How to use the roofing calculator
- Measure the house footprint including the overhangs (length and width), and enter the roof pitch — or enter a roof area you have measured.
- Choose a waste allowance: 10% for simple roofs, 15% for complex ones.
- Check how many bundles per square your shingle needs (on the wrapper).
- Optionally add the eave + rake and ridge + hip lengths for starter strip and ridge cap.
Roofing formulas
Pitch factor = √(1 + (pitch ÷ 12)²) Roof area = Footprint area × Pitch factor Squares = Roof area ÷ 100 ft² Bundles = Squares × (1 + waste %) × bundles per square → round up
Worked example
A 40 × 30 ft footprint (with overhangs) and a 6/12 gable roof:
- Pitch factor = √(1 + 0.5²) = 1.118
- Roof area = 1,200 × 1.118 = 1,342 ft² = 13.42 squares
- +10% waste = 14.76 squares × 3 = 44.3 → 45 bundles
Pitch factor table
| Pitch | Factor | Pitch | Factor |
|---|---|---|---|
| 3/12 | 1.031 | 8/12 | 1.202 |
| 4/12 | 1.054 | 9/12 | 1.250 |
| 5/12 | 1.083 | 10/12 | 1.302 |
| 6/12 | 1.118 | 12/12 | 1.414 |
| 7/12 | 1.158 | 14/12 | 1.537 |
Other materials for a re-roof
- Underlayment: synthetic underlayment rolls often cover about 10 squares; traditional #15 felt about 4 squares and #30 felt about 2 squares per roll. Check the label.
- Ice and water shield: along eaves (often 2–3 ft up the roof) and in valleys in cold climates, as local code requires.
- Drip edge: along eaves and rakes, sold in 10 ft lengths.
- Nails: usually 4 per shingle, or 6 in high-wind areas, as the shingle maker specifies.
- Vents, flashing and pipe boots as needed.
3-tab vs architectural shingles
3-tab shingles are flat, single-layer and the cheapest option, usually rated around 60–70 mph winds. Architectural (laminated or dimensional) shingles have two bonded layers that give a textured look, higher wind ratings and longer warranties, and they are now the most common choice in the US. Both are normally sold 3 bundles to a square. Premium designer shingles mimic slate or shake, weigh more and may need 4–5 bundles per square — always check the wrapper before you calculate.
Measuring a roof safely
You don’t need to climb onto the roof. Measure the house from the ground, add the overhang on each side, and measure the pitch from the attic or a gable end (see our Roof Pitch Calculator). Satellite measurement services and many roofing suppliers can also produce a roof report.
Tips
- Use the pitch of each roof plane; if a house has more than one pitch, calculate each section separately and add them.
- Count tear-off layers. Most codes allow at most two layers of shingles; tearing off adds disposal cost.
- Buy from one lot so colours match, and keep a bundle for repairs.
- Safety first: roofs over 6/12 need harnesses and roof jacks.