Roof Pitch Calculator
Convert rise and run into pitch, angle, slope multiplier and true roof surface area.
Pitch is written as rise over run: a 6/12 roof climbs six inches for every twelve inches it travels horizontally. That ratio is what determines which roofing materials are permitted, how much surface area you actually have to cover, and how much labour the job takes.
The multiplier is the useful number. It converts a flat footprint into roof surface, and it grows faster than most people expect — a 4/12 roof multiplies by 1.05, a 6/12 by 1.12, a 9/12 by 1.25, and a 12/12 by 1.41. On a 1,200 square foot footprint, going from 4/12 to 9/12 adds about 240 square feet of shingles.
How this calculator works
Every field is editable and the result updates as you type — nothing is submitted anywhere, all the maths runs in your browser. That matters for a number you are about to spend money on: you can see exactly which input moved the answer, and by how much.
What this calculation assumes
Assumes a simple gable roof measured over the building footprint. Hips, valleys, dormers, overhangs and any change of pitch add roof surface that a single multiplier cannot capture.
Treat the result as an order-of-magnitude check rather than a final quantity. Product packaging changes, regional labour rates vary by a factor of two, and no calculator has seen your actual site.
Frequently asked questions
What pitch is too steep for standard shingles?
Asphalt shingles need at least a 2/12 pitch, and anything below 4/12 also needs a doubled underlayment. Above 9/12 most crews want roof jacks and extra labour, and above 12/12 many will not walk the roof at all without scaffolding.
How do I measure pitch from the ground?
Hold a level horizontally against the gable end, mark 12 inches along it, then measure vertically down to the roof line at that mark. The vertical measurement in inches is the rise. In a loft, lay a level on the joist and measure up to the rafter the same way.
Does pitch change the number of squares I order?
It changes the roof area, and therefore the squares. Order from the sloped surface figure, not the footprint, then add 10% for hips, valleys and waste. Ordering from the footprint on a steep roof can leave you short by a fifth of the job.
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