Roofing
calculator.
Work out the real surface area of a roof from its footprint and pitch, or measure it plane by plane, then turn it into squares, bundles of shingles and rolls of underlayment with a waste allowance.
A 1,503 sq ft roof is 15.03 squares; with 10% waste you need 50 bundles of shingles and 2 rolls of underlayment.
- Roof area
- 1,503 sq ft (139.6 m²)
- Squares (100 sq ft)
- 15.03
- Squares with 10% waste
- 16.53
- Shingle bundles
- 50 bundles (3 per square)
- Underlayment
- 2 synthetic rolls
- Pitch
- 6/12 = 26.57°
(40 + 2 × 1) × (30 + 2 × 1) = 1,344 sq ft plan; × √(1 + (6/12)²) = × 1.11803 = 1,502.6 sq ft
1,502.6 ÷ 100 = 15.03 squares × 1.1 = 16.53 × 3 = 49.59 → 50 bundlesFootprint × pitch,
then squares.
- Roof area = plan area × √(1 + (rise ÷ 12)²). This is exact for any roof whose planes all share one pitch, including hip roofs, because each plane's area is its plan area divided by the cosine of the slope angle.
- In footprint mode the overhang is added to every side: (length + 2 × overhang) × (width + 2 × overhang). In plane mode, enter each plane measured on the roof itself and no pitch factor is applied.
- A square is 100 sq ft of roof. Waste is added to the squares first; bundles and underlayment rolls are then rounded up to whole units.
- From 2/12 up to 4/12, two layers of underlayment are counted, and below 2/12 the calculator warns that asphalt shingles are not allowed, following the International Residential Code (R905.2.2) as adopted by local building departments.
How do you calculate roof area from the footprint?
Measure the building's length and width, add the overhang to each side, and multiply by the pitch factor. A 40 × 30 ft house with a 1 ft overhang covers (42 × 32) = 1,344 sq ft from above. At a 6/12 pitch the factor is √(1 + 0.5²) = 1.1180, so the roof surface is 1,344 × 1.1180 = 1,502.6 sq ft.
The same factor works for a hip roof, because every plane is tilted by the same angle. It does not hold if parts of the roof have different pitches; measure those parts separately, or use the plane-by-plane mode.
plan area = (L + 2 × overhang) × (W + 2 × overhang)
roof area = plan area × √(1 + (rise ÷ 12)²)What is the pitch multiplier for my roof?
The multiplier is the length of the slope for every foot of horizontal run: √(1 + (rise ÷ 12)²). A 4/12 roof has a multiplier of 1.0541 (18.4°), 6/12 is 1.1180 (26.6°), 8/12 is 1.2019 (33.7°) and 12/12 is 1.4142 (45°). If you know the angle instead, the multiplier is 1 ÷ cos(angle): 1 ÷ cos 30° = 1.1547.
multiplier = √(1 + (rise ÷ 12)²) = 1 ÷ cos(angle)
angle = arctan(rise ÷ 12)How many bundles of shingles do I need?
Divide the roof area by 100 to get squares, add waste, and multiply by the bundles per square. Owens Corning packages its architectural shingles at three bundles per square (about 33.3 sq ft per bundle), and most 3-tab and architectural shingles follow the same pattern. The 1,502.6 sq ft roof above is 15.03 squares; with 10% waste that is 16.53 squares, and 16.53 × 3 = 49.6, so you buy 50 bundles.
Some heavier designer shingles are packed with more bundles to the square. Change the bundles-per-square setting to match the label on the product you are buying.
squares = roof area ÷ 100 × (1 + waste ÷ 100)
bundles = squares × bundles per square, rounded upHow much waste should I add for a roof?
Owens Corning advises adding 10% to 15% to all material totals, and more for complicated roofs because of the cut shingles at hips, valleys, walls and edges. Their worked example adds 2.4 squares to a 24-square roof, which is 7 or 8 more bundles: 26.4 × 3 = 79.2, or 80 bundles. A plain gable roof sits near the low end; a cut-up roof with dormers and valleys needs the high end or more.
How many rolls of underlayment do I need?
Underlayment covers the same deck area as the shingles, plus waste. Owens Corning gives synthetic rolls as 10 squares and #15 felt as 4 squares. The 16.53-square example needs 2 synthetic rolls (1.65 rounded up) or 5 rolls of #15 felt (4.13 rounded up).
Building departments that follow the IRC require double underlayment on slopes from 2/12 up to 4/12, so the calculator doubles the roll count there. Check the roll label for its actual net coverage: some rolls cover a different area, and you can enter that as a custom roll size.
Worked examples.
Gable house from the footprint
A 40 × 30 ft house with a 1 ft overhang and a 6/12 roof, 10% waste, synthetic underlayment.
50 bundles: 1,503 sq ft, 15.03 squares, and 2 rolls of synthetic underlayment.
Load this exampleTwo measured planes (Owens Corning's example)
Two roof planes, each 40 × 30 ft measured on the roof, with 10% waste.
80 bundles: 2,400 sq ft is 24 squares, 26.4 with waste, and 3 synthetic rolls.
Load this exampleHip roof from the plan area
2,000 sq ft under the roof as seen from above, 4/12 pitch on every side, 15% waste, #15 felt.
73 bundles: 2,108 sq ft of roof and 7 rolls of #15 felt.
Load this exampleLow-slope porch roof
A 300 sq ft porch at 3/12, 10% waste, #15 felt. Between 2/12 and 4/12 the IRC requires two layers of underlayment.
11 bundles and 2 rolls of #15 felt for the double layer.
Load this exampleMetric house with a 30° roof
A 12 × 9 m house with a 0.5 m overhang and a 30° pitch, 12% waste.
55 bundles: 1,616 sq ft (150.1 m²) of roof and 2 synthetic rolls.
Load this exampleSteep roof priced per square
1,000 sq ft of plan area at 12/12, 15% waste, shingles at $130 a square.
49 bundles (16⅓ squares bought) for $2,123.33.
Load this exampleWhat this calculator
does not do.
- It estimates field shingles and underlayment only. Starter strip, hip and ridge caps, drip edge, ice barrier, flashing, vents and nails are sold by length or count and are not included.
- The footprint and plan modes assume every plane has the same pitch. For mixed pitches, dormers or curved roofs, measure each plane and use plane mode.
- The building-code slope checks follow the model International Residential Code. Your local code, the shingle maker's instructions and wind or ice rules can be stricter.
- Prices and bundle coverage vary by product. Use the numbers printed on the bundle and roll you are buying.
Common questions.
How many squares is a 2,000 sq ft roof?
20 squares, because a square is 100 sq ft of roof surface. With 10% waste you would order 22 squares, which is 66 bundles at three per square.
Is roof area the same as the house's square footage?
No. The roof is larger: it overhangs the walls and is tilted. A 6/12 roof over a 1,344 sq ft plan has about 1,503 sq ft of surface.
How many bundles are in a square?
Three for most 3-tab and architectural shingles; Owens Corning packs its architectural shingles three bundles to the square. Some heavier shingles need more, and the bundle label says how many.
What is the minimum pitch for asphalt shingles?
2/12 under the International Residential Code, and slopes from 2/12 up to 4/12 need double underlayment. Below 2/12 use a low-slope roofing system.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches of level run, using a level and tape on the roof or in the attic. If you know the angle, enter degrees instead.
- Owens Corning: DIY roofing, how to measure a roof — squares, 3 bundles per square, 10–15% waste, synthetic 10-square and #15 felt 4-square rolls, slope correction factors
- City of Hopkins, Minnesota: Asphalt shingles (IRC R905.2) — 2:12 minimum slope and double underlayment from 2:12 to 4:12
The engine computes the plan area, multiplies by √(1 + (rise ÷ 12)²) (or 1 ÷ cos of the angle), divides by 100 for squares, adds waste, and rounds bundles and rolls up to whole units, ignoring floating-point dust so an exact 30.0 bundles stays 30. Tests reproduce Owens Corning's 24-square worked example, check footprint, plan and plane modes against each other, confirm feet and meters give the same roof, and check that the multiplier equals 1 ÷ cos(angle) across 300 random roofs.
Built and reviewed by CalculatorLab against the sources above; every worked example on this page is re-checked by our automated test suite before each release. How we build and check calculators. Found a problem? Tell us.