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Roofing Calculator

Give us the footprint and the pitch and you'll get the roofing squares, bundles, the whole accessory list, and a price range for both buying the material and having the job done.

  • Squares and bundles
  • Underlayment, cap, and starter
  • Installed price range

Project measurements

What kind of roof? Pick the closest shape

A ridge down the middle and two slopes, and the most common roof there is.

Measurements Pitch drives everything else

How did you measure it?

ft
ft
ft
in
ft

Waste allowance How cut up is it?

Material Sets the price and the whole shopping list

How this number is built

The roofing math is easy, but mistakes are expensive. These are the rules and numbers behind the result.

A square is 100 square feet of roof surface

Not 100 ft² of floor. Shingles are sold by the square, and a square is 10 ft × 10 ft measured on the roof itself, which is why the same house has more squares than it has floor.

The pitch multiplier applies to the footprint only

Plan area × √(1 + (rise ÷ 12)²) gives true roof surface. If you measured along the slope instead, that number is already surface area. Multiplying it again is the double-multiplier error, and the order comes out too big, which is why this page asks which one you did.

Below 2:12, shingles are not allowed at all

IRC R905.2.2 permits asphalt shingles only at 2:12 or steeper. Under that, the correct products are low-slope membranes (modified bitumen, TPO, or EPDM), and this page says so instead of quoting shingles for a pitch that cannot take them.

2:12 to 4:12 needs two layers of underlayment

IRC R905.1.1 requires double underlayment in that band, which doubles the roll count. At 4:12 and above, one layer lapped 2 inches is the requirement.

Above 4:12 is a steep roof, and a professional job

OSHA defines a steep roof as steeper than 4:12 (29 CFR 1926.500(b)) and requires fall protection above 6 ft; California requires it above 7:12 at any height. A low pitch is not the easier job: below 2:12 the shingles have to change, and the fall protection rules apply at every pitch. Whichever slope you have, this is work for a roofing professional.

Pitch, angle, and multiplier

Pitch Angle Multiplier
2/12 9.5° 1.014
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
11/12 42.5° 1.357
12/12 45.0° 1.414

The multiplier is √(1 + (rise ÷ 12)²), computed here, so it is exact for a pitch that is not in the table. Bundles are 3 per square on standard and architectural shingles but 4–5 on premium and designer lines, and the waste percentage covers shingles only. Starter, cap, underlayment, valley metal, and flashing are separate line items.

Measuring the footprint first? The square footage calculator works out the plan area of any shape and hands it straight to this page.

Where these numbers come from

  • GAF's own Timberline HDZ product specification (Bundles/Square: 3; 33.3 ft² per bundle, ~63 pieces per square). Owens Corning Duration and CertainTeed Landmark are specified the same way.
  • IRC R905.2.2 (2:12 minimum for asphalt shingles), IRC R905.1.1 and Table R905.1.1(2) (double underlayment at 2:12–4:12), read via municipal code bulletins (Long Beach IB-014, Des Moines, Firestone).
  • OSHA 29 CFR 1926.500(b) and 1926.501(b)(10) to (13): steep roof defined as greater than 4:12, with fall protection above 6 ft. California Title 8 §1731 requires it above 7:12 at any height.
  • 2026 roofing cost guides (secondary; they disagree widely, so this page shows ranges). Bundle prices, not per-square, because bundles are what the supply house actually sells in.
  • BLS PPI series WPU1361 (prepared asphalt and tar roofing and siding products): +5.5% from 2023 to 2026 so far, the calmest material inflation of the ten calculators.
  • GAF's estimating language ("add up to 10% overage") plus the conventional 10/15/20% split by roof complexity. Note the waste percentage never covers accessories.