Roofing Calculator
Estimate a shingle job from the ground up. Enter the footprint (length × width) and the pitch, and the calculator applies the slope factor to get the true roof area, then converts it to roofing squares and bundles of shingles (3 per square) with a waste factor, material cost and underlayment — plus a roof diagram that redraws with the pitch.
45 bundles · 14.8 squares
Roof area: 1,342 ft² (13.4 squares) · pitch 6/12 → ×1.1 slope factor
With 10% waste: 1,476 ft² · 1305 shingles · $1,771 material
Underlayment: 2 synthetic roll(s) or 8 × 30 lb felt
Slope factor by pitch (1/12 → 12/12)
| Pitch | Slope factor | Roof area for this footprint |
|---|---|---|
| 1/12 | ×1.003 | 1,204 ft² |
| 2/12 | ×1.014 | 1,217 ft² |
| 3/12 | ×1.031 | 1,237 ft² |
| 4/12 | ×1.054 | 1,265 ft² |
| 5/12 | ×1.083 | 1,300 ft² |
| 6/12 | ×1.118 | 1,342 ft² |
| 7/12 | ×1.158 | 1,389 ft² |
| 8/12 | ×1.202 | 1,442 ft² |
| 9/12 | ×1.250 | 1,500 ft² |
| 10/12 | ×1.302 | 1,562 ft² |
| 11/12 | ×1.357 | 1,628 ft² |
| 12/12 | ×1.414 | 1,697 ft² |
A roofing square = 100 ft²; standard 3-tab and architectural bundles cover ~33.3 ft² (3 per square). Add-ons like ridge cap, starter and drip edge vary by roof shape — measure on-site before ordering. How we calculate →
How to calculate roof area from footprint and pitch
You can't shingle the floor plan — a sloped roof has more surface than the ground it covers. Enter your building's footprint (length × width, the area seen from above) and the pitch, and the calculator multiplies the footprint by the slope factor to get the true roof area. The slope factor updates live as you change the pitch, and both the footprint and the slope-adjusted area are shown so you can see exactly how much the slope adds.
For an L-shaped or multi-section roof, measure each rectangle's footprint, add them up, and enter the total in the override footprint field.
Roof pitch and the slope factor
Pitch is the rise over a 12-inch run — a 6/12 roof climbs 6 inches for every 12 horizontal. The slope factor converts flat footprint into actual area and equals √(rise² + 12²) ÷ 12: about 1.054 at 4/12, 1.118 at 6/12, 1.25 at 9/12 and 1.414 at 12/12. The reference table above lists every common pitch and shows what each one does to your specific footprint, so a steeper roof clearly needs more material.
How many bundles of shingles you need
Roofers measure in squares — one square = 100 ft² of roof. Standard 3-tab and architectural shingles come 3 bundles to the square, each bundle covering about 33.3 ft² (≈29 shingles). The calculator divides your waste-adjusted roof area by the bundle coverage and rounds up to whole bundles, and shows the total shingle count. Premium or specialty shingles cover less per bundle, so edit the coverage field if you're using them.
Why you add a waste factor
Cuts at hips, valleys, rakes and around penetrations all create offcuts, and you need starter and ridge pieces too. Add 10% for a simple gable, 15% for hips and valleys, and up to 20% for a complex, cut-up roof. The waste selector recalculates the squares, bundles and cost so you order enough the first time without a second delivery — and a little leftover for future repairs is normal.
Estimating material cost and underlayment
Set a price per square (the way roofing material is sold) and the calculator multiplies it by your waste-adjusted squares for a quick material estimate. It also sizes underlayment — one synthetic roll covers about 10 squares, a roll of 30 lb felt about 2. This is materials only; labor, tear-off, flashing, drip edge and disposal are separate, so treat it as a starting figure and get a contractor quote for the full job.
Frequently asked questions
How many bundles of shingles do I need?
Divide your slope-adjusted roof area (with a 10–15% waste factor) by the bundle coverage — about 33.3 ft² for standard shingles — and round up. The rule of thumb is 3 bundles per roofing square (100 ft²). The calculator does this and shows the exact bundle and shingle counts for your roof.
What is a roofing square?
A roofing square equals 100 square feet of roof surface — the unit roofers and suppliers price by. About 3 bundles of standard shingles cover one square. The calculator reports your roof in both squares and square feet.
How do I calculate roof area from the footprint?
Measure the footprint (length × width as seen from above), then multiply by the pitch slope factor to get the true sloped area. For example, a 1,200 ft² footprint at 6/12 pitch (×1.118) is about 1,342 ft² of roof. The tool applies the slope factor automatically.
What is roof pitch and the slope factor?
Pitch is the vertical rise per 12 inches of horizontal run, like 6/12. The slope factor, √(rise² + 144) ÷ 12, converts flat footprint into actual roof area — 1.118 at 6/12, 1.414 at 12/12. Steeper roofs have a higher factor and need more material.
How many shingles are in a bundle?
A standard bundle holds about 29 shingles (12"×36") and covers roughly 33.3 ft², so three bundles make a 100 ft² square. Premium and specialty shingles are heavier and cover less per bundle — sometimes around 20 ft² — so check the wrapper and adjust the coverage input.
Do I need to add extra shingles for waste?
Yes. Add about 10% for a simple gable roof, 15% when there are hips and valleys, and up to 20% for a complex, cut-up roof. The waste covers cuts, starter strip and ridge caps; the calculator's waste selector folds it into the bundle and cost totals.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →