How to use the roof calculator
- Enter the building length and width measured outside the walls. You can measure this on the ground, without climbing on the roof.
- Enter the overhang, the horizontal distance the eaves and rakes stick out past the walls (often 12 to 18 inches).
- Enter the pitch as rise per 12 inches of run. If you do not know it, the roof pitch calculator explains how to measure it.
- Pick the roof style and waste: about 10% for a simple gable, 15% for a hip roof with more cut shingles along the hips.
The roof area formula
Footprint = (length + 2 × overhang) × (width + 2 × overhang)
Roof area = footprint × √(1 + (pitch ÷ 12)²)
Squares = roof area ÷ 100 · bundles = squares × (1 + waste) × 3
For a simple gable or hip roof where every plane has the same pitch, the sloped area is the flat footprint times the pitch factor, regardless of how many planes there are. A roofing square is 100 sq ft. Most standard asphalt shingles are packaged three bundles to a square; check the wrapper, because some heavy designer shingles come four or five bundles per square.
Pitch factors
- 3/12: 1.031 · 4/12: 1.054 · 5/12: 1.083
- 6/12: 1.118 · 7/12: 1.158 · 8/12: 1.202
- 9/12: 1.250 · 10/12: 1.302 · 12/12: 1.414
Worked example: 40 × 30 ft gable roof
12 inch overhang, 6/12 pitch, 10% waste:
- Footprint: 42 × 32 = 1,344 sq ft
- Roof area: 1,344 × 1.118 = 1,503 sq ft = 15.03 squares
- With 10% waste: 16.53 squares × 3 = 50 bundles
- Drip edge: eaves 2 × 42 = 84 ft plus four rakes of 16 × 1.118 = 71.6 ft, so 156 ft, or 16 pieces of 10 ft
- Ridge: 42 ft, for ridge cap shingles
The same house with a hip roof has the same 1,503 sq ft but needs drip edge only along the 148 ft of eaves, and its ridge is shorter (42 − 32 = 10 ft), with hip caps running down the four corners.
What else a re-roof needs
- Drip edge: the International Residential Code (section R905.2.8.5) requires drip edge at eaves and rake edges of asphalt shingle roofs.
- Underlayment: buy by the roll's coverage printed on the label; synthetic rolls commonly cover several squares each.
- Starter strip and ridge cap: starter runs along eaves (and often rakes); ridge cap covers the ridge and hips.
- Ice barrier: required along the eaves in cold climates where the code calls for it.
- Decking: if sheathing is being replaced, the plywood calculator converts the same area into sheets.
Tips for an accurate estimate
- Split complex roofs into rectangles (main house, garage, porch), run each one, and add the results.
- Add dormers and valleys separately or raise waste to 15–20%.
- Compare with your roofer's measurement; satellite reports and tape measurements usually agree within a few percent.
- Working on a roof is dangerous. Measure from the ground where possible and use fall protection if you go up.
Frequently asked questions
How do I calculate the square footage of my roof?
Multiply the house length and width including overhangs to get the footprint, then multiply by the pitch factor. A 42 × 32 ft footprint at 6/12 pitch is 1,344 × 1.118 = 1,503 sq ft.
How many squares is my roof?
Divide the roof area by 100. A 1,503 sq ft roof is about 15 squares, or 16.5 squares once 10% waste is added.
How many bundles of shingles do I need?
Most standard shingles are 3 bundles per square. Multiply squares with waste by 3 and round up: 16.53 squares needs 50 bundles.
How much waste should I add for roofing?
About 10% for a simple gable roof and 15% for hip roofs or roofs with valleys and dormers, because more shingles are cut.
How big is a 2,000 sq ft house roof?
It depends on the pitch and overhangs. A 2,000 sq ft single-story footprint with 1 ft overhangs is roughly 2,200 sq ft of plan area, or about 2,450 sq ft of roof at 6/12.