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Roofing Calculator
Shingles are sold by the bundle and counted by the square — and the roof you pay for is bigger than the house footprint by exactly the slope factor of your pitch. Enter the footprint and pitch for the true roof area, squares, bundles to buy, underlayment and ridge cap.
Your results
Shopping list — buy this much
Bundles assume 3 per square, standard for architectural shingles. Three-tab shingles also run 3 per square; heavyweight designer lines can run 4–5 — check the wrapper of the shingle you're buying. Underlayment is figured at 1,000 sq ft per synthetic roll; a felt roll covers only ~400 sq ft.
How it works
True roof area = footprint × slope factor
Squares = roof area ÷ 100 | Bundles = squares × 3 × (1 + waste %)
Roof pitch, angle and slope factor
Pitch is rise over a 12 in run. If you know any one of these, you know them all:
| Pitch | Angle | Slope factor | Common on |
|---|---|---|---|
| 4/12 | 18.4° | 1.054 | Ranches, low-slope additions |
| 5/12 | 22.6° | 1.083 | Many production homes |
| 6/12 | 26.6° | 1.118 | The residential default |
| 7/12 | 30.3° | 1.158 | Steeper traditional styles |
| 8/12 | 33.7° | 1.202 | Cape Cods, A-frame-ish |
| 9/12 | 36.9° | 1.250 | Tudors, steep gables |
| 10/12 | 39.8° | 1.302 | Steep; staging gets serious |
| 12/12 | 45.0° | 1.414 | True 45° roofs |
Measuring pitch without a gauge: in the attic, hold a 12 in level against a rafter, mark 12 in of level run, and measure the vertical rise from the rafter to the mark. That rise number is your pitch — and it's the input above. (A dedicated roof pitch calculator that works from rise and run measurements is on the build list; for now the table converts most of what you'll find.)
This calculator handles simple gable and single-plane roofs from the footprint. Complex roofs — hips, valleys, multiple wings — are best measured plane by plane: treat each rectangle separately with our square footage calculator, apply its slope factor, and add them up.
Worked example
A 40×30 ft roof footprint at a 6/12 pitch: slope factor √(1 + 0.5²) = 1.118, so the true roof area is 1,200 × 1.118 = 1,342 sq ft = 13.42 squares. At 3 bundles per square with 10% waste: 45 bundles, plus 2 rolls of synthetic underlayment. Shingles at $35 a bundle: about $1,575 in material — the number to compare against any roofer's line item.
Frequently asked questions
How many bundles of shingles do I need for 1,000 sq ft?
1,000 sq ft of roof is 10 squares; at 3 bundles per square that's 30 bundles, 33 with a 10% waste allowance. Note that's 1,000 sq ft of roof surface — a 1,000 sq ft footprint at 6/12 pitch is really 1,118 sq ft of roof, so measure the roof, not the floor plan.
What is a roofing square?
100 square feet of roof area — the unit every shingle price, quote and bundle count is built on. A typical 1,400 sq ft ranch roof is roughly 15–16 squares once the pitch factor is applied. When a roofer quotes 'per square,' this is the square.
How do I find my roof's pitch?
Three easy ways: a pitch gauge or phone level app on the roof surface; in the attic with a 12 in level and a tape (rise over 12 in of run); or from the ground, measure the gable's height above the eave and half the house width — rise ÷ run × 12. The attic method is the most accurate and keeps you off the roof.
How much does it cost to replace a roof?
Materials for a standard architectural-shingle roof run roughly $1.10–1.50 per sq ft of roof (shingles, underlayment, ridge, fasteners) — about $1,600–2,100 for the 13.4-square example above. Installed prices are a different universe: $4–8 per sq ft in most markets once tear-off, labor and disposal are in. This calculator prices the material side so you can read a quote line by line.
Can I shingle over an existing roof?
Most codes allow one overlay (two layers total), and it saves the tear-off cost — but it adds ~250 lb per square to the structure, shortens the new roof's life, voids some warranties, and hides deck damage. Most roofers and manufacturers recommend tear-off. Either way, the shingle count is the same.
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About the author
Harrison Alley is a web publisher and data enthusiast based in Dallas, Texas. He built Urbarama’s remodeling calculators and maintains them.
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