How this roofing calculator works
The roofing calculator has two parts. Roof Area from House Size takes the footprint of the house, the roof pitch or angle, and the eaves overhang, and works out the roof square footage, the number of roofing squares, and an optional cost. Materials from Roof Area skips straight to the material list when you already know the roof area. Both list the shingle bundles, roll roofing, and felt underlayment you need, in US units with square meters alongside.
For example, a 2,000 sq ft house with a 6/12 roof and 1 ft eaves has about 2,461 sq ft of roof. That is 28 roofing squares with 10% extra, and 74 bundles of shingles.
Roof square footage calculator: the formula
First the eaves are added to the footprint. The calculator treats the footprint as a square, adds a strip as wide as the overhang on all four sides, and adds 10% to that strip for the extra roof at corners and gable ends. Then it multiplies by the slope factor, because a sloped roof is larger than the floor below it:
Roof area = footprint with eaves × √(1 + (pitch ÷ 12)²)
With the roof angle instead of the pitch, the slope factor is 1 ÷ cos(angle). A flat roof has a slope factor of 1.
Roof pitch and slope factor
Roof pitch is the rise in inches for every 12 inches of horizontal run, so a 6/12 roof climbs 6 in per foot. Measure it with a level and tape in the attic or on a rafter, or use the angle from your plans.
| Pitch | Angle | Slope factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Shingle calculator: roofing materials
Roofers price and order by the square, 100 sq ft of roof. The roofing squares count adds 10% for waste and rounds up. The material counts use the roof area itself, rounded up to whole units:
- Composition (asphalt) shingles: 3 bundles per square, about 33⅓ sq ft per bundle
- Roll roofing: a 36 in × 36 ft roll covers 1 square once overlaps are allowed for
- #15 felt: a 36 in × 144 ft roll covers 4 squares
- #30 felt: a 36 in × 72 ft roll covers 2 squares
Frequently asked questions
How many bundles of shingles do I need?
Divide the roof area in square feet by 100 to get squares, then multiply by 3. A 2,461 sq ft roof is 24.6 squares, or 74 bundles. Add 10–15% for cuts, hips, and valleys, and check the coverage printed on the bundle, since heavy designer shingles can take 4 or more bundles per square.
How many squares is my roof?
Roof area ÷ 100. A 1,500 sq ft single-story house with a 6/12 roof and 1 ft eaves has about 1,872 sq ft of roof, or 18.7 squares. The calculator shows 21 squares because it adds 10% for waste and rounds up.
How do I estimate roofing cost?
Enter an installed price per square foot (or per square meter, square yard, and so on) in the Price field. The roofing cost is the roof area times that price, so 2,461 sq ft at $4.00 per sq ft comes to $9,844. Roofers often quote per square; divide that price by 100 to get the price per square foot. For a roof replacement cost, use a price that includes tear-off, disposal, and labor, not just materials.
Is this a roof pitch calculator?
It uses the pitch rather than finding it. To get the pitch, measure the rise over a 12 in level run. The table above converts common pitches to angles and slope factors.
Does the roofing calculator work for hip and complex roofs?
The slope factor applies to any roof whose planes all share one pitch, including hip roofs. For roofs with sections at different pitches, dormers, or large overhangs, measure each section separately and use the Materials from Roof Area tab with the total.
Limitations
This is an estimate for planning and comparing quotes. It assumes one pitch for the whole roof and a roughly square footprint for the eaves, and it does not include ridge caps, starter strips, drip edge, flashing, nails, or labor. A roofer's measurement or an aerial roof report will be more accurate.
What's changed
- Launched.