Roofing Calculator
Work out how much roofing a roof needs — surface area from the footprint and pitch, roofing squares with wastage, and underlayment rolls.
A roof is bigger than its footprint — the pitch stretches it. This calculator turns the footprint and the pitch into the true surface area, then into roofing squares and underlayment rolls.
For shingles specifically, use the roof shingle calculator; for metal, the metal roofing calculator.
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 the roof footprint (length × width) from the ground or a plan.
- Enter the pitch — rise per 12 in of run.
- Set wastage at 10–15% (more for hips and valleys).
- Enter the underlayment roll coverage from the product.
Worked example
| Footprint | 30 ft × 40 ft = 1,200 ft² |
|---|---|
| Pitch | 6/12 → slope factor √(1 + 0.25) = 1.118 |
| Roof area | 1,200 × 1.118 = 1,341.64 ft² |
| Squares | 1,341.64 ÷ 100 = 13.42 squares |
| With 10% wastage | 13.42 × 1.10 = 14.76 squares |
| Underlayment (400 ft² rolls) | ⌈1,341.64 ÷ 400⌉ = 4 rolls |
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
What is a roofing square?
A roofing square is 100 ft² (9.29 m²) of roof surface. Roofing materials — shingles, metal, underlayment — are sold and quoted by the square. This calculator converts your roof area into squares.
How do I measure roof pitch?
Measure the rise over a 12 in run: hold a level horizontal from the roof surface, measure 12 in along it, then measure down to the roof. That vertical distance is the pitch (e.g. 6 in = 6/12). The roof pitch calculator does the maths from rise and run.
How much wastage should I allow for roofing?
10% for a simple gable roof, 15% for hips, valleys and dormers. Cutting around penetrations (chimneys, vents, skylights) uses more material than a plain rectangle.
Is this calculator an engineering approval?
No. It estimates materials only. Structural sizing, roof work and building-code compliance must be verified by qualified professionals — see the safety notice on this page.
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.