FixerCalc

Roof Area Calculator

A 40×30 house does not have a 1,200 sq ft roof. Pitch stretches the surface. This tool applies the standard multiplier used in roofing takeoffs for simple planes.

Last reviewed 2026-08-25. How we calculate

Results update as you type. Units stay imperial inside the math.

Footprint
Pitch

Use 2 if you have two same-size detached roofs (house + garage) with the same pitch.

Leave 0 for net area. Add 10–15% when you will order shingles from this number.

Results

  • Roof surface area

    1,342 sq ft

  • Area with waste

    1,342 sq ft

  • Pitch factor

    1.118

  • Plan / footprint area

    1,200 sq ft

  • Squares (with waste if set)

    13.4

  • This models a simple rectangular footprint with one pitch. Dormers, hips, valleys, and turrets add area this rectangle misses.
  • Measure eaves in the footprint. Living-area square footage is the wrong starting number.

How this calculation works

  1. Plan area is length × width of the roof as seen from above, including overhangs.
  2. The pitch factor is the hypotenuse of the rise/run right triangle: √(1 + (rise/run)²). A 6:12 roof uses rise 6 and run 12, factor ≈ 1.118.
  3. Optional copies let you add a garage with the same geometry. Waste is optional and is for ordering, not for describing geometry.

area = length × width × √(1 + (rise/run)²) × copies × (1 + waste%)

What to do next

  • If the roof is hip or has valleys, add waste or split the roof into rectangles and add the results.
  • Pass this area into the shingle calculator to convert to squares and bundles.
  • Walk the attic or use a pitch gauge; a wrong pitch is a systematically wrong order.

FAQ

Does a gable roof need to be doubled?
No. Length × width is the full plan rectangle covering both slopes. The pitch factor converts that plan into surface area for both sides together.
What about a hip roof?
Plan area × pitch factor is still a decent first pass if the pitch is constant. Hips and valleys consume extra shingles; use a waste percentage rather than pretending the geometry is a perfect rectangle.

Sources

  • Roof surface ≈ plan area × √(1 + (rise/run)²) for a single regular plane or simple gable

Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.