Roofing Calculator
Roof area, roofing squares and shingle bundles from the house footprint and roof pitch.
- Footprint area (sq ft)1,200
- Roof surface with 10% waste (sq ft)1,476
- Roofing squares14.8
- Shingle bundles (3 per square)45
Inputs used
- Footprint length40 ft
- Footprint width30 ft
- Roof pitch6/12 — average
About this calculator
A pitched roof is bigger than the footprint beneath it: the pitch multiplier converts flat area to true roof surface. A 6/12 pitch (6 inches of rise per foot of run) multiplies the footprint by about 1.118, so a 40 × 30 house carries roughly 1,342 square feet of actual roof.
Roofers measure in "squares" — one square is 100 square feet — and standard architectural shingles come three bundles to the square. The bundle count below already includes 10% for waste: ridge caps, valleys and cut-offs at edges consume more than flat math suggests, and complex roofs with many hips and valleys can need 15%.
Enter the footprint (the area the roof covers when viewed from above), not the wall dimensions — include eave overhangs, which typically add a foot or two on each side.
Quick reference
Roofing squares by footprint size — 6/12 pitch
Roofing squares (including 10% waste) for an average 6/12 pitch, by footprint length and width in feet.
| Footprint length | 20 ft | 25 ft | 30 ft | 40 ft | 50 ft |
|---|---|---|---|---|---|
| 20 ft | 4.9 | 6.1 | 7.4 | 9.8 | 12.3 |
| 30 ft | 7.4 | 9.2 | 11.1 | 14.8 | 18.4 |
| 40 ft | 9.8 | 12.3 | 14.8 | 19.7 | 24.6 |
| 50 ft | 12.3 | 15.4 | 18.4 | 24.6 | 30.7 |
| 60 ft | 14.8 | 18.4 | 22.1 | 29.5 | 36.9 |
Shingle bundles by footprint length and pitch — 30 ft wide
Shingle bundles (3 per square, 10% waste included) for a 30 ft wide footprint, by length and roof pitch.
| Footprint length | Flat | 4/12 — low slope | 6/12 — average | 8/12 — steep | 10/12 — very steep | 12/12 — 45 degrees |
|---|---|---|---|---|---|---|
| 20 ft | 20 | 21 | 23 | 24 | 26 | 28 |
| 30 ft | 30 | 32 | 34 | 36 | 39 | 42 |
| 40 ft | 40 | 42 | 45 | 48 | 52 | 56 |
| 50 ft | 50 | 53 | 56 | 60 | 65 | 70 |
| 60 ft | 60 | 63 | 67 | 72 | 78 | 84 |
Frequently asked questions
How do I calculate roof area from the house footprint?
Multiply the footprint area by the pitch multiplier, which accounts for the slope. A 6/12 pitch multiplies by about 1.118, so a 40 × 30 ft footprint (1,200 sq ft) carries roughly 1,342 square feet of actual roof before any waste allowance.
What is a roofing square?
A roofing square is 100 square feet of roof surface — the unit roofers and suppliers use for pricing and materials. A 1,500 square foot roof is 15 squares.
How many bundles of shingles do I need for a 40x30 house?
At an average 6/12 pitch the roof works out to about 14.8 squares once 10% waste is added, and standard architectural shingles come three bundles to the square — so about 45 bundles. Steeper pitches and complex roofs with many valleys need more.
Do I measure the house or the roof footprint?
Use the footprint — the area the roof covers seen from above — and include the eave overhangs, which usually extend a foot or two past the walls on each side. Wall dimensions alone will undercount the roof.
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