Roofing Calculator

Roof area from the footprint and pitch, squares, shingle bundles or roof tiles, and underlay rolls, in feet or metres.

Roof area is the plan area of the roof, the footprint seen from above including the overhangs, multiplied by the slope factor for its pitch. That works for any gable or hip roof where every plane has the same pitch. A 40 × 30 ft footprint is 1,200 ft²; at 6/12 the factor is 1.118, so the roof is 1,342 ft², about 14.8 squares with 10% extra. At 3 bundles per square that's 45 bundles of shingles and 2 rolls of 10-square underlayment. In metric, a 12 × 9 m footprint at 22.5° is 116.9 m², or about 1,290 tiles at 10 per m² with 10% extra. Enter the footprint and pitch and the calculator does the rest.

Units
Roof plan
ft
ft
Feet and inches work too: type 12'6 or 12.5.

Measure the building's footprint including the overhangs, as seen from above.

/12
%
Roof area
1,342 ft²

1,200 ft² of plan × 1.118

Squares
14.76

incl. 10% extra

Shingle bundles
45

3 per square

Underlay rolls
2

10 squares each

How it was worked out
  1. Roof plan: 40 ft × 30 ft = 1,200 ft²
  2. Roof area = plan area × slope factor 1.118 (6/12) = 1,342 ft²
  3. Plus 10% for waste, valleys and cuts = 1,476 ft²
  4. 1,476 ft² ÷ 100 = 14.76 squares × 3 bundles = 44.27, rounded up to 45 bundles
  5. Underlay: 1,342 ft² + 10% for laps ÷ 10 squares per roll = 2 rolls

For roofs where every plane has the same pitch, gable or hip, the roof area is the plan area times the slope factor. Allow 10% extra for a simple gable and 15% or more for hips and valleys. Starter strips, ridge caps, flashing and drip edge are measured by length along the eaves, rakes, ridges and hips.

Common use cases

  • Estimate shingles for a re-roof
  • Count roof tiles and membrane for a new roof
  • Work out the roof area for a quote or insurance claim
  • Order underlayment by the roll

How to use

  1. Choose your units.
  2. Enter the roof's footprint including overhangs; add areas for wings and porches.
  3. Enter the pitch (x in 12, or degrees in metric) and the extra for waste.
  4. Check bundles per square or tiles per m² and the underlay roll size, then read the results.

FAQ

What is a square of roofing?

100 ft² of roof surface. Most asphalt shingles are packed 3 bundles to the square, though some heavier designer shingles take more, so check the wrapper.

How much extra should I add for waste?

About 10% for a simple gable roof and 15% or more for hips, valleys, dormers and other cut-up roofs.

Why do I need the pitch?

A sloping roof has more surface than the footprint under it. The slope factor converts one to the other: 1.054 at 4/12, 1.118 at 6/12, 1.202 at 8/12 and 1.414 at 12/12.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. Sizes, spans, depths and reinforcement for anything structural must come from span tables, your building code or an engineer.