Roof Area Calculator
Work out the true surface area of a roof — from the footprint and pitch, with up to three sections for L-shaped or multi-part roofs.
Most roofs are not one rectangle — they are a main roof plus a garage, an extension, or a wing, each with its own pitch. This calculator adds up to three sections into one total area.
Measure each section's footprint and pitch, and the total and the squares follow.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure each roof section's footprint (length × width).
- Enter each section's pitch.
- Leave sections 2 and 3 blank if the roof is a single rectangle.
- Read the total area and the roofing squares.
Worked example
| Section 1 | 30 ft × 40 ft, 6/12 → 1,200 × 1.118 = 1,341.64 ft² |
|---|---|
| Section 2 | 20 ft × 15 ft, 4/12 → 300 × 1.054 = 316.23 ft² |
| Total | 1,341.64 + 316.23 = 1,657.87 ft² |
| Squares | 1,657.87 ÷ 100 = 16.58 squares |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
Why is the roof area bigger than the floor area?
Because the roof is sloped: the surface area is the footprint multiplied by the slope factor. A 6/12 roof is 11.8% bigger than its footprint; a 12/12 roof is 41.4% bigger.
How do I measure a roof with multiple sections?
Split it into rectangles — main roof, garage, extension — and enter each as a section with its own pitch. This calculator adds them up. L-shaped roofs are two rectangles.
Should I add the overhangs?
Yes — measure to the edge of the roof, including the eaves overhang, not just the wall line. The overhang is roof surface too.
What is a roofing square?
100 ft² (9.29 m²) of roof surface. Materials are sold by the square; this calculator converts the total area into squares for ordering.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.