QuikCalcTools
Roof geometry · Step 2

Roof Square Footage Calculator

Calculate sloped roof square footage from building length, width, overhangs and roof pitch. See plan area, pitch multiplier and roofing squares.

  • Footprint plus overhangs
  • Saved roof pitch
  • Sloped surface area

Step 2 · Surface area

Calculate sloped roof area

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Your result

Sloped roof area

1,502.64 sq ft

Plan area multiplied by the pitch factor

Roofing squares

15.026

Before adding a waste allowance

Plan area

1,344 sq ft

42 ft × 32 ft

Pitch multiplier

1.118×

6/12 pitch

This footprint method assumes all roof planes use the same pitch and fully cover the rectangular plan area. Measure separate sections for additions, dormers, or mixed pitches.

Convert this area to roofing squares

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Formula, intermediate values and rounding rules are visible.

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Inputs, units and limitations stay visible so you can verify each estimate.

How roof square footage is calculated

A tape measure on the ground gives a horizontal plan area, not the larger sloped surface area. For a simple rectangular roof with one uniform pitch, first add the overhangs to the footprint and then apply the pitch multiplier.

This method works for a simple gable or hip roof whose planes cover one rectangular footprint at the same pitch. Break additions, dormers, intersecting roofs and mixed-pitch sections into separate measurements instead of forcing them into one rectangle.

Plan area

(length + 2 × end overhang) × (width + 2 × eave overhang)

The horizontal area covered by the roof.

Pitch multiplier

√(1 + (rise ÷ 12)²)

Converts horizontal plan area to sloped area.

Roof area

plan area × pitch multiplier

Estimated roof surface area in square feet.

How to use this calculator

  1. 1Measure the building footprint at eave level.
  2. 2Measure the roof overhang on each side.
  3. 3Enter the pitch as inches of rise per 12 inches of run.
  4. 4For a complex roof, repeat the calculation by section and add the results.

Common mistakes

  • • Using interior floor area instead of the exterior roof footprint.
  • • Leaving out overhangs or adding a per-side overhang only once.
  • • Applying one pitch to roof sections that have different slopes.
  • • Adding a waste percentage here and then adding it again in the material estimate.

Use this calculator when

  • You know the exterior footprint and a uniform roof pitch.
  • You need a planning area before converting to squares or packages.

Use another method when

  • You already measured the sloped surface of every plane.
  • The roof has sections with different pitches that have not been separated.

Worked example

40 ft × 30 ft building at 6/12

With no overhangs, the plan area is 1,200 sq ft. A 6/12 pitch has a 1.1180 multiplier.

1,200 × 1.1180 = 1,341.6 sq ft of roof surface

Frequently asked questions