Roofing Calculator
Calculate roof area, shingle bundles, and material quantities based on roof dimensions and pitch. See also Square Footage Calculator and Brick Calculator.
Input Method
Cost Estimate (optional)
How to Measure a Roof for Shingles
Measuring a roof requires accounting for the pitch (slope), which increases the actual surface area compared to the flat footprint. Measure the building footprint from the ground or use blueprints, then multiply by the pitch multiplier to get the true roof area. Divide by 100 to get the number of squares (roofing is sold in squares, where 1 square = 100 sq ft). Each square requires 3 bundles of standard three-tab or architectural shingles.
Roofing Calculation Formula
Footprint Area = Length x Width (or measured from ground)
Roof Area = Footprint Area x Pitch Multiplier
Squares = Roof Area / 100
Shingle Bundles = Squares x 3
Ridge Cap = Ridge Length / 35 (bundles)
Underlayment = Roof Area / 400 (rolls)
Pitch Multiplier = sqrt(1 + (rise/12)^2)
Example: 6/12 pitch = sqrt(1 + 0.25) = 1.118
Example Calculation
House: 40 ft x 30 ft, 6/12 pitch
Footprint = 40 x 30 = 1,200 sq ft
Roof area = 1,200 x 1.118 = 1,341.6 sq ft
Squares = 1,341.6 / 100 = 13.4 squares
Bundles = 13.4 x 3 = 41 bundles (rounded up)
Understanding Roof Pitch
Roof pitch is expressed as rise over run — for example, 6/12 means the roof rises 6 inches for every 12 inches of horizontal run. Steeper pitches shed water and snow better but require more material and are harder to work on. Most residential roofs range from 4/12 to 8/12. Pitches below 3/12 are considered low-slope and may require different roofing materials.
Roof Pitch Multiplier Table
| Pitch | Multiplier | Degrees | % Increase | Category |
|---|---|---|---|---|
| 2/12 | 1.014 | 9.5 | 1.4% | Low slope |
| 3/12 | 1.031 | 14.0 | 3.1% | Low slope |
| 4/12 | 1.054 | 18.4 | 5.4% | Moderate |
| 5/12 | 1.083 | 22.6 | 8.3% | Moderate |
| 6/12 | 1.118 | 26.6 | 11.8% | Standard |
| 7/12 | 1.158 | 30.3 | 15.8% | Standard |
| 8/12 | 1.202 | 33.7 | 20.2% | Steep |
| 9/12 | 1.250 | 36.9 | 25.0% | Steep |
| 10/12 | 1.302 | 39.8 | 30.2% | Steep |
| 12/12 | 1.414 | 45.0 | 41.4% | Very steep |
Frequently Asked Questions
How many bundles of shingles do I need?
Divide your roof area by 100 to get squares, then multiply by 3. A 1,500 sq ft roof needs 15 squares or 45 bundles. Add 10-15% for waste, especially for complex roof shapes with many hips and valleys.
What is a roofing square?
A roofing square is a unit of measurement equal to 100 square feet of roof area. Roofing materials (shingles, underlayment) are priced and sold per square. Three bundles of standard shingles cover one square.
How do I determine my roof pitch?
Place a level horizontally against the roof (or in the attic against a rafter). Measure 12 inches along the level, then measure the vertical distance from that point to the roof surface. That vertical measurement is your rise — expressed as rise/12.
How much does a new roof cost?
A new asphalt shingle roof costs $300-$500 per square installed (materials + labor). For a 20-square roof, expect $6,000-$10,000 total. Premium materials (metal, tile, slate) cost $500-$1,500+ per square. Tear-off of old shingles adds $100-$150 per square.
How long do asphalt shingles last?
Three-tab shingles last 15-20 years. Architectural (dimensional) shingles last 25-30 years. Premium architectural shingles can last 40-50 years. Actual lifespan depends on climate, ventilation, and maintenance.
Do I need to remove old shingles before re-roofing?
Most building codes allow up to two layers of shingles. However, removing old shingles (tear-off) is recommended because it allows inspection of the roof deck for damage, reduces weight on the structure, and provides a better surface for new shingles.
What underlayment do I need?
Synthetic underlayment is the modern standard — it is lighter, stronger, and more water-resistant than felt paper. Use ice and water shield membrane in valleys, around penetrations, and along eaves in cold climates. Standard felt (15 lb or 30 lb) is still acceptable in many areas.