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Roof area and material estimate.
The roofing calculator estimates the actual sloped surface area of a roof from its flat footprint and pitch, then converts that area into roofing 'squares' and shingle bundles. Because a pitched roof is larger than the ground it covers, you cannot order materials from the footprint alone.
It gives a quick material estimate for gable-style roofs so you can price shingles, underlayment and labour, which are all commonly quoted per square (100 ft²).
A 6/12 pitch has a multiplier of about 1.118, so it adds roughly 12% to the flat area. Steeper roofs add more.
Pitch is the vertical rise for every 12 units of horizontal run. To measure it, hold a level horizontally against the roof and measure the vertical drop 12 inches out along the level — that drop in inches is your rise, e.g. 6/12.
A sloped surface is longer than its horizontal projection. The pitch multiplier accounts for that: the steeper the roof, the larger the multiplier and the more material you need for the same footprint.
Most asphalt shingles are packaged three bundles to a square (100 ft²). Heavier or specialty shingles can be four or five bundles per square, so check the specific product before ordering.
Yes. Add roughly 10% for a simple gable roof, and more for cut-up roofs with many valleys and hips, where off-cuts and starter/ridge courses increase consumption.