Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Roofing Calculator works
This calculator turns a roof pitch into its angle and slope factor, then works out a simple gable roof: rafter length, roof area on both sides, roofing squares (100 sq ft each) and how many bundles of shingles to buy. Enter the pitch as X/12 (6, 6/12 or 6:12) or in degrees.
The slope factor is √(1 + (pitch ÷ 12)²): how much longer the sloped roof is than its flat footprint. Rafter length = (half the span + overhang) × slope factor. Roof area = (span + 2 overhangs) × (length + 2 overhangs) × slope factor. Squares = area ÷ 100; add waste, then bundles = squares × 3, rounded up.
Example: a 24' × 40' house with a 6/12 pitch, 12" overhangs and 10% waste. Slope factor = √(1 + 0.5²) = 1.118 (26.57°). Rafter = (12' + 1') × 1.118 = 14.534 ft, or 14' 6 7/16". Area = 26' × 42' × 1.118 = 1,092 × 1.118 = 1,221 sq ft = 12.21 squares. With 10% waste that is 13.43 squares × 3 = 40.29, so 41 bundles.
Most 3-tab and architectural shingles come 3 bundles per square; some heavier lines are 4 or 5, so check the wrapper. Allow about 10% waste on a simple gable, around 15% on a hip roof and more with valleys and dormers. Rafter length runs to the ridge center, before deducting half the ridge board. Figure starter strip, ridge cap, underlayment and drip edge separately. The IRC (R905.2.2) allows asphalt shingles only on slopes of 2/12 or steeper, with double underlayment from 2/12 up to 4/12; local codes and the shingle maker's instructions may differ.
Find the roof area (the plan area, overhangs included, times the slope factor for your pitch), divide by 100 and add waste. A 24' × 40' house with 12" overhangs and a 6/12 pitch has about 1,221 sq ft of roof, or 12.21 squares; with 10% waste plan on 13.43 squares, which is 41 bundles.
How many bundles of shingles are in a square?
Most 3-tab and architectural asphalt shingles are packed 3 bundles per square, so each bundle covers about 33.3 sq ft. Some heavy or designer shingles take 4 or 5 bundles per square; the coverage is printed on the wrapper.
How do I find the pitch of my roof?
Pitch is the inches of rise in 12" of level run. Hold a level horizontally with one end touching the roof (or the underside of a rafter in the attic), and at the 12" mark on the level measure plumb to the roof surface. That distance is the rise: 6" means a 6/12 pitch, which is 26.57°.
How do you calculate rafter length?
Take half the building width, add the horizontal overhang and multiply by the slope factor. For a 24' span with a 12" overhang at 6/12, that is (12' + 1') × 1.118 = 14' 6 7/16" from the ridge center to the end of the tail. Deduct half the ridge board thickness at the top before you cut.
What is the minimum roof pitch for asphalt shingles?
The IRC (R905.2.2) allows asphalt shingles on slopes of 2/12 or steeper, and from 2/12 up to 4/12 it requires double underlayment. Below 2/12 use a low-slope roofing system. Local codes and the manufacturer's instructions may be stricter.