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Trowelly

Roofing Calculator

Enter your house footprint, overhang and roof pitch to estimate roof area, roofing squares and bundles of shingles.

Units
ft
ft
in

A 4/12 roof rises 4 in for every 12 in across. Most homes are 4/12 to 9/12.

%

Use 10% for simple gable roofs, 15% for hips and valleys.

Most asphalt shingles are 3 bundles per 100 ft².

Add prices for a cost estimate (optional)
per bundle

Results

Bundles of shingles
50 bundles
Roofing squares (incl. extra)
16.53 squares
Roof surface area
1,503 ft²
Pitch multiplier
1.118 ×
  • For a roof with one pitch, the surface area is the footprint (with overhangs) times the pitch multiplier, whatever its shape.
  • Dormers, mixed pitches and steep roofs need a professional measurement.

Where to buy

Quantities from your result, with links to search each store.

Your ZIP code stays on this device. We use it to show stores and suppliers in your area.

You may also need

Links search each store’s website. Check prices and stock before you go.

How to use the roofing calculator

  1. Enter the House length (footprint) and House width (footprint).
  2. Enter the Eave overhang in inches.
  3. Choose the Roof pitch (rise per 12 in), for example 6/12.
  4. Set the Extra for cuts, ridges & starter (10% for simple gables, 15% for hips and valleys) and Bundles per square.
  5. Optionally enter a price per bundle to estimate cost.

How it works

The footprint including overhangs is multiplied by a pitch multiplier to get the sloped roof area. For a roof with a single pitch, this works whatever the roof’s shape.

A roofing square is 100 ft². Most asphalt shingles come 3 bundles per square, the default. Dormers, mixed pitches and steep roofs need a professional measurement.

Pitch multiplier = √(1 + (rise ÷ 12)²)

Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch multiplier

Squares = roof area × (1 + extra %) ÷ 100

Bundles = squares × bundles per square, rounded up

Worked example

With the default inputs, a 40 × 30 ft house with 12 in overhangs has a 42 × 32 ft footprint, or 1,344 ft². At a 6/12 pitch the multiplier is 1.118, giving about 1,503 ft² of roof.

Adding 10% gives 16.53 squares. At 3 bundles per square that is 49.6, so buy 50 bundles.

Tips before you buy

  • Underlayment, drip edge, flashing, ridge vents and nails are not included; estimate them separately.
  • Check the shingle wrapper for bundles per square, since some heavier shingles use 4.
  • Working on a roof is dangerous. Use fall protection, or have a professional measure steep or complex roofs.
  • Reroofing usually needs a permit; check your local building code and any limits on layering over old shingles.

Frequently asked questions

What is a roofing square?

A roofing square is 100 ft² of roof surface. Shingles, underlayment and labor are often priced per square.

How many bundles of shingles are in a square?

Most asphalt shingles come 3 bundles per square, but some heavier styles use 4. Check the wrapper.

How does roof pitch affect roof area?

A steeper roof has more surface than its footprint. The multiplier is √(1 + (rise ÷ 12)²): 1.054 for a 4/12 roof and 1.118 for a 6/12 roof.

How many bundles of shingles do I need for a 40×30 house?

With 12 in overhangs, a 6/12 pitch and 10% extra, the calculator gives 16.53 squares, or 50 bundles at 3 per square.

Does the waste allowance cover ridge caps and starter strips?

The extra allowance is meant to cover cuts, ridges and starter courses. Many roofers buy dedicated ridge cap and starter shingles, so check your shingle maker’s instructions.

Results are estimates based on the measurements you enter and typical material properties. Products vary, so check quantities with your supplier and follow product labels and your local building codes. Read the full disclaimer. Last reviewed October 9, 2026.