Roofing calculator
Measure the footprint of the roof including overhangs, then pick the pitch. The pitch factor converts flat footprint area to actual roof surface.
Materials takeoff
Roofing Calculator · https://estimahome.com/roofing-calculator/
Roof length, incl. overhang: 40 ftRoof width, incl. overhang: 30 ftPitch (rise per 12 in): 6/12Roof style: Gable (two slopes)
- Pitch factor
- 1.118
- Roof area
- 1342 sq ft
- Squares
- 13.42
- Ridge + hips
- 40 lin ft
| No. | Item | Qty | Unit | Unit cost | Amount |
|---|---|---|---|---|---|
| 1 | Architectural shingles | 45 | bundle | $38.00 | $1,710.00 |
| 2 | Synthetic underlayment, 1000 sq ft | 2 | roll | $95.00 | $190.00 |
| 3 | Hip and ridge cap shingles | 2 | bundle | $60.00 | $120.00 |
| 4 | Drip edge, 10 ft | 15 | ea | $9.00 | $135.00 |
- Subtotal
- $2,155.00
- Likely range
- $1,831.75 – $2,478.25
- Footprint × pitch factor assumes every slope has the same pitch. Measure dormers and additions separately.
- Not included: starter strip, nails, ice and water shield, vents, flashing, tear-off and disposal.
Buy these materials
- 45 bundles of shinglesAmazon search for bundles of shingles (opens in new tab)
- 2 rolls of roof underlaymentAmazon search for rolls of roof underlayment (opens in new tab)
- 2 bundles of ridge capAmazon search for bundles of ridge cap (opens in new tab)
- 15 pieces of drip edge (10 ft)Amazon search for pieces of drip edge (10 ft) (opens in new tab)
Links open a store search for each item. Check sizes and grades against your plan.
Quick reference
- One roofing square is 100 sq ft of roof surface.
- Pitch factor = √(1 + (rise ÷ 12)²). At 6/12 it is 1.118; at 12/12 it is 1.414.
- Most architectural shingles come 3 bundles to the square.
How it's calculated
⌈x⌉ means round up to the next whole unit; ⌊x⌋ means round down. Every quantity you buy is rounded up.
Pitch factor = √(1 + (rise ÷ 12)²)Roof area = length × width × pitch factorSquares = roof area ÷ 100Bundles = ⌈ squares × (1 + waste ÷ 100) × bundles per square ⌉Underlayment rolls = ⌈ roof area × 1.1 ÷ roll coverage ⌉ (10% for laps)Gable ridge = length. Hip: ridge = length − width, plus 4 hips of √(2 × (width ÷ 2)² + (width ÷ 2 × rise ÷ 12)²)Ridge cap bundles = ⌈ ridge + hips ÷ lin ft per bundle ⌉Drip edge = ⌈ (eaves + rakes) ÷ 10 ⌉. Gable: 2 × length + 4 × (width ÷ 2) × pitch factor. Hip: 2 × (length + width).Cost = Σ (quantity × unit price); range = cost × (1 ± range % ÷ 100)
Worked example
A 40 × 30 ft gable roof at 6/12 has a pitch factor of √(1 + (6/12)²) = 1.118. Roof area is 1200 × 1.118 = 1342 sq ft, or 13.42 squares. With 10% waste at 3 bundles per square, that is 45 bundles. Underlayment with 10% for laps is 2 rolls. The 40 ft ridge takes 2 bundles of cap. Eaves and rakes total 147.08 ft, 15 pieces of drip edge. At the default prices: $2,155.00.
Questions
How many bundles of shingles per square?
Three for most architectural and 3-tab shingles. Some heavy designer shingles are four or five. It is printed on the wrapper.
How do I find my roof pitch?
Hold a level horizontally against a rafter or the roof, measure 12 in along it, and measure straight up to the roof surface. That rise in inches is the pitch, such as 6/12.
Why measure the footprint instead of the roof?
It is safer and simpler. When every slope has the same pitch, footprint area × pitch factor gives the true roof area for both gable and hip roofs.
How much waste for shingles?
About 10% on a simple gable roof and 15% or more on hip roofs and roofs with valleys and dormers.
Cite: “Roofing Calculator.” EstimaHome, https://estimahome.com/roofing-calculator/. Updated 2026-10-20. Formulas in machine-readable form: /api/calculators.json.