How to use the roofing calculator
- Choose US (feet) or metric (meters).
- Enter the house length and width, measured around the outside walls.
- Enter the overhang: how far the roof sticks out past the walls at the eaves and gable ends.
- Enter the pitch as inches of rise per 12 inches of run (a 6/12 roof rises 6 in for every foot), or switch to degrees.
- Pick gable or hip. Leave waste blank to use 10% for a gable roof or 15% for a hip roof, or type your own.
- Choose the underlayment and, if you have one, a cost per square.
How roof area is calculated
A sloped roof covers more area than the ground it sits over, and the pitch factor says how much more. It works for every plane of a gable roof and, as long as all the planes share one pitch, for a hip roof too, so both roof types have the same area over the same footprint. What differs is the cutting: hip roofs have more angled edges, so they waste more shingles, and they need ridge cap along the hips as well as the ridge. A square is 100 square feet of roof, and most asphalt shingles are packed 3 bundles to a square.
Worked example
The defaults are a 40 × 30 ft house with a 1 ft overhang, a 6/12 gable roof and the automatic 10% waste:
- Plan area with overhang: (40 + 2) × (30 + 2) = 42 × 32 = 1,344 sq ft
- Pitch factor: √(1 + (6 ÷ 12)²) = √1.25 = 1.118
- Roof area: 1,344 × 1.118 = 1,503 sq ft (139.6 m²)
- With 10% waste: 1,652.9 sq ft, or 16.53 squares
- Bundles: 16.53 × 3 = 49.6, rounded up to 50 bundles
- Synthetic underlayment at about 10 squares a roll: 2 rolls
The same house with a hip roof has the same 1,503 sq ft, but 15% waste raises it to 17.28 squares and 52 bundles. At $150 per square for materials, the gable version costs about 16.53 × $150 = $2,479.
Common roof pitches
| Pitch | Angle | Pitch factor | Typical use |
|---|---|---|---|
| 3/12 | 14.0° | 1.031 | Low slope; shingles need extra underlayment |
| 4/12 | 18.4° | 1.054 | Ranch homes, porches |
| 6/12 | 26.6° | 1.118 | Common on many houses |
| 8/12 | 33.7° | 1.202 | Steeper suburban roofs |
| 12/12 | 45.0° | 1.414 | Steep, A-frame and Cape Cod styles |
Most shingle makers require a slope of at least 2/12, with a double layer of underlayment between 2/12 and 4/12. Check the installation instructions on the bundle wrapper.
Underlayment coverage
A roll of #15 asphalt felt covers about 4 squares, a roll of heavier #30 felt about 2 squares, and a typical synthetic roll about 10 squares, after the overlaps the manufacturer specifies. Roll sizes vary, so check the net coverage printed on the label. The calculator applies the same waste allowance to underlayment as to shingles.
Frequently asked questions
How many bundles of shingles are in a square?
Three, for most 3-tab and architectural asphalt shingles. Some heavy designer shingles come 4 or 5 bundles to a square, so read the wrapper before you order.
How do I find my roof pitch?
Hold a level horizontally against the underside of a rafter or on the roof surface, measure 12 inches along it, then measure straight up to the roof. That distance in inches is the rise per 12. You can also measure from the attic or use a phone app with an inclinometer.
Does the calculator include ridge cap and starter shingles?
It shows the ridge and hip length so you can buy ridge cap, which is sold by linear feet of coverage per bundle. Starter strips run along the eaves and rakes; use the drip edge length shown as a guide. Neither is included in the bundle count.
How much waste should I allow for roofing?
About 10% for a simple gable roof and 15% for a hip roof. Add a few percent more for valleys, dormers, skylights and chimneys, which all need extra cuts.
Related calculators
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- Stair Calculator — Number of steps, riser height, total run, stringer length and angle, checked against US IRC stair limits.
- Concrete Calculator — Concrete for slabs, footings, round columns, circular pads and steps, in cubic yards or m³, plus bags needed.
- Slope Calculator — Slope of a line from two points, or its equation from a point and a slope, with intercepts, angle and distance.
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