Before ordering asphalt shingles, estimate the roof’s actual sloped surface rather than relying only on the home’s floor area. Roof pitch, overhangs and waste all affect the amount of material required. This guide explains a preliminary calculation for a simple symmetrical gable roof.
1. Measure the building footprint
Measure the building length along the ridge and the overall width across the eaves. Add the planned overhang to both ends of each dimension. Use consistent feet measurements for the footprint.
2. Convert roof pitch into a slope multiplier
Roof pitch is commonly written as rise per 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every 12 inches of horizontal distance. The slope multiplier is the square root of 1 plus the square of rise divided by 12.
Slope multiplier = √(1 + (rise ÷ 12)²)
3. Calculate the roof surface area
For this simple gable estimate, multiply the adjusted footprint area by the slope multiplier. For example, a 40 ft × 30 ft footprint with a 1 ft overhang on every edge becomes 42 ft × 32 ft. With a 6:12 pitch, the multiplier is about 1.118, producing roughly 1,503 square feet of roof surface before waste.
4. Convert the area into roofing squares and bundles
One US roofing square equals 100 square feet. Divide the roof area by 100 to estimate squares. Add a cutting allowance for valleys, edges, starter courses and off-cuts. Then divide the order area by the shingle coverage per bundle printed on the packaging and round up to whole bundles.
Use the Calclio Roofing Calculator to enter roof dimensions, pitch, overhang, waste and actual bundle coverage. The optional price field estimates shingle material cost.
What this simple estimate does not include
- Hips, valleys, dormers, intersecting roof planes and complex roof geometry.
- Underlayment, flashing, drip edge, ridge caps, starter strips, ventilation, nails and delivery.
- Structural checks, fall protection, local building code and manufacturer’s installation requirements.
Frequently asked questions
How many square feet are in a roofing square?
One roofing square equals 100 square feet of roof surface.
Does roof pitch change material quantity?
Yes. A steeper roof has more surface area than the horizontal building footprint.
How many bundles cover one square?
It depends on the product and exposure. Use the bundle coverage stated by the manufacturer rather than assuming every shingle has the same coverage.
For other home project estimates, try the Flooring Calculator or Paint Coverage Calculator.