Roofing calculator

How to use the roofing calculator

Estimate roof surface area, roofing squares, shingle bundles, underlayment rolls, and drip edge from a building footprint and roof pitch. It supports Feet / inches and Metres / cm inputs, with waste settings for simple or complex roofs.

1. Choose Feet / inches or Metres / cm.
2. Enter the footprint Length and Width, including overhangs, or enter Or a known area to override the dimensions.
3. Enter Pitch (rise per 12), then choose the roof Complexity.
4. Review the bundle count, or change Bundles per square and Waste % override (optional).
5. Keep Include underlayment selected if needed, then choose Synthetic or Felt underlayment.

What to enter

Feet / inches or Metres / cm

Choose the unit system. Major dimensions use feet or metres. The controls labelled plus inches use inches in imperial mode and centimetres in metric mode.

Length

Building footprint length, including overhangs. Default: 40 ft in imperial mode or 40 m in metric mode.

plus inches

Adds to Length. Default: 0. Use inches in imperial mode or centimetres in metric mode.

Width

Building footprint width, including overhangs. Default: 30 ft in imperial mode or 30 m in metric mode.

plus inches

Adds to Width. Default: 0. Use inches in imperial mode or centimetres in metric mode.

Or a known area

Optional footprint plan area in sq ft or m². When filled, it overrides Length × Width. The perimeter is then estimated as a square footprint.

Pitch (rise per 12)

Enter the roof rise for every 12 units of run. Default: 6, shown as 6/12. Allowed range: 0 to 24.

Complexity

Choose simple or complex. Simple uses a 10% waste default. Complex uses a 15% waste default.

Bundles per square

Choose 3 or 4 bundles per 100 sq ft square. Default: 3.

Waste % override (optional)

Leave blank to use the Complexity default, or enter 0% to 50% to replace it.

Include underlayment

Default: selected. Clear it if you do not want an underlayment roll estimate.

Underlayment

Choose synthetic, with a tool coverage constant of 400 sq ft per roll, or felt, with 200 sq ft per roll. Default: synthetic.

Example: 40 ft × 30 ft simple roof at 6/12 pitch

Use the default imperial settings except for the example dimensions: Length = 40 ft, Width = 30 ft, Pitch = 6/12, Complexity = simple, Bundles per square = 3, and synthetic underlayment. Footprint: 40 × 30 = 1,200 sq ft. The slope factor is √(1 + (6 ÷ 12)²) = √1.25 = 1.1180339887… Roof surface area: 1,200 × 1.1180339887… = 1,341.640786… sq ft → 1,341.64 sq ft. Roofing squares: 1,341.640786… ÷ 100 = 13.416407… → 13.42 squares. Simple complexity applies 10% waste, so the purchase area is 1,341.640786… × 1.10 = 1,475.804865… sq ft → 1,475.8 sq ft. Exact bundles: (1,475.804865… ÷ 100) × 3 = 44.274145… bundles → 45 bundles after rounding up. Synthetic underlayment: 1,475.804865… ÷ 400 = 3.689512… rolls → 4 rolls. Drip edge from the rectangular footprint: 2 × (40 + 30) = 140 ft. The main results are 13.42 squares, 45 bundles, 4 synthetic rolls, and 140 ft of drip edge.

How the roofing calculator computes results

The tool converts metric inputs to its internal units, then calculates the footprint. With dimensions, footprint area = Length × Width and perimeter = 2 × (Length + Width). If Or a known area is supplied, that area overrides the dimensions and perimeter = 4 × √footprint area, treating the footprint as square for the perimeter estimate. The pitch slope factor is √(1 + (rise ÷ 12)²). Roof surface area = footprint area × slope factor. Roofing squares = roof surface area ÷ 100, because the tool defines 1 square as 100 sq ft. Effective waste is the Waste % override when entered; otherwise it is 10% for simple or 15% for complex. Area to purchase = roof surface area × (1 + waste ÷ 100). Exact bundles = (area to purchase ÷ 100) × Bundles per square, then the tool rounds up to a whole bundle. Underlayment rolls, when included, equal area to purchase ÷ 400 for synthetic or ÷ 200 for felt, rounded up. Drip edge uses the footprint perimeter. Displayed calculated values are generally rounded to two decimals, while the pitch slope factor result is rounded to four decimals. The area calculation uses the unrounded slope factor before display rounding.

Reading the results

Roofing squares show the roof surface area divided by 100, before waste. Shingle bundles include the selected waste and are rounded up to a whole bundle. Area to purchase shows the waste-adjusted area. Underlayment rolls also include waste and are rounded up. Drip edge is shown in feet or metres based on the selected unit system. In metric mode, area results use m² and length results use m.

Limitations and common mistakes

• The estimator uses footprint and pitch. It does not model each roof plane, so hips, valleys, dormers, and other geometry are represented only through the simple or complex waste choice.
• If you enter a known area, the tool estimates the perimeter as a square footprint. For a rectangular footprint, enter Length and Width instead.
• Drip edge is based on the footprint perimeter. The tool notes that rake edges on steep roofs can be slightly longer.
• Starter material and ridge cap are not included. The waste setting is limited to 0% through 50%, and the underlayment coverages are the tool's synthetic and felt defaults.

Frequently asked questions