How to Calculate Square Footage for Flooring, Paint, and Tiling
Before you buy flooring, paint, or tile, you need one number: the square footage of the area you are covering. Get it wrong and you either run short mid-project or waste money on materials you cannot return.
This guide walks through measuring rectangular rooms, handling L-shapes and odd layouts, adding the right waste factor, and converting between square feet and square metres. It is the practical companion to the Square Footage Calculator.
✨Key takeaways
- Rectangle: Area = Length × Width. Always measure in feet (or metres) and multiply.
- Odd shapes: break the room into rectangles, calculate each, and add them together.
- Add 10% waste for flooring and tile; 5–10% for paint.
- For paint: calculate wall area (perimeter × height), subtract doors and windows.
Measuring a rectangular room
Measure the length and width of the room in feet using a tape measure. Measure at the longest points in case the walls are not perfectly straight.
Example: a room is 12 feet long and 10 feet wide. Area = 12 × 10 = 120 sq ft.
If you measured in inches, divide by 144 to convert to square feet (since 12 × 12 = 144). A room 144 inches × 120 inches = 17,280 sq in ÷ 144 = 120 sq ft.
Handling L-shaped and irregular rooms
Break the room into smaller rectangles. An L-shaped room might split into a 10×12 rectangle and a 6×8 rectangle.
Calculate each: 120 + 48 = 168 sq ft total.
For rooms with alcoves, closets, or bump-outs, add each section separately. For circular areas (bay windows), use πr². A semicircular bay with a 4-foot radius: π × 4² ÷ 2 = 25.1 sq ft.
Square footage for paint
For walls, you need the perimeter times the wall height, minus door and window openings.
Example: a 12×10 room with 8-foot ceilings. Perimeter = 2(12+10) = 44 feet. Wall area = 44 × 8 = 352 sq ft. Subtract one door (21 sq ft) and two windows (2 × 15 = 30 sq ft). Paintable area = 352 − 51 = 301 sq ft.
One gallon of paint covers about 350–400 sq ft (one coat). For two coats, double the coverage needed: 301 × 2 = 602 sq ft → 2 gallons.
Waste factor for flooring and tile
Always buy more than the exact area. For straightforward rectangular rooms with planks running parallel: add 10%. For diagonal installation or complex patterns: add 15%. For natural stone with colour variation: add 15–20%.
Our 120 sq ft room with 10% waste: 120 × 1.10 = 132 sq ft of flooring needed.
Use the Square Footage Calculator to get the base area, then apply the waste percentage manually or in the calculator.
Common mistakes
Mixing up feet and inches. Measuring 10 feet 6 inches is 10.5 feet, not 10.6 feet. The decimal conversion is: inches ÷ 12. So 6 inches = 0.5 feet, 3 inches = 0.25 feet.
Forgetting closets. If you are laying flooring, you probably need to cover the closet too. Measure it separately and add.
Not accounting for obstacles. Kitchens have islands and cabinets. Measure the total room and subtract the footprint of permanent fixtures.
Try the calculators referenced in this guide
Put the maths into practice — every calculator is free and runs entirely in your browser.
Frequently Asked Questions
How do I convert square feet to square metres?
Divide by 10.764. So 120 sq ft = 120 ÷ 10.764 = 11.15 sq metres. The Unit Converter handles this conversion automatically.
How many boxes of flooring do I need?
Check the box — most list the coverage per box (e.g., 20 sq ft). Divide your total needed area (with waste) by the per-box coverage and round up. 132 sq ft ÷ 20 = 6.6, so buy 7 boxes.
Should I measure the room myself or trust the builder's plans?
Always measure yourself. Builder plans are often nominal dimensions and may not account for as-built variations, added drywall thickness, or post-construction changes.
The Precision Calculator Editorial Team
The editorial team at Get Precision Calculator writes practical, formula-driven guides that explain the maths behind every calculator on this site. All content is reviewed for accuracy before publishing.
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