Free online roofing calculator

How to use

1. Enter roof length and width (the flat footprint).
2. Select the roof pitch from the dropdown.
3. Choose architectural or 3-tab shingles.
4. Set the waste factor (10% for a simple gable).
5. Read sloped area, squares, and bundle count.

Formulas

pitchFactor = sqrt(1 + (rise/run)²)
  e.g. 6/12 → sqrt(1 + 0.5²) = sqrt(1.25) ≈ 1.118

baseSqFt    = roofLength × roofWidth
slopedSqFt  = baseSqFt × pitchFactor

squares     = slopedSqFt × (1 + waste/100) / 100

bundles     = ceil(squares × bundlesPerSquare)
  3-tab:           3 bundles/square
  architectural:   4 bundles/square

ridgeLength ≈ roofWidth / 2
ridgeBundles = ceil(ridgeLength / 35)

nails (lbs) = ceil(squares × 2.5)

Worked example

40 ft × 30 ft roof, 6/12 pitch, architectural shingles, 10% waste:

pitchFactor = sqrt(1 + (6/12)²) = sqrt(1.25) ≈ 1.118
baseSqFt    = 40 × 30 = 1,200 ft²
slopedSqFt  = 1,200 × 1.118 ≈ 1,342 ft²

squares     = 1,342 × 1.10 / 100 ≈ 14.76 squares
bundles     = ceil(14.76 × 4) = 60 bundles

ridge       = 30 / 2 = 15 ft → ceil(15/35) = 1 bundle
nails       = ceil(14.76 × 2.5) = 37 lbs

Notes

• Measure the flat footprint from the ground using the exterior wall dimensions. The pitch factor does the rest.
• Order a few extra bundles beyond the estimate; shingles from different lots may not match exactly if you run short mid-job.
• Drip edge runs along the eaves (roof length × 2) and rakes (roof width × 2 × pitchFactor) — order separately.