How this calculator works
Asphalt shingles cover the sloped surface of a roof, but you usually measure its flat footprint in square feet or square metres - the shadow it casts on the ground. The bridge between the two is the pitch’s slope factor, the same multiplier the roof pitch calculator gives. Multiply the footprint by it for the real area to cover, then convert to squares and bundles:
slope factor = sqrt(1 + (rise ÷ run)²) (a 6:12 pitch = sqrt(1 + 0.5²) = 1.118)
sloped area = footprint area × slope factor
squares = sloped area in sq ft ÷ 100
bundles = ceil(squares × bundles-per-square × waste)
A square is 100 sq ft of roof, the unit shingles are sold in, packed about 3 bundles to the square. Underlayment is sized from the same sloped area, while starter strip and ridge cap come off the linear run of the eaves and the ridges plus hips. The waste allowance is applied to every material at once.
Worked example
This example follows the unit system you pick in the calculator above.
A 96 m² footprint at a 6 : 12 pitch, 3 bundles per square, 10% waste:
- Slope factor for 6 : 12: √(1 + 0.5²) = 1.118.
- Sloped area: 96 × 1.118 = 107.3 m² of roof to cover.
- Squares: 107.3 m² = 1155 sq ft ÷ 100 = 11.55 squares.
- Bundles: 11.55 × 3 × 1.10 = 38.1, rounded up to 39 bundles.
- Starter: 24 m of eaves ÷ 33 m per bundle (× 1.10) = 1 bundle.
- Ridge cap: 12 m of ridge ÷ 6 m per bundle (× 1.10) = 3 bundles.
Slope factor and bundles by pitch
Footprint to sloped area is a single multiply by the slope factor for your pitch. This table also shows the bundles for a 10-square footprint (1000 sq ft, or 92.9 m²) at 3 bundles per square with 10% waste, so you can sanity-check the calculator. It follows the unit system you pick in the calculator above:
| Pitch (x : 12) | Slope factor | Sloped area of 92.9 m² footprint | Squares | Bundles (3/sq, +10%) |
|---|
| 3 : 12 | 1.031 | 95.8 m² | 10.31 | 35 |
| 4 : 12 | 1.054 | 97.9 m² | 10.54 | 35 |
| 5 : 12 | 1.083 | 100.6 m² | 10.83 | 36 |
| 6 : 12 | 1.118 | 103.9 m² | 11.18 | 37 |
| 8 : 12 | 1.202 | 111.7 m² | 12.02 | 40 |
| 9 : 12 | 1.250 | 116.1 m² | 12.50 | 42 |
| 12 : 12 | 1.414 | 131.4 m² | 14.14 | 47 |
The flatter your roof, the closer the order is to the footprint; a steep roof can need a quarter more material than its plan area suggests. For the full pitch-to-slope-factor breakdown, see the roof pitch chart.
How many bundles of shingles do I need?
Most shingles come three bundles to the square, so the quick answer is your roofing squares times three, rounded up after waste - a 20-square roof (2000 sq ft, or 186 m²) is about 66 bundles with 10% waste. The calculator does this exactly for your pitch and waste setting. For the full method by roof size, and how the packing changes for heavier architectural shingles, see how many bundles of shingles do I need. To turn a ground measurement into roof squares, see how to measure a roof for shingles.
Shingles for hip, gable and other roof shapes
The footprint-times-slope-factor method works for any roof shape, because the footprint captures the outline and the pitch adds the steepness back. What changes between shapes is how you measure the footprint and how much waste to expect:
- Gable - two rectangular planes meeting at a ridge. The footprint is the building outline, and waste is low (10%) because the cuts run straight along the eaves and ridge.
- Hip - two triangular ends and two trapezoidal sides. Add each end as a Triangle plane (a trapezoid is one rectangle plus one triangle). The diagonal hips mean more cut shingles, so use 15% waste and add the hip lengths to your ridge-cap run.
- Gambrel (barn) - two pitches per side. Measure each pitch band as its own plane (lower steep, upper shallow) and add them, since the slope factor differs between the two.
- Shed / lean-to - a single slope across the full width. Enter the one plane and its pitch; the shed roof pitch calculator works out the slope.
- Flat / low-slope - under about 2:12 the slope factor is nearly 1, but most shingles are not rated this low, so check the product before ordering.
Add each plane as a Rectangle or a Triangle (base × height to the apex) and the calculator sums the footprints, or switch to Total area and type the measured roof square footage straight in. There is no need to subtract chimneys, skylights or vents - the waste allowance covers the cut-around, and deducting them risks under-ordering.
Field shingles, underlayment, starter and ridge cap
A shingle order is more than the field:
- Field shingles cover the open roof - the headline bundle count above.
- Underlayment goes down first over the whole sloped area. A synthetic roll covers about 10 squares, 15 lb felt about 4 and 30 lb felt about 2 once the overlap is allowed for.
- Starter strip runs along the eaves (and often the rakes) to lock down the first course. Size it from the eave run and the bundle’s stated linear coverage.
- Ridge cap bends over the ridges and hips. Size it from the combined ridge and hip length.
Enter the runs and coverages and the calculator counts each one, applying the same waste allowance throughout.
Tips for ordering shingles
- Buy all your shingles in one lot where you can - colour can shift slightly between production runs, and you want the dye lots to match across the roof.
- Keep two or three spare bundles back for repairs; a roof outlives the product line, and matching shingles years later is hard.
- The footprint to enter is the plan area, the flat outline of the roof. The slope factor adds the steepness back in - do not measure the slope and then multiply again, or you will double-count.
- For a roof with valleys, hips and dormers, lean towards the 15% waste setting; the diagonal cuts add up fast.
- Always work to your shingle manufacturer’s printed coverage and minimum-pitch requirements - this calculator estimates quantities, not warranty compliance.
