How to use the roofing calculator
- Measure the outside length and width of the house at ground level, including any attached garage under the same roof.
- Estimate the eave overhang; 12 in (30 cm) is common.
- Enter the pitch as rise per 12 (imperial) or as an angle in degrees (metric). The roof pitch calculator can convert either.
- Use 10% waste for a simple gable roof and about 15% for hip roofs, valleys and dormers.
- Add shingle and underlayment prices for materials, and low and high installed prices from local quotes for the range.
How the roof area is worked out
Pitch multiplier = 1 ÷ cos(angle) = √(1 + (rise ÷ 12)²)
Roof area = plan area × multiplier; with waste = roof area × (1 + waste %)
Squares = area with waste ÷ 100 sq ft; bundles = squares × 3, rounded up
The multiplier works for gable and hip roofs alike, as long as every slope has the same pitch, because each slope rises over the same plan area. Roofs with several different pitches should be measured section by section.
Worked example
A 40 × 30 ft house with a 12 in overhang all round, a 6/12 roof, 10% waste, shingles at $40 a bundle and installed prices of $450 to $700 per square:
- Plan area with overhang: 42 × 32 = 1,344 sq ft.
- A 6/12 pitch has a multiplier of 1.118, so the roof area is 1,344 × 1.118 = 1,503 sq ft.
- With 10% waste that is 1,653 sq ft, or 16.5 squares, which needs 50 bundles and 2 rolls of synthetic underlayment.
- Installed cost range: 16.53 squares × $450 to $700 = $7,438 – $11,570.
Tips and common mistakes
- Stay off the roof. Measure from the ground or from plans; working at height is dangerous. A licensed, insured roofer will measure and quote properly.
- Count the extras. Starter strips, ridge caps, flashing, vents and drip edge are separate items; the calculator lists drip edge from the perimeter.
- Ask about tear-off. Removing old layers, replacing damaged decking and disposal often add a large share of the final price.
- Cold climates need more. Many northern codes require ice and water shield membrane along eaves and in valleys.
- Check permits and warranties. Most re-roofs need a permit, and manufacturer warranties often require approved installers.
US, UK and Canada differences
In the US and Canada, roofing is measured in squares of 100 sq ft, asphalt shingles are packed about three bundles per square, and quotes are given per square. In the UK and much of Europe, pitched roofs are mostly concrete or clay tiles or natural slate, laid on battens over a breathable membrane, and roofers price per square metre. The area result in metric mode suits those quotes, while the bundle count mainly applies to North American shingles.
Metric units and other countries
In metric mode, enter the footprint in metres, the overhang in centimetres and the pitch in degrees; the area is in square metres and installed prices are per m². Many homes in South Asia and the Gulf have flat concrete roofs; for those, the area is simply the footprint and waterproofing is quoted per square metre. Type your local prices.