Roofing Calculator
Enter the footprint your roof covers and its pitch. The calculator finds the actual sloped roof area, converts it to roofing 'squares' (100 ft² each), and returns the number of shingle bundles to buy — plus the underlayment, starter strip, hip-and-ridge cap, drip edge and nails to go with them. Add a waste allowance under advanced options for cuts; 10% is set by default for a simple roof. Don't know your pitch? The roof pitch calculator finds it.
Your measurements
Footprint length along the ridge (the long way). Include the eave overhang.
Footprint width, eave to eave (the short way, across the ridge). Include overhangs.
How many inches the roof rises per 12 inches across — a "6-in-12" roof. Steeper roofs have more surface, so you buy more. Use the roof pitch calculator to measure it.
Shingle bundles
45 bundles
≈ 14.8 squares · incl. 10% waste
What to buy
- Shingles
- 45 bundles
- Underlayment
- 2 rolls(synthetic, ~10 sq/roll)
- Starter strip
- 1 bundles(80 lin ft eaves)
- Hip & ridge cap
- 2 bundles(40 lin ft ridge)
- Drip edge
- 15 pieces(147 lin ft · 10 ft pieces)
- Roofing nails
- 37 lb(~2.5 lb per square)
3 bundles cover one square (100 ft² of roof).
Water-resistant layer rolled over the deck before shingles go on.
A special first row along the eaves that locks down the bottom shingles against wind.
Pre-cut caps that fold over the peak where two roof planes meet.
Metal edging along the eaves and rakes that guides water off the roof past the fascia board.
Estimates only. Verify against your supplier's coverage figures before ordering.
Footprint
Footprint to scale
Roof size
- Footprint area
- 1,200 ft²(length × width)
- Roof pitch
- 6-in-12(1.118× slope)
- Actual roof area
- 1,341.64 ft²(footprint × slope)
- Squares to order
- 14.8 squares
The slope multiplier is how much bigger the sloped roof is than the flat footprint underneath it.
1 square = 100 ft² of roof — the unit roofers price and order shingles by.
The formula
Slope multiplier = √(rise² + 12²) ÷ 12 · Roof area = footprint × multiplier · Squares = roof area ÷ 100 · Bundles = ⌈squares × 3 × (1 + waste %)⌉
Example: A 40 × 30 ft footprint (1,200 ft²) at a 6-in-12 pitch (×1.118) is about 1,342 ft² of roof — 13.4 squares. With 10% waste that's ~14.8 squares, or 45 bundles of architectural shingles.
How it works
- 1Measure the roof's footprint — the flat length and width of the area it covers, including the eave overhangs.
- 2Find the slope multiplier from the pitch: √(rise² + 12²) ÷ 12. A 6-in-12 roof is 1.118, so the real roof is 11.8% bigger than the footprint.
- 3Roof area = footprint × multiplier. Divide by 100 to get roofing squares (1 square = 100 ft²).
- 4Add a waste allowance (10% for a simple roof), then multiply squares by 3 for the bundles of asphalt shingles to buy.
- 5Add the supporting materials — underlayment, starter strip, hip & ridge cap, drip edge and nails — sized off the roof area and edges.
Frequently asked questions
How many bundles of shingles do I need?
Multiply your roof area by the pitch's slope factor, divide by 100 for squares, then multiply by 3 bundles per square and add ~10% for waste. A 1,200 ft² footprint at a 6-in-12 pitch is about 13.4 squares, or roughly 45 bundles with waste.
How many shingles are in a square?
A roofing square is 100 square feet of finished roof. Standard asphalt shingles — both 3-tab and most architectural — take 3 bundles to cover one square. A few heavy premium shingles need 4 bundles per square.
How do I measure my roof without getting on it?
Measure the building's footprint (length × width, plus overhangs) from the ground, then find the pitch with the roof pitch calculator or a level photo of the gable. Multiply the footprint by the slope factor to get the actual roof area — no climbing required.
How much waste should I add for roofing?
About 10% for a simple gable roof, and 15% or more for roofs with lots of hips, valleys, dormers or chimneys, where there are more cuts. Steeper and more complex roofs waste more material.