How to use the roofing calculator calculator
- 1
Measure the footprint
Measure the roof area as seen from above, one section per roof plane or per part of the building. Include overhangs, since they are roofed too.
- 2
Find the pitch
Roof pitch is expressed as the rise in inches for every 12 inches of horizontal run. A roof that rises 6 inches over 12 inches of run is a 6/12. Enter 0 for a flat roof.
- 3
Read the sloped area
The calculator multiplies the footprint by the pitch multiplier to give the real surface area, then divides by 100 for squares.
- 4
Add an allowance
Hips, valleys, starter courses, and ridge caps all consume material beyond the plane area. How much depends on how cut up the roof is, so the figure is yours to choose.
- 5
Price it if you want a total
Enter your price per square for an estimated material total. Roofing is commonly quoted by the square.
Formula
Roof area = footprint(sq ft) x sqrt(12² + rise²) / 12, and squares = roof area / 100
The multiplier is the hypotenuse of the pitch triangle over its run. A roofing square is 100 square feet by definition.
Worked example
A 1,500 square foot roof footprint at a 6/12 pitch
- 1Pitch multiplier: sqrt(12² + 6²) / 12 = sqrt(180) / 12 = 1.118.
- 2Sloped roof area: 1,500 x 1.118 = 1,677 square feet.
- 3Squares: 1,677 / 100 = 16.77.
- 4Round up: 17 squares.
The roof is about 1,677 square feet of surface, which is 17 squares before any allowance for hips, valleys, and starters.
Worked examples
How much the pitch changes things
The same 1,500 square foot footprint at three different pitches
- 1Flat (0/12): multiplier 1.000, area 1,500 sq ft, 15 squares.
- 24/12: multiplier 1.054, area 1,581 sq ft, 15.81 squares.
- 36/12: multiplier 1.118, area 1,677 sq ft, 16.77 squares.
- 412/12: multiplier 1.414, area 2,121 sq ft, 21.21 squares.
The same building footprint spans 15 to 21 squares depending on pitch. Ignoring the pitch on a steep roof under-orders by more than a quarter.
A roof with two different planes
A 1,200 square foot main roof at 6/12 and a 300 square foot porch at 3/12
- 1Main roof: 1,200 x 1.118 = 1,342 square feet.
- 2Porch: multiplier sqrt(144 + 9) / 12 = 1.031, so 300 x 1.031 = 309 square feet.
- 3Total: 1,342 + 309 = 1,651 square feet.
- 4Squares: 16.51, so 17.
Planes at different pitches have to be calculated separately, because one multiplier cannot describe both. Add each plane as its own section.
Why the footprint is not the roof area
A pitched roof plane is the sloping surface over a flat area, which makes it the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. The hypotenuse is always longer than the base, so the roof is always larger than the footprint beneath it.
The pitch multiplier is just that relationship expressed as a ratio. With the run fixed at 12 inches, the multiplier is the square root of 12 squared plus the rise squared, all divided by 12. A 6/12 roof gives 1.118, meaning the surface is about 12 percent larger than the flat area.
The effect grows quickly with pitch. At 4/12 it is about 5 percent, at 6/12 about 12 percent, at 9/12 about 25 percent, and at 12/12 about 41 percent. On a steep roof, working from the footprint is not a small error.
How to find the pitch
Pitch is expressed as rise over a 12 inch run. The straightforward way to measure it is to hold a level horizontally against the roof line, mark 12 inches along it, and measure straight down from that mark to the roof surface. That vertical distance is the rise.
It can also be measured from inside the attic against a rafter, which is often safer and easier than getting onto the roof. The same method applies: 12 inches along the level, then measure down.
If the roof has more than one pitch, measure each. Porches, dormers, and additions frequently differ from the main roof, and each needs its own section in the calculator because one multiplier cannot describe two different slopes.
What a square is, and why roofing uses it
A roofing square is 100 square feet. It is a definition rather than a product measure, and it exists because roofing materials have long been sold and quoted in that unit.
That makes conversion simple: divide the sloped area by 100. A 1,677 square foot roof is 16.77 squares, which rounds up to 17 for ordering purposes.
How many bundles or pieces make a square is a product question that varies by material and manufacturer, so this page stops at squares. Take the coverage per bundle from the product documentation for whatever you are buying.
Common mistakes
Where roofing estimates go wrong:
- Using the building footprint as the roof area and skipping the pitch entirely.
- Applying one pitch to a roof that has several.
- Measuring the footprint without including the overhangs.
- Forgetting that squares are 100 square feet, and treating them as 10 by 10 feet of plan area on a sloped roof.
- Ordering the bare plane area on a roof full of hips and valleys.
- Assuming a fixed number of bundles per square instead of reading the product coverage.
Sources and methodology
Every figure on this page comes from geometry rather than from a product database. Areas use the standard area formulas, counts divide an area by the size of one unit, and unit conversions use exact factors: 12 inches to a foot, 9 square feet to a square yard, and 100 square feet to a roofing square.
YardCalc publishes no product dimensions, coverage rates, or prices. Unit sizes differ between manufacturers and regions, and a figure that is right for one product is wrong for another, so the calculator asks you for the size of the product you are actually buying. Prices are yours alone.
Results are planning estimates, not order quantities. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Pitch multipliers and roof area by pitch
The multiplier for each pitch, with the sloped area and squares it gives for a 1,500 square foot footprint. Multipliers are exact geometry, not product data.
| Pitch | Multiplier | Roof area for 1,500 sq ft | Squares |
|---|---|---|---|
| 0/12 | 1 | 1,500 sq ft | 15 |
| 1/12 | 1 | 1,505.2 sq ft | 15.05 |
| 2/12 | 1.01 | 1,520.7 sq ft | 15.21 |
| 3/12 | 1.03 | 1,546.2 sq ft | 15.46 |
| 4/12 | 1.05 | 1,581.1 sq ft | 15.81 |
| 5/12 | 1.08 | 1,625 sq ft | 16.25 |
| 6/12 | 1.12 | 1,677.1 sq ft | 16.77 |
| 7/12 | 1.16 | 1,736.6 sq ft | 17.37 |
| 8/12 | 1.2 | 1,802.8 sq ft | 18.03 |
| 9/12 | 1.25 | 1,875 sq ft | 18.75 |
| 10/12 | 1.3 | 1,952.6 sq ft | 19.53 |
| 12/12 | 1.41 | 2,121.3 sq ft | 21.21 |
Frequently asked questions
- How do I calculate roof area?
- Measure the footprint from above, then multiply by the pitch multiplier, which is the square root of 12 squared plus the rise squared, divided by 12. A 1,500 square foot footprint at 6/12 gives 1,677 square feet of roof.
- How many squares is my roof?
- Divide the sloped roof area by 100. A 1,677 square foot roof is 16.77 squares, which rounds up to 17 for ordering.
- What is a roofing square?
- 100 square feet. It is a unit of measure rather than a product, used because roofing has long been quoted and sold that way.
- How do I measure roof pitch?
- Hold a level horizontally against the roof or a rafter, mark 12 inches along it, and measure straight down to the surface. That vertical distance in inches is the rise, giving a pitch of rise over 12.
- Can I just use the house footprint?
- Only for a flat roof. Any pitch makes the roof larger than the footprint, by about 5 percent at 4/12, 12 percent at 6/12, and 41 percent at 12/12. Using the footprint under-orders every pitched roof.
- How much extra should I order for hips and valleys?
- It depends on how cut up the roof is. A simple gable wastes very little; a roof with several hips, valleys, and dormers wastes considerably more. Judge it from the roof and enter your own allowance.
Roof area is your measured footprint multiplied by the exact pitch multiplier, and squares are that area divided by 100, which is the definition of a roofing square. No pitch, material, coverage rate, allowance, or price is assumed. Counts come from exact geometry using the product dimensions you enter. Confirm the product size, the layout, cutting and breakage, and the final order quantity with your supplier before purchasing.
Results are planning estimates. Last reviewed 2026-08-30.