About the Roofing Calculator
This roofing calculator estimates how much material a roof needs from measurements you can take on the ground. Enter the length and width of the house, the eave overhang and the roof pitch; it multiplies the footprint by the pitch factor to find the true sloped area, adds waste, and converts the result into roofing squares, shingle bundles and rolls of underlayment. Enter a price per square to see an estimated cost.
It works for gable and hip roofs with a single, uniform pitch — for those shapes the sloped area is the same whichever way the planes face. Homeowners use it to sanity-check a roofing quote; DIYers use it to build a materials list.
A roofing square is 100 sq ft. Three-tab and most architectural shingles come three bundles to the square, though some heavier products use four. Allow about 10% waste for a simple gable roof and 15% or more for hips, valleys and dormers.
With the default inputs, the roofing squares (incl. waste) is 16.53. Change any value above to recalculate instantly.
How to use the roofing calculator
- 1Measure the length and width of the house footprint.
- 2Enter the eave overhang and the roof pitch.
- 3Set waste — 10% for a plain gable, 15% for hips and valleys.
- 4Choose bundles per square and your underlayment type.
- 5Add a price per square to estimate the total cost.
Formula and method
The footprint is the house length and width plus the overhang on each side. Multiplying it by the pitch factor √(1 + (pitch/12)²) converts the flat plan area into the true sloped surface area; this holds for any gable or hip roof where every plane has the same pitch.
The sloped area is increased by the waste allowance and divided by 100 to give roofing squares. Bundles are squares × bundles per square, rounded up, and underlayment rolls are the area with waste divided by the roll coverage, rounded up. Ridge cap, starter strip, drip edge and flashing are extra.
- L, W
- House length and width (ft)
- OH
- Overhang on each side (ft)
- pitch
- Rise in inches per 12 inches of run
Worked examples
40 × 30 ft house, 6/12 pitch, 1 ft overhang
With 1 ft of overhang on each side the footprint is 42 × 32 = 1,344 sq ft. The 6/12 multiplier of 1.118 gives 1,502.6 sq ft of roof, and 10% waste brings it to 16.53 squares — 50 bundles and 2 rolls of synthetic underlayment, about $7,438 at $450 a square.
50 × 28 ft hip roof, 8/12, 18 in overhang
The footprint is 53 × 31 = 1,643 sq ft. At 8/12 the multiplier is 1.2019, so the roof is 1,974.6 sq ft. With 15% waste for the hips that is 22.71 squares, 69 bundles and 3 underlayment rolls.
30 × 24 ft garage, 4/12, no overhang
A 720 sq ft footprint at 4/12 (multiplier 1.054) gives 758.9 sq ft of roof. Adding 10% waste is 8.35 squares, or 26 bundles of shingles.
Frequently asked questions
What is a square in roofing?+
A roofing square is 100 square feet of roof surface. Roofers price jobs and order materials by the square, so a 2,000 sq ft roof is 20 squares.
How many bundles of shingles are in a square?+
Most three-tab and architectural shingles are packaged three bundles per square, each covering about 33.3 sq ft. Some heavier designer shingles are four or five bundles per square — check the wrapper.
How do I calculate roof area without climbing on the roof?+
Measure the house footprint including overhangs, then multiply by the pitch multiplier for your roof pitch. For a 6/12 roof the multiplier is 1.118, so a 1,344 sq ft footprint is about 1,503 sq ft of roof.
How much waste should I add for roofing?+
About 10% for a simple gable roof and 15–20% for hip roofs or roofs with many valleys, dormers and penetrations, because more shingles are cut and discarded.
What does a new roof cost per square?+
Installed prices vary widely with material, tear-off layers, roof complexity and region. Get several local quotes per square and enter them here to compare totals.