Roofing calculator
You do not have to climb on the roof to estimate it. Enter the building footprint, the overhang, and the pitch, and the calculator converts the flat area into true roof area, then into bundles and rolls.
How the roofing calculator works
Roofs are measured in squares, where one square is 100 square feet of roof surface. The calculator starts with the building footprint plus overhangs, which is the area the roof covers when seen from above. It then multiplies by a pitch factor to account for the slope, because a steeper roof has more surface than its footprint.
The pitch factor is the length of the slope per foot of horizontal run: for a 6/12 roof it is √(1 + 0.5²) = 1.118. Shingles are counted at three bundles per square, underlayment by the roll coverage you select, and ridge cap at about 33 linear feet per bundle. Drip edge is based on the eave and rake lengths.
Formula
pitch factor = √(1 + (rise ÷ 12)²)
roof area = (length + overhangs) × (width + overhangs) × pitch factor
bundles = roof area ÷ 100 × 3 × (1 + waste)
Example: a 40 × 28 ft house with 12 in overhangs covers 42 × 30 = 1,260 sq ft. At 6/12 pitch the roof area is 1,260 × 1.118 = 1,409 sq ft, or 14.1 squares. With 10 percent waste, that is 15.5 squares × 3 = 47 bundles, and 2 rolls of synthetic underlayment.
Pitch multipliers
| Pitch | Multiplier | Pitch | Multiplier |
|---|---|---|---|
| 2/12 | 1.014 | 7/12 | 1.158 |
| 3/12 | 1.031 | 8/12 | 1.202 |
| 4/12 | 1.054 | 9/12 | 1.250 |
| 5/12 | 1.083 | 10/12 | 1.302 |
| 6/12 | 1.118 | 12/12 | 1.414 |
To find your pitch from the ground, hold a level against the underside of a rafter in the attic or a gable overhang, measure 12 in along it, and measure the vertical distance to the roof surface. That vertical number is the rise.
Common questions
How many bundles of shingles do I need per square?
Three bundles per square for standard 3-tab and most architectural shingles. Some thick designer shingles need four or five. The bundle wrapper states the coverage.
How many squares is a 2,000 square foot house?
The living area does not tell you the roof size, since it depends on the footprint, overhangs, and pitch. A single-story 2,000 sq ft home with a 6/12 gable roof typically has 24 to 26 squares. A two-story home with the same living area has about half that.
How much roofing waste should I add?
10 percent for a simple gable roof, 15 percent for hips and valleys, and up to 20 percent for roofs with many dormers or angles. Waste covers cut shingles at edges, valleys, and starter courses.
How many nails per square of shingles?
About 320 nails per square with four nails per shingle, or 480 in high-wind areas with six. That is roughly 2 pounds of 1-1/4 in roofing nails per square.