Square footage is the number that drives almost every material order in construction. Get it right and your estimate is solid. Get it wrong and you’re either making two trips to the supply house or sitting on material you paid for and don’t need.
The math behind square footage is not hard. Area is just length times width for a rectangle. But real jobs are not all rectangles. Triangular cuts in a yard, circular patios, angled additions, odd-shaped rooms - these are everyday situations that trip people up. This guide covers every common shape with the formulas, worked examples, and unit conversions you actually need on the job.
Use the square footage calculator to run the math as you follow along.
Quick Answer: Square Footage Formulas by Shape
The fastest reference on the page. Find your shape, apply the formula.
| Shape | Formula | Variables |
|---|---|---|
| Rectangle / Square | L x W | Length x Width |
| Triangle | (B x H) / 2 | Base x Height / 2 |
| Circle | pi x r^2 (approx. 3.1416 x r^2) | Radius squared |
| Trapezoid | ((A + B) / 2) x H | Average of parallel sides x Height |
| L-Shape | Rect 1 + Rect 2 | Break into rectangles, add areas |
| Irregular / Polygon | Sum of triangles or rectangles | Break into simple shapes |
These six shapes cover the vast majority of what you’ll encounter on any residential or light commercial job.
Why Square Footage Matters in Construction
Square footage is the common currency of construction estimating. Before you can order material for almost any trade, you need it.
- Flooring - Tile, hardwood, LVP, carpet. All priced and sold per square foot.
- Paint - Coverage rates are in square feet per gallon.
- Concrete - Slab area combined with thickness gives you cubic yards needed.
- Drywall - Sheet counts depend on wall and ceiling square footage.
- Roofing - Sold in “squares” (100 sq ft each).
- Insulation, landscaping, siding - All use square footage as the base unit.
Labor estimates run the same way. Most subcontractor bids come in as cost per square foot. If you don’t know the area, you can’t evaluate the bid.
I’ve been bidding residential remodels for over 20 years. The estimate always starts with accurate measurements. Everything downstream gets more accurate when the square footage is right. There’s no shortcut around that first step.
How to Calculate Square Footage: Rectangle and Square
This covers the majority of spaces you’ll measure.
Formula: Length x Width = Square Feet
Measure in feet. Convert any inches to decimal feet first.
- 6 inches = 0.5 ft
- 3 inches = 0.25 ft
- 9 inches = 0.75 ft
- 4 inches = 0.333 ft
Example: Standard bedroom
A bedroom measures 13 feet 6 inches by 11 feet 3 inches.
- 13 ft 6 in = 13.5 ft
- 11 ft 3 in = 11.25 ft
- 13.5 x 11.25 = 151.9 sq ft
Round up to 152 sq ft for material ordering. Add your waste factor on top of that (10% for most flooring).
Example: Full house square footage
A single-story home: living room (18x20=360), kitchen (12x14=168), master (14x13=182), two bedrooms (132+110), hallway (96), two baths (104).
Total: 1,152 sq ft
That number drives the flooring order, the paint estimate, the HVAC load calculation, and the real estate listing. It is the foundation of the whole estimate.
How to Calculate Square Footage: Triangle
Triangular areas come up more often than people expect. Shed dormers, triangular yard sections, gable end walls, and angled rooms all involve triangle math.
Formula: (Base x Height) / 2
Height is the perpendicular distance from the base to the opposite point. Not the slant length - the straight-up-and-down height.
Example: Triangular garden bed
A triangular planting area at the corner of a yard has a base of 14 feet and a perpendicular height of 9 feet.
(14 x 9) / 2 = 126 / 2 = 63 sq ft
If you’re ordering mulch at 1 cubic foot per 4 sq ft at 3-inch depth, that’s about 16 cubic feet of mulch for this one bed. Use the square footage calculator to confirm the area, then run your mulch or material quantities.
Example: Gable end wall for siding
A triangular gable end has a base of 28 feet (the width of the house) and a ridge height of 6 feet above the top plate.
(28 x 6) / 2 = 168 / 2 = 84 sq ft
Add that to your rectangular wall area below to get total siding square footage for that elevation.
How to Calculate Square Footage: Circle
Circular patios, round planters, circular rooms (less common but they exist), and cylindrical footings.
Formula: pi x r^2
Where r is the radius (half the diameter). Pi is approximately 3.1416.
Example: Round concrete patio
A circular patio with a 12-foot diameter: radius = 6 ft, area = 3.1416 x 36 = 113.1 sq ft.
Use the concrete calculator to convert that area and slab thickness to cubic yards.
Example: Circular fountain pad
An 8-foot-diameter concrete pad: radius = 4 ft, area = 3.1416 x 16 = 50.3 sq ft. At 4-inch slab thickness: 50.3 x 0.333 = 16.75 cubic feet = 0.62 cubic yards. A few bags of premix handles this one.
How to Calculate Square Footage: Trapezoid
A trapezoid has two parallel sides of different lengths. Angled lot lines, sloped yard sections, and some room layouts create trapezoids.
Formula: ((Side A + Side B) / 2) x Height
Side A and Side B are the two parallel sides. Height is the perpendicular distance between them.
Example: Irregular yard section
A section of yard along a fence line has one edge that runs 30 feet, a parallel edge on the other side that runs 22 feet, and the perpendicular distance between them is 18 feet.
((30 + 22) / 2) x 18 = (52 / 2) x 18 = 26 x 18 = 468 sq ft
That’s a common landscaping scenario. New sod, gravel, mulch, or artificial turf all need this number before you can estimate material.
How to Calculate Square Footage: L-Shaped and Irregular Rooms
This is where most people slow down. An irregular shape just means you haven’t broken it into simple shapes yet.
Method 1: Add rectangles. Find a natural break point, divide into two rectangles, measure each, add the areas.
Method 2: Subtract from the bounding rectangle. Measure the largest rectangle that contains the whole space. Measure the missing piece. Subtract.
Example: L-shaped open floor plan
Bounding rectangle: 26 x 18 = 468 sq ft. Missing corner: 10 x 8 = 80 sq ft. Net area: 388 sq ft.
Check with Method 1: living section 16 x 18 = 288, dining bump 10 x 10 = 100. Total = 388. Same answer.
If the two methods don’t match, one of your measurements is off.
For flooring specifically, the flooring calculator handles waste-adjusted quantities once you’ve calculated your raw square footage.
Unit Conversions: Square Feet to Other Units
Different trades and different countries use different units. Here’s the conversion reference you need.
| From | To | Multiply By |
|---|---|---|
| Square feet | Square yards | 0.1111 (divide by 9) |
| Square feet | Square meters | 0.0929 |
| Square yards | Square feet | 9 |
| Square meters | Square feet | 10.764 |
| Square feet | Acres | 0.000023 (divide by 43,560) |
| Acres | Square feet | 43,560 |
Quick conversion notes:
Sq ft to sq yards: Divide by 9. Carpet is still sold by the yard. 250 sq ft = 27.8 sq yd, order 28.
Sq ft to sq meters: Multiply by 0.0929. Useful when working with metric suppliers. 400 sq ft = 37.2 sq m.
Sq ft to acres: Divide by 43,560. A 10,000 sq ft lot is 0.23 acres.
Sq ft to roofing squares: Divide by 100. A 2,000 sq ft roof = 20 squares of shingles before waste.
Worked Example 1: Whole House Square Footage for Flooring
A client wants new LVP throughout a 3-bedroom, 2-bath house. Here’s how I’d run the measurement.
Room-by-room measurements:
| Room | Dimensions | Square Feet |
|---|---|---|
| Living room | 19 x 15 | 285 |
| Kitchen/dining | L-shape: 14x12 + 10x10 | 268 |
| Master bedroom | 14 x 12 | 168 |
| Master closet | 6 x 5 | 30 |
| Bedroom 2 | 11 x 11 | 121 |
| Bedroom 3 | 10 x 10 | 100 |
| Hallway | 3.5 x 22 | 77 |
| Bath 1 | 8 x 7 | 56 |
| Bath 2 | 6 x 6 | 36 |
| Laundry | 6 x 7 | 42 |
| Total | 1,183 sq ft |
Waste factor for LVP across multiple rooms: 10%
1,183 x 1.10 = 1,301 sq ft to order
If the LVP comes in boxes covering 22 sq ft each: 1,301 / 22 = 59.1 boxes. Order 60 boxes.
Run the final numbers through the flooring calculator to double-check and get underlayment quantities at the same time.
Worked Example 2: Irregular Yard for Sod and Mulch
A backyard combines shapes. Main lawn: 40 x 28 = 1,120 sq ft rectangular. Triangular corner behind the garage: (16 x 12) / 2 = 96 sq ft. Circular fire pit area excluded from sod: radius 5 ft, area = 3.1416 x 25 = 78.5 sq ft.
Total sod area: 1,120 + 96 - 78.5 = 1,137.5 sq ft
Planting beds along the perimeter: a trapezoidal strip at ((42 + 40) / 2) x 3 = 123 sq ft, plus a rectangular strip 40 x 4 = 160 sq ft. Total: 283 sq ft of beds.
At 3 inches of mulch: 283 x 0.25 ft depth = 70.75 cubic feet = 2.6 cubic yards. Order 3 yards with a little cushion.
Square Footage for Specific Trades
Paint
Paint square footage is wall and ceiling area, not floor area.
Wall area formula: (Room perimeter x ceiling height) - (doors x 21 sq ft) - (windows x 15 sq ft)
A 12 x 14 room with 9-foot ceilings, one door, two windows: perimeter = 52 ft, wall area = 52 x 9 = 468 - 21 - 30 = 417 sq ft of paintable wall. Ceiling: 12 x 14 = 168 sq ft. Total: 585 sq ft per coat. One gallon covers 350-400 sq ft, so plan two gallons per coat.
The paint calculator handles this automatically.
Concrete Slab
Area x thickness (in feet) = cubic feet. Divide by 27 for cubic yards.
A 16 x 20 patio at 4 inches thick: 320 x 0.333 = 106.6 cubic feet = 3.95 cubic yards. Order 4 yards. Running short mid-pour is a real problem.
Drywall
A standard 4 x 8 sheet covers 32 sq ft. Measure all wall and ceiling surfaces, add 10-15% waste, divide by 32 for sheet count. The drywall calculator converts room dimensions directly to sheet count.
Frequently Asked Questions
How do I calculate the square footage of a room?
Measure the length and width of the floor in feet. Multiply them. For irregular rooms, break the space into rectangles, calculate each section, and add the totals.
What is the formula for square footage?
Rectangle: L x W. Triangle: (B x H) / 2. Circle: 3.1416 x r^2. Trapezoid: ((A + B) / 2) x H. Irregular: break into simple shapes and add areas.
How many square feet is a 10x10 room?
100 square feet. That’s why “100 sq ft” is used as a standard benchmark in construction estimating.
How do I convert square feet to square yards?
Divide by 9. A 180 sq ft room = 20 square yards.
How do I convert square feet to square meters?
Multiply by 0.0929. A 500 sq ft space = 46.45 square meters.
Is square footage measured inside or outside the walls?
For interior work (flooring, paint, drywall): inside the walls. For real estate listings: exterior dimensions of living area are typical under ANSI standards, though it varies by state. When measuring for materials, always measure where the material goes.
Should I include closets in square footage?
Yes, if material is going there. Include closets in flooring measurements every time. For paint, only include closets if you’re painting the interior walls. For real estate: closets count as part of the room.
A Note on Measurement Accuracy
The formulas in this guide are only as good as the measurements you feed them. I learned early in my career that the most important tool on a job is a tape measure used correctly.
A few habits worth keeping:
- Measure twice. Any measurement driving a material order should be taken twice and compared.
- Sketch first. Draw a rough sketch before measuring irregular spaces. It prevents the “which dimension was that?” problem at your desk.
- Convert before you calculate. Write inches as decimal feet before multiplying. Mixed units cause errors every time.
- Add waste on top. Your measured area is the net. Material orders need waste added: 10% for most hard flooring, 15% for diagonal tile, 10% for paint and drywall.
Old carpenter saying: “Measure twice, cut once.” It applies just as much to a material order as to a board. A bad measurement before a tile order wastes a full pallet.
Try EstimationPro free to put these measurements to work in a complete project estimate with material quantities, labor costs, and a professional PDF for your client.
The fastest path is to take your measurements, verify with the square footage calculator, and let the tools build the material list from there. The math is straightforward. The discipline is measuring it right the first time.
Try EstimationPro free and see how fast you can go from room measurements to a complete, professional estimate.
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