125 sq ft.
Meters: multiply to get square meters, then multiply by 10.764. 3.5 m × 4.2 m = 14.7 m² × 10.764 = 158.23 sq ft.
Yards: multiply to get square yards, then multiply by 9.
The square feet conversion guide has full tables for every unit.
Calculating several rooms
Calculate each room separately and add the totals. Hallways and closets count as their own small rectangles. Keep a running list like this:
| Area | Size | Sq ft |
|---|---|---|
| Living room | 18 × 14 ft | 252 |
| Hallway | 12 × 3.5 ft | 42 |
| Bedroom | 14.5 × 11.25 ft | 163.13 |
| Total | 457.13 |
In the Areo app, each shape you draw is saved, so a multi-room total takes seconds.
Common mistakes to avoid
- Mixing units: multiplying feet by inches gives a meaningless number. Convert first.
- Treating inches as decimals: 10′6″ is 10.5 ft, not 10.6 ft.
- Rounding too early: keep two decimal places until the final answer.
- Forgetting waste: add 5–15% for material that gets cut, broken or mismatched. See our flooring guide for exact percentages.
- Assuming walls are square: old houses often aren’t. Measure both ends of each wall and use the larger number.
How to estimate cost from square footage
Once you know the area, cost is one multiplication:
Total cost = Square footage × (1 + waste %) × Price per sq ft
For 252 sq ft of flooring at $4.50/sq ft with 10% waste: 252 × 1.10 × 4.50 = $1,247.40.
The faster way: draw it
Formulas get tedious when a room has five or six corners. In the Areo app you trace the outline with your finger, enter the side lengths, and it calculates the exact area of any polygon, plus square yards, square meters, acres and total cost. It also saves shapes you measure often, like an L-shaped living room or a standard driveway, so repeat quotes take seconds.
Skip the formulas next time
Draw any shape in Areo and get square feet, square meters, acres and the total cost instantly. Try it free for 3 days.