Roofing Calculator

Calculate roof area, roofing squares and shingle bundles from the building footprint and roof pitch — with waste factor and optional cost estimate.

Roof details

ft
ft
Enter the full building width — the calculator uses the full footprint.
:12
Common: 4:12 ranch · 6:12 standard · 8:12 steep
%
10 % gable · 15 % hip · 20 % complex cut-up
Standard 3-tab asphalt: 3 bundles. Architectural: 3–4.
$
Material cost per 100 ft² (1 roofing square).

Calculation results

Roofing squares

— squares

Shingle bundles

— bundles

Actual roof area

— ft²

Pitch

— :12

Squares are rounded up to the next whole square. Material cost covers shingles only — add underlayment, flashing, fasteners and labour separately.

Informational only. These results are material estimates. Roof replacement should be performed by a licensed roofing contractor who can assess decking condition, flashing, ventilation and code compliance.

Understanding the formula

The footprint (plan area) must be multiplied by a pitch factor to get the actual sloped surface area. One roofing square = 100 ft².

pitch factor = √(1 + (x/12)²)  ·  roof area = footprint × factor  ·  squares = ⌈ ordered / 100 ⌉

A 6:12 pitch gives a factor of √(1 + 0.25) = 1.118 — meaning the actual roof surface is 11.8 % larger than the floor footprint. A flat roof (0:12) has a factor of exactly 1.

Worked example

A 24 × 40 ft house, 6:12 pitch, 10 % waste, 3 bundles/square:

  • Footprint = 24 × 40 = 960 ft²
  • Roof area = 960 × 1.118 = 1,073 ft²
  • Ordered = 1,073 × 1.10 = 1,180 ft²
  • Squares = ⌈ 1,180 / 100 ⌉ = 12 squares
  • Bundles = ⌈ 11.80 × 3 ⌉ = 36 bundles

Common mistakes & tips

  • Enter the full footprint, not per-slope area. The calculator handles both sides of the gable automatically via the pitch factor.
  • Increase waste for complex roofs. Every valley, hip, dormer and penetration adds waste cuts. For a straightforward gable, 10 % is fine. A hip roof with dormers may need 20 %.
  • Check the bundle count for your shingle. Standard 3-tab asphalt uses 3 bundles per square. Architectural (dimensional) shingles often use 4 per square — verify the product label.
  • Factor in underlayment and starter strip. Roof area for underlayment (15 lb or synthetic) is about the same as shingle area. Starter strip covers the eaves and rakes — measure their total linear footage separately.
  • Don’t forget ridge cap. Ridge cap shingles cover the peak. Measure the total ridge and hip length and order one bundle per 25–33 lin ft (product-dependent).

Frequently asked questions

How many shingles do I need for a 1,500 sq ft roof?
1,500 ft² ÷ 100 = 15 squares. With 10 % waste: 16–17 squares. At 3 bundles per square: 48–51 bundles of standard 3-tab asphalt shingles.
What is a roofing square?
One roofing square equals 100 square feet of roof surface. It is the standard unit roofers use for estimating material quantities. A typical US home has 15–25 squares of roof.
How do I calculate roof area from the ground?
Measure the building footprint (length × width). Multiply by the pitch factor for your roof pitch. For a 6:12 pitch, multiply by 1.118. For a hip roof or complex layout, add 10–20 % extra waste factor.
How long does a shingle roof last?
Standard 3-tab asphalt: 20–25 years. Architectural (dimensional): 30 years. Premium architectural: 40–50 years. Metal: 40–70 years. Lifespan depends heavily on ventilation, installation quality and climate.
What is the pitch factor for a 4:12 roof?
The pitch factor = √(1 + (4/12)²) = √(1 + 0.111) = √1.111 = 1.054. A 4:12 roof surface is about 5 % larger than its footprint.
How much does a new roof cost per square?
Material only: $90–150/square for standard 3-tab asphalt; $150–350/square for architectural; $400–900+/square for metal, slate or tile. Installed cost including labour is typically 2–3× material cost.