Roofing Calculator

How to use this calculator

Measure the footprint of the roof from the ground or from plans: the building’s length and width plus the overhangs on each side. Enter the plan area and the roof pitch in inches of rise per 12 inches of run. The pitch multiplier converts the flat footprint into the real, sloped surface area; this works for any roof where all planes have the same pitch, including hips.

The default 10% waste allowance suits a simple gable. Use 15% for hip roofs and 15–20% for roofs with many valleys, dormers or skylights. If you know the roof area already — from a satellite measurement report, say — enter it directly to get squares and bundles. Use the roof pitch calculator if you only know the rise and run.

The formula

k = √(1 + (pitch / 12)²)

A_roof = A_plan × k

SQ = A_roof × (1 + w%) / 100 ft²

B = 3 × SQ

A_plan: roof footprint (plan area incl. overhangs)
x/12: pitch (inches of rise per 12 in of run)
k: pitch multiplier
A_roof: roof surface area
w%: waste allowance (hips, valleys, starter)
SQ: roofing squares (100 ft² each)
B: shingle bundles (3 per square)

Worked example

A house with a 1,500 ft² roof footprint (including overhangs) has a 6/12 gable roof. How many bundles of architectural shingles are needed?

Pitch multiplier for 6/12 = √(1 + (6/12)²) = 1.118.
Roof area = 1,500 × 1.118 = 1,677 ft².
Squares = 1,677 × 1.10 ÷ 100 = 18.4; bundles = 3 × 18.4 = 55.3, so 56 bundles.

Order about 18½ squares — 56 bundles of three-bundle-per-square shingles.

Pitch multipliers

Pitch	Multiplier	Pitch	Multiplier
3/12	1.031	8/12	1.202
4/12	1.054	9/12	1.250
5/12	1.083	10/12	1.302
6/12	1.118	12/12	1.414
7/12	1.158	14/12	1.537

Assumptions and limits

All roof planes share one pitch. For mixed pitches, calculate each section separately.
Three bundles per square is standard for three-tab and most architectural shingles; some heavy designer shingles are four or five bundles per square — check the wrapper.
Asphalt shingles generally need a pitch of at least 2/12, with extra underlayment below 4/12.

Frequently asked questions

What is a roofing square?

100 square feet of roof surface. Shingles, underlayment and labor are usually priced per square.

How many bundles of shingles are in a square?

Usually three for standard architectural and three-tab shingles, each covering about 33.3 ft².

Does a hip roof have more area than a gable?

Not for the same footprint and pitch — the multiplier is the same. Hip roofs do waste more material in cuts, so use a larger allowance.

Roofing Calculator (Squares & Bundles)