How this calculation works
Measure the roof’s horizontal footprint, including overhangs if required, and use the uniform pitch to calculate the slope factor. Apply the entered extra-area allowance once. Divide by the product’s stated coverage per bundle, then round the final bundle count upward. A roofing square means 100 square feet, not one bundle or one physical square shingle.
Sloped area = plan length × plan width × √(1 + (rise/12)²); bundles = ceil(order area / bundle coverage)A worked example
A 50 by 25 ft footprint at 6:12 pitch has about 1,397.542486 ft² of sloped area. A 10% allowance gives about 1,537.296735 ft², or 15.37296735 roofing squares. At 100/3 ft² per bundle, the purchase estimate is 47 bundles.
Good to know
One rectangular plan and one uniform pitch. Do not count the entire horizontal footprint twice for two slopes. Product coverage, waste and overhangs must be entered deliberately. No valley/dormer layout, structural design, fasteners, underlayment or installation approval.
Reference: NIST: conversion factors and unit definitions (opens in a new tab) ↗
A footprint and a sloping surface are different areas
A 6:12 pitch means six units of rise over twelve horizontal units, not a six-degree angle. The slope multiplier comes from a right triangle: the sloped side divided by the horizontal run is the square root of one plus the squared rise/run ratio. The roof pitch calculator explains that geometry independently of material quantities.
A gable roof has two sides, but the complete horizontal footprint already includes the horizontal runs below both sides. Multiplying the whole footprint by the slope factor and then doubling again overcounts that simple roof. Overhangs must be represented deliberately in the input dimensions; the tool does not guess them from a building’s wall dimensions.
Coverage is a product assumption, not a unit definition
A roofing square is exactly a 100-square-foot area measure. A bundle is a package whose coverage depends on the product. Three bundles per square is a common example, not an identity that holds for every shingle. Enter the actual coverage stated for the chosen material before relying on a purchase count.
Apply the area allowance before dividing by coverage and round only the final package count upward. The tile quantity calculator uses the same final-rounding idea for a different covering material, but a roof’s valleys, dormers and cutting pattern need additional work. A precise arithmetic result does not supply a roof layout, structural design or complete bill of materials.
A few questions, answered
Does 6:12 mean six degrees?
No. It is a ratio of rise to horizontal run.
Should I double the full footprint for a gable roof?
No. A full rectangular footprint already covers both horizontal halves of the simple uniform-pitch roof.
Is every bundle one third of a roofing square?
No. Bundle coverage is product-specific.