Roofing Calculator: Squares and Shingle Bundles

This roofing calculator estimates how many roofing squares and shingle bundles you need for a simple roof, using the house footprint, the roof pitch, a waste allowance and the number of bundles per square. It also gives a rough count of underlayment rolls, so you can plan a reroof budget or check a contractor’s material list before you sign.

Roofing Calculator

Estimates only. Check product labels and local conditions before buying.

How to Use the Roofing Calculator

  1. Measure the length and width of the house in feet. Measuring the outside walls at ground level is the easiest way to get the footprint.
  2. Choose the roof pitch. Pitch is the rise in inches for every 12 inches of horizontal run, written as 6/12, 8/12 and so on. If you are unsure, see the tips below for ways to find it.
  3. Pick a waste allowance: 10% for a plain gable roof, or 15% for a hip roof with more cuts along the hips and valleys.
  4. Choose bundles per square. Most standard architectural shingles come 3 bundles to a square. Some heavier products come 4 to a square, so check the wrapper.
  5. Enter the coverage per roll of underlayment from the product label, or leave the default of 1,000 square feet.
  6. Read the bundle count, squares, roof area and underlayment rolls in the results.

How the Calculation Works

The calculator starts with the footprint, which is length times width. A sloped roof has more surface than the flat footprint under it, so the footprint is multiplied by a pitch multiplier. The multiplier is the square root of 1 plus (pitch / 12) squared. For a 6/12 roof that works out to about 1.118, meaning the roof surface is roughly 11.8% larger than the footprint.

The waste allowance is then added to the roof area to cover cut shingles, starter rows and ridge caps. The total is divided by 100 to get roofing squares, since one square covers 100 square feet. Squares are multiplied by bundles per square and rounded up to a whole bundle. Underlayment rolls are the roof area (before waste) divided by the coverage per roll, rounded up.

Worked example with the default values: a 40 x 30 foot house with a 6/12 pitch, 10% waste, 3 bundles per square and 1,000 square foot underlayment rolls.

  • Footprint: 40 x 30 = 1,200 square feet
  • Pitch multiplier: square root of (1 + (6 / 12) x (6 / 12)) = square root of 1.25 = 1.118
  • Roof surface area: 1,200 x 1.118 = 1,342 square feet
  • With 10% waste: 1,342 x 1.10 = 1,476 square feet
  • Squares: 1,476 / 100 = 14.8 squares
  • Bundles: 14.76 x 3 = 44.3, rounded up to 45 bundles
  • Underlayment: 1,342 / 1,000 = 1.34, rounded up to 2 rolls

If you switch to the 15% hip allowance, the total rises to about 15.4 squares and 47 bundles.

Roof Pitch Multipliers

These are the multipliers the calculator uses for each pitch option. You can also use them to check any roof area figure by hand: footprint times multiplier equals roof surface area.

Pitch Multiplier Extra area over footprint Common description
2/12 1.014 About 1% Low slope, often needs special underlayment
3/12 1.031 About 3% Low slope
4/12 1.054 About 5% Common on ranch homes
5/12 1.083 About 8% Moderate
6/12 1.118 About 12% Moderate, very common
7/12 1.158 About 16% Moderate to steep
8/12 1.202 About 20% Steep, harder to walk
9/12 1.250 About 25% Steep
10/12 1.302 About 30% Steep, usually needs roof jacks
12/12 1.414 About 41% Very steep, 45 degrees

Tips and Common Mistakes

  • Add the overhangs. The footprint of the walls does not include eaves and rake overhangs. Add twice the overhang to each dimension, so a 1 foot overhang turns a 40 x 30 house into 42 x 32.
  • Dormers, porches and garages are separate. The calculator assumes one simple roof over one rectangle. Measure attached garages, porch roofs and dormers on their own and add their squares.
  • Find the pitch safely. You can often read it from the attic by measuring how much a rafter rises over 12 inches of level run. Phone level apps held against a gable rake from a ladder also work. Avoid walking the roof just to measure.
  • Ridge cap and starter strip are extra. Many shingle lines use separate ridge cap and starter products. The waste allowance helps, but count ridge and eave lengths in linear feet and check the coverage on those packages.
  • Underlayment needs overlap. The roll count uses the plain roof area with no allowance for side laps and end laps. If your rolls are close to the limit, buy one more.
  • Low slopes have different rules. Many shingle makers limit standard installation below 4/12 and require extra underlayment at 2/12 to 4/12. Check the instructions on the wrapper.

Frequently Asked Questions

What is a roofing square?
A square is 100 square feet of roof surface. Roofers price materials and labor by the square, so a 1,500 square foot roof is called a 15 square roof.

How many bundles of shingles cover a square?
Standard architectural and three tab shingles are usually packed 3 bundles to a square. Some heavier designer shingles are 4 or even 5 bundles per square. The coverage is printed on each bundle.

Should I use 10% or 15% waste?
Use about 10% for a straightforward gable roof with two flat planes. Use about 15% for hip roofs, roofs with valleys, or roofs with many penetrations, since each cut edge throws away more shingle.

Can I measure the roof from the ground?
Yes, for a rough estimate. Measure the house footprint, add the overhangs, and apply the pitch multiplier. Satellite measurement reports and a contractor’s site visit give more accurate numbers for complex roofs.

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