Roofing Calculator
Roof area, squares and shingle bundles, scaled by the pitch. A 6/12 roof is twelve per cent larger than the ground it covers, and ignoring that is how people under-order.
Roofing needed
14.76squares
- Roof area
- 1,476 sq ft
- Squares
- 14.76
- Shingle bundles
- 45
- Pitch factor
- 1.1180
- Footprint
- 1,200 sq ft
- Square metres
- 137.1
- Includes a 10% waste allowance. The bare figure is 1,341.64 sq ft.
- A 6/12 pitch makes the roof 11.8% larger than the footprint beneath it.
- A roofing square is 100 square feet, and shingles run three bundles to the square. This covers the field only — ridge, hip and starter courses are extra.
- Measure the footprint the roof covers, not the walls. Overhangs count.
About the roofing calculator
A roof is always bigger than the footprint beneath it, and how much bigger depends entirely on its pitch. The multiplier comes straight out of Pythagoras: for every twelve inches of horizontal run, a roof rising R inches travels the square root of R squared plus 144.
For a 6/12 pitch that works out at 1.118, so the roof is about twelve per cent larger than the ground. A 12/12 pitch is 1.414 — forty-one per cent larger. On a 2,000 square foot footprint that is the difference between 21 and 28 squares of shingles.
Results are given in squares, the unit roofers and suppliers quote in. A square is 100 square feet and shingles run three bundles to the square. This covers the field of the roof only — ridge caps, hip caps and starter strips are sold by the linear foot and ordered separately.
What it works out
- Roof area, squares and shingle bundles
- The pitch factor it applied, shown explicitly
- Footprint against actual roof surface
The formula
Roof area = Footprint × √(Rise² + 144) ÷ 12
A roof is always bigger than the ground it covers, and by how much depends entirely on its pitch. The multiplier comes straight out of Pythagoras: for every 12 inches of horizontal run, a roof with a rise of R inches travels √(R² + 144) inches of actual surface.
For a 6/12 pitch that is √(36 + 144) ÷ 12, or 1.118 — so the roof is about 12 per cent larger than the footprint. A 12/12 pitch works out at 1.414, meaning a roof 41 per cent larger. Skipping this step is the single most common way people under-order shingles.
The result is usually expressed in squares. A roofing square is 100 square feet, and shingles are sold three bundles to the square, so a 2,000 square foot roof is 20 squares or 60 bundles before waste.
- Footprint
- The plan area the roof covers, measured including overhangs rather than to the walls.
- Rise
- Inches of vertical rise per 12 inches of horizontal run. A 6/12 roof rises six inches per foot.
- Square
- 100 square feet of roof. The unit shingles are quoted in.
A worked example
A forty by thirty foot footprint under a 6/12 gable roof.
That works out to 14.76 squares.
- Roof area
- 1,476 sq ft
- Squares
- 14.76
- Shingle bundles
- 45
- Pitch factor
- 1.1180
- Footprint
- 1,200 sq ft
- Square metres
- 137.1
Questions
How many bundles of shingles do I need per square?
Three. That holds for both 3-tab and most architectural shingles, though a few heavyweight designer products run four or five bundles to the square — check the wrapper before ordering.
What is a roofing square?
One hundred square feet of roof surface. Roofers and suppliers quote in squares rather than square feet, so a 2,400 square foot roof is 24 squares.
How do I find my roof pitch?
Hold a level out horizontally against the roof line, mark 12 inches along it, then measure straight down from that mark to the roof. That drop in inches is the rise, so a 6-inch drop is a 6/12 pitch.
Does pitch really change how much I need?
Substantially. A 4/12 roof is 5 per cent larger than its footprint, a 6/12 is 12 per cent, a 9/12 is 25 per cent and a 12/12 is 41 per cent. On a 2,000 square foot footprint that is the difference between 21 and 28 squares.
How much waste should I allow for roofing?
Ten per cent on a simple gable roof. Fifteen where there are hips and valleys, since every course meeting a valley is cut at an angle and the offcuts are waste.
Does this include the ridge and starter courses?
No. This is the field of the roof only. Ridge caps, hip caps and starter strips are separate products sold by the linear foot, so measure your ridges and eaves and order those on top.