What this calculator does
Shingle quantities start from the true roof area, not the plan area. A 12 by 8 metre roof plan at a slope factor of 1.15 has 110.4 square metres of surface, which with 10% waste is 121.4 and needs 40 bundles.
Forgetting the slope factor is the expensive mistake. Ordering to the 96 square metre plan area would leave you five bundles short on a modestly pitched roof, and a steeper roof at 1.5 would leave you seventeen short. The steeper the roof, the worse the shortfall.
The formula
The plan area is multiplied by the slope factor to give the true surface, then by the waste allowance, then divided by the coverage per bundle. The roofing square, an American unit of 100 square feet or 9.29 square metres, is reported alongside because shingles are often specified in squares with three bundles to a square.
| Term | Meaning |
|---|---|
| Slope factor | True area divided by plan area. Depends on the pitch and always exceeds 1. |
| Roofing square | 100 square feet, or 9.29 m². Three bundles typically cover one square. |
| Bundle | The packaging unit for shingles, covering around 3.1 m². |
| Starter and ridge | Additional courses at eaves and ridge, not included in the field area. |
The inputs explained
| Field | What to enter |
|---|---|
| Roof plan length (m) | Roof plan length in metres, measured as though looking straight down. |
| Roof plan width (m) | Roof plan width. |
| Slope factor | Slope factor for the pitch. A 30 degree roof is about 1.15; calculate it from the rise and run if unsure. |
| Waste allowance (%) | Waste allowance. 10% for a simple gable, more for hips, valleys and dormers. |
| Bundle coverage (m²) | Coverage per bundle in m², from the packaging. |
When to use it
Ordering shingles
Bundles are the purchase unit, and getting the slope factor right is what makes the order correct.
Budgeting a reroof
Shingles are the main material cost, though underlay, flashings and fasteners add up.
Checking a quote
A roofer bundle count can be sanity-checked against the roof dimensions.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does the pitch change the order?
The same roof plan at a range of slope factors.
| Slope factor | Shingle bundles needed | Roof area (with slope) | Roofing squares |
|---|---|---|---|
| 1× | 35 | 96.0 m² | 11.37 |
| 1.15× | 40 | 110.4 m² | 13.07 |
| 1.3× | 45 | 124.8 m² | 14.78 |
| 1.5× | 52 | 144.0 m² | 17.05 |
Questions
What is a roofing square?
100 square feet, or 9.29 square metres. Shingles are traditionally sold and quoted by the square, with three bundles covering one square. The unit persists in roofing even in metric markets, which is why it is reported here.
How do I find the slope factor?
Divide the rafter length by the run, or calculate it from the pitch angle as one over the cosine. A 30 degree roof gives about 1.15 and a 45 degree roof 1.41. The roof pitch calculator works it out from the rise and run directly.
How much waste should I allow?
10% for a simple gable roof. Hips, valleys and dormers all produce angled cuts and need 15% or more. Complex roofs with many intersections can waste 20%, since almost every course needs trimming at both ends.
Does this include starter and ridge shingles?
No, it covers the field area only. Starter courses at the eaves and cap shingles along ridges and hips are additional, and are usually calculated by linear metre rather than by area.
For the slope factor itself, see the roof pitch calculator. For the structure beneath, see the roof truss calculator.