Roofing Calculator

How to use the roofing calculator

1. Measure the footprint. Measure the length and width of the house from the ground, then add the overhang on each side. You do not need to climb on the roof.
2. Find the pitch. Hold a level horizontally against the roof or rafter, measure 12 inches along it, then measure straight up to the roof surface. That height in inches is the rise, so 6 inches means a 6/12 pitch. Plans and roof inspection reports often list the pitch too.
3. Choose the roof shape. This sets a typical waste allowance, which you can change.
4. Adjust the extras (optional). Set the bundles per square for your shingle brand, and add a price per square to estimate the cost.
5. Read the results. You get the roof area, squares and bundles to order, rolls of underlayment, drip edge pieces and the cost, plus a table showing how pitch changes the area.

This calculator is for estimating. For a roof with several sections, add each section's footprint separately, or ask a roofer to measure. Working on a roof is dangerous, so use proper fall protection or hire a professional.

How to calculate roof area

A sloped roof has more surface than the ground it covers. Start with the footprint (length × width, including the overhangs), then multiply by the pitch multiplier:

Roof area = Length × Width × Pitch multiplier

Pitch multiplier = √(1 + (Rise ÷ 12)²)

Common multipliers:

• 4/12 pitch: 1.054
• 6/12 pitch: 1.118
• 8/12 pitch: 1.202
• 10/12 pitch: 1.302
• 12/12 pitch: 1.414

Roofing squares and bundles

Roofers measure in squares. One square covers 100 square feet. Most asphalt shingles come 3 bundles to a square, so each bundle covers about 33.3 sq ft.

Squares = Roof area × (1 + Waste %) ÷ 100

Bundles = Squares × Bundles per square, rounded up

Worked example

A house with a 40 × 30 ft footprint and a 6/12 gable roof:

1. Footprint: 40 × 30 = 1,200 sq ft
2. Roof area: 1,200 × 1.118 = 1,342 sq ft (13.42 squares)
3. Add 10% waste: 13.42 × 1.10 = 14.76 squares
4. Shingles: 14.76 × 3 = 44.3, so buy 45 bundles

How much waste to allow

Cutting shingles to fit edges, hips and valleys always leaves offcuts:

• Simple gable roof: about 10%
• Hip roof: about 15%
• Complex roof with valleys or dormers: about 20%

Other materials

• Underlayment: a synthetic roll typically covers about 1,000 sq ft (10 squares), while traditional 15 lb felt covers about 400 sq ft per roll.
• Drip edge: runs along the eaves and rakes, sold in 10 ft lengths. The calculator uses the footprint perimeter as a guide.
• Starter strip, ridge cap and nails are extra. Ask your supplier to check the full list against your roof plan.