Roofing Calculator (Roof Squares)
Enter your house footprint and roof pitch to see the true sloped roof area, how many roofing squares and shingle bundles to order, and the materials around the edges.
Pitch multiplier table
| Pitch | Angle | Multiplier | Your roof area |
|---|
How to use the Roofing Calculator (Roof Squares)
- Choose the roof style: gable, hip, shed or flat.
- Enter the footprint (length, width and overhang) or the footprint area if you already know it.
- Select the pitch in inches of rise per 12 inches of run, using the menu or slider.
- Adjust waste and materials: bundles per square, underlayment type and an optional price per square.
- Read the results: squares, bundles, underlayment rolls, drip edge and ridge cap. Press Copy result to save the estimate.
Roof pitch multiplier chart
The pitch multiplier converts a flat footprint area into the actual sloped roof area: multiplier = √(1 + (rise ÷ 12)²). Multiply the footprint (including overhangs) by it. The same multiplier applies to gable, hip and shed roofs as long as every plane has the same pitch.
| Pitch | Angle | Multiplier | Roof area per 100 ft² of footprint |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 101 ft² |
| 3/12 | 14.0° | 1.031 | 103 ft² |
| 4/12 | 18.4° | 1.054 | 105 ft² |
| 5/12 | 22.6° | 1.083 | 108 ft² |
| 6/12 | 26.6° | 1.118 | 112 ft² |
| 7/12 | 30.3° | 1.158 | 116 ft² |
| 8/12 | 33.7° | 1.202 | 120 ft² |
| 9/12 | 36.9° | 1.250 | 125 ft² |
| 10/12 | 39.8° | 1.302 | 130 ft² |
| 12/12 | 45.0° | 1.414 | 141 ft² |
| 14/12 | 49.4° | 1.537 | 154 ft² |
| 16/12 | 53.1° | 1.667 | 167 ft² |
| 18/12 | 56.3° | 1.803 | 180 ft² |
| 24/12 | 63.4° | 2.236 | 224 ft² |
Worked example
A 40 × 30 ft house with a 12-inch overhang on every side has a roof footprint of 42 × 32 = 1,344 ft². With a 6/12 gable roof the multiplier is 1.118, so the roof area is 1,344 × 1.118 = 1,502.6 ft². Adding 10% waste gives 1,652.9 ft², or 16.53 squares. At 3 bundles per square you need 16.53 × 3 = 49.6, so buy 50 bundles. The drip edge runs along both eaves (2 × 42 ft) and the four rakes (2 × 32 × 1.118), about 155.6 ft, and the ridge is 42 ft long.
Linear materials and accessories
- Drip edge goes along the eaves (under the underlayment) and the rakes (over it). Sold in 10 ft lengths; allow for 2-inch overlaps.
- Ridge cap covers the ridge and, on hip roofs, every hip. A hip on a rectangular roof is (span ÷ 2) × √(2 + (pitch ÷ 12)²) long. Pre-made ridge cap bundles typically cover about 20–33 linear ft.
- Starter strip runs along every eave (and often the rakes): use the eave length from the drip edge figure.
- Ice and water shield is required in many cold climates along eaves (usually at least 24 inches inside the warm wall) and in valleys.
- Underlayment: synthetic rolls commonly cover 10 squares, #15 felt about 4 squares and #30 felt about 2 squares.
Low-slope roofs
Asphalt shingles are normally installed only on roofs of 2/12 pitch and above, and roofs between 2/12 and 4/12 need a double layer of underlayment under most manufacturers' instructions. Flatter roofs need a membrane system such as EPDM, TPO or modified bitumen, which is also sold and priced by the square.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet (about 9.29 m²) of roof surface. Shingles, underlayment and labor are usually priced per square, so a 2,000 ft² roof is 20 squares.
How many bundles of shingles are in a square?
Most 3-tab and standard architectural shingles come 3 bundles to a square, each bundle covering about 33.3 ft². Some heavy designer shingles need 4 or 5 bundles per square, so check the wrapper.
How do I find my roof pitch?
Pitch is the rise in inches over 12 inches of horizontal run. Hold a level horizontally against the roof (or a rafter in the attic), measure 12 inches along it and then measure straight up to the roof surface. A 6-inch rise is a 6/12 pitch.
Why is the roof area bigger than the house footprint?
A sloped roof is longer than the flat distance it spans. The pitch multiplier, √(1 + (pitch/12)²), converts the horizontal footprint into the real sloped area. A 6/12 roof is about 11.8% larger than its footprint and a 12/12 roof about 41.4% larger.
How much waste should I add for a roof?
About 10% is typical for a simple gable roof and 15% or more for hip roofs, which have more cuts along the hips. Add more for roofs with many valleys, dormers or skylights.
Does this work for complex roofs?
For roofs with several sections, valleys or dormers, measure each rectangular section separately and add the squares together, or use the footprint area mode with the total area including overhangs.