This roofing calculator estimates the surface area of a pitched roof from its footprint and pitch. It then adds an allowance for cuts and overlaps, so you can see how much roofing to buy and how many roofing squares that makes.
It works best on a simple roof where every face has the same pitch. Enter the footprint with the overhang included, because the roof reaches past the walls. The results are a planning estimate, so check them against your own measurements before you order.
How to use the roofing calculator
- Measure the length of the roof footprint in feet, including the overhang on both ends, and enter it.
- Measure the width of the roof footprint in feet, including the overhang on both sides, and enter it.
- Enter the roof pitch as the rise in inches for every 12 inches of run.
- Optionally, enter an extra percentage for cuts, valleys, ridges and overlaps. Leave it blank if you do not want an allowance.
- Read the roof surface, the slope factor, the roofing to buy in square feet, and the roofing squares.
- Compare the roofing squares with what your supplier sells, and round up if needed.
The formula
- Slope factor = √(1 + (rise ÷ 12)²), where rise is the pitch in inches per 12 inches of run
- Roof surface = footprint length × footprint width × slope factor (sq ft)
- To buy = roof surface × (1 + extra % ÷ 100)
- Roofing squares = square feet to buy ÷ 100 (a roofing square covers 100 square feet)
- Assumes a simple roof where every face has the same pitch
Worked example
The figures below are made up to show the maths. They are not current prices, rates or product figures.
| Footprint length (with the overhang) | 40 ft |
|---|---|
| Footprint width (with the overhang) | 30 ft |
| Roof pitch (rise per 12 of run) | 6 in |
| Extra for cuts and overlaps | 10% |
- Roofing squares 14.76 squares (100 sq ft each)
- Roofing to buy 1,475.8 sq ft
- Roof surface 1,341.6 sq ft
- Slope factor 1.118
Tips for an accurate answer
- Include the overhang in both footprint measurements, or the surface will be underestimated.
- Use the pitch from a level and a tape, or from your plans, rather than guessing.
- Add an allowance for cuts and overlaps, especially on roofs with hips, valleys or many corners.
- If your roof has faces with different pitches, run the calculator for each section and add the results together.
- Check the final quantity against your supplier's coverage information before buying.
Questions people ask
Can I use a metal roofing calculator with this tool?
This calculator works from the footprint, the pitch and an optional waste allowance, so it gives the roof surface and roofing squares for any roofing type. Enter your measurements and the extra percentage you want, and read the roofing to buy. Confirm the panel layout with your supplier, since this tool does not account for panel widths.
How do I use a residential roofing calculator for a house roof?
Measure the footprint length and width of the roof with the overhang included, then enter the roof pitch in inches per 12 inches of run. Add an optional allowance for cuts and overlaps. The calculator then shows the roof surface and how many roofing squares to buy.
What roof measurements do I need to enter?
You need the footprint length and width in feet, including the overhang, and the roof pitch as rise per 12 inches of run. The calculator works out the roof surface from these three values. An optional percentage can be added for waste.
How many square feet of roofing do I need?
Enter the footprint length, width and pitch to get the roof surface in square feet. The roofing to buy figure adds your chosen waste allowance on top of that surface. Use that figure as your starting quantity.
What is the roof pitch input?
Roof pitch is the vertical rise of the roof for every 12 inches of horizontal run. Enter the rise in inches, for example 6 for a 6 in 12 pitch. The calculator uses this to work out how much longer the sloped surface is than the flat footprint.
Why do I need to include the overhang?
The roof extends beyond the walls, and that extra area is covered with roofing. Measuring the footprint with the overhang gives a roof surface that matches what you will actually cover.
What is a roofing square?
A roofing square is a unit equal to 100 square feet. The calculator divides the square feet you need to buy by 100 to give the number of squares.
What does the extra percentage cover?
It adds an allowance for cuts, overlaps, valleys and other waste. Using it helps make sure you do not run short partway through the job.
Sources
- 1 foot = 0.3048 metre exactly, so 1 inch (a twelfth of a foot) = 2.54 cm. Source: NIST, Revised unit conversion factors (the international foot).
- 1 yard = 0.9144 metre (3 feet). Source: NIST, NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically.
An estimate from the numbers you enter, worked out with the formula shown. Check quantities with your supplier before you order. Formulas and examples reviewed 24 Oct 2026. Read the disclaimer