Footprint first, then pitch
Measure or take from drawings the building length along the ridge and the width from eave to eave. That rectangle is the footprint, not the shingle area.
Pitch is written as rise over a 12-inch run. A 6/12 roof rises 6 inches for every 12 inches horizontally. The sloped area is footprint × √(1 + (rise/12)²). The Roofing Calculator applies that factor for you.
Squares, bundles, and packs
In the US, a square is 100 square feet of roof. Asphalt shingles are commonly three bundles per square. In the UK, tiles and felt are sold by coverage per pack — convert the sloped area to square metres and divide by the pack coverage on the product sheet.
Ridge, hips, starter course, and valleys are extra. A simple gable with 10% waste is a planning default. Hips, dormers, and lots of penetrations often need 15% or more.
What this estimate leaves out
Underlayment, ice-and-water shield, flashing, ventilation, and labour are separate. Tear-off of the old roof is a labour and disposal line, not a shingle count.
If the roof is not a simple rectangle — L-shapes, multiple ridges — split it into rectangles, run the calculator for each, and add the results. When the geometry is messy, a roofer's takeoff from drawings is the right next step.
Using the number in a budget
Put the material estimate into the Project Cost Estimator alongside labour if you have a quote. Keep the total labelled as an estimate. Roofing is a safety-critical trade; confirm quantities and specification with a qualified roofer before you order.
Use a calculator for this
Frequently asked questions
Can I estimate pitch from the street?
Roughly, yes, but it is easy to misread. If you cannot see the rise clearly, use a conservative (steeper) pitch for planning or wait for a measured pitch from a roofer.
Do architectural shingles use the same bundle count?
Many still pack three bundles per square, but always check the wrapper. Some products differ. The calculator's bundle count is a typical asphalt assumption, not a universal rule.