Skip to content
ChalkCalculator

Home

Roofing Calculator

Roof area, roofing squares and shingle bundles from your building footprint and roof pitch.

Enter the footprint your roof covers and pick its pitch.

Building footprint

Measure the area the roof covers on the ground, not the sloped surface. The pitch below handles the rest.

Roof and materials

Rise in inches per 12 inches of horizontal run.

10% simple, 15% complex

Optional

Printed on the shingle wrapper.

Estimate. A real-world figure based on the assumptions listed below, which you can change.

How this calculator works

A roof is bigger than the building underneath it, and the difference is the pitch. Pitch is quoted as rise per 12 inches of horizontal run, and the amount by which the sloped surface exceeds the flat footprint is the pitch multiplier — simply the hypotenuse of the slope triangle divided by its base.

A 6/12 roof has a multiplier of 1.118, so it has almost 12% more surface than the ground it covers. On a 2,000 square foot footprint that is 236 extra square feet, or more than two roofing squares of material.

A roofing square is 100 square feet. Three bundles per square is standard for three-tab and most architectural shingles, but heavier designer products come four or five to a square — the figure is on the wrapper, so it is an input here rather than a constant.

Waste covers starter courses, hip and ridge cuts, and valleys. 10% is normal for a simple gable roof; complex roofs with several valleys and dormers commonly run 15%.

The formula

Exactly what happens to your numbers, step by step.

  1. Pitch multiplier

    multiplier = √(rise² + 12²) ÷ 12

    For a 6/12 roof: √(36 + 144) ÷ 12 = 13.416 ÷ 12 = 1.118.

  2. Roof area and squares

    roof area = footprint area × multiplier
squares   = roof area ÷ 100

  3. Bundles to order

    bundles = ⌈squares × (1 + waste %) × bundles per square⌉

A worked example

A 40 × 30 foot house with a 6/12 pitch roof and a 10% waste allowance.

What you enter

Footprint
40 ft × 30 ft = 1,200 sq ft
Pitch
6/12
Waste
10%
Bundles per square
3

The working

Pitch multiplier
1.1180
Roof area
1,200 × 1.1180 = 1,341.6 sq ft
Roofing squares
13.42
With 10% waste
14.76 squares
Bundles
⌈14.76 × 3⌉ = 45

45 bundles, about 15 squares

Ignoring the pitch entirely would give 12 squares and 36 bundles — nine bundles short. That is the difference between finishing the job and a second trip to the supplier.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • The footprint you enter is the actual area the roof covers, including any overhang.
  • The whole roof is at one pitch.
  • Three bundles per square unless you change it.
  • Waste covers cuts and starter courses, not damage or a change of plan.

What this cannot tell you

  • **This is a material estimate, not a roof measurement.** Dormers, varying pitches, complex valleys and overhangs all change the answer, and no calculator can see your roof. Get an on-site measurement before ordering or signing a contract.
  • It does not include ridge cap, drip edge, flashing, vents, nails or the cost of tearing off the old roof.
  • Labour is not estimated, and on most re-roofing jobs labour exceeds materials.

Questions people ask

What is a roofing square?

One hundred square feet of roof area — a 10 by 10 foot patch. Roofing is quoted and sold in squares rather than square feet, which is why the conversion is worth knowing.

How many bundles of shingles are in a square?

Three for most three-tab and architectural shingles. Heavier designer shingles come four or five to a square because a bundle has to stay light enough to carry up a ladder. The wrapper states it.

How do I measure my roof pitch?

Hold a level horizontally against the roof surface, measure 12 inches along it, then measure straight down to the roof. That drop in inches is the rise: a 6-inch drop over 12 inches of run is a 6/12 pitch.

Can I put shingles on a low-slope roof?

Most asphalt shingles are not rated below 2/12, and between 2/12 and 4/12 manufacturers require a doubled underlayment. Check the specific product’s instructions — installing outside them typically voids the warranty.

Related calculators

Sources and review

This calculator uses standard arithmetic with no external rules or published rates, so there is nothing to cite beyond the formulas shown above.

Methodology version 1.0.0 · Last reviewed