Slope of Roof Calculator
Roofers say “slope” where others say “pitch”. Enter the rise and run to get the slope as a percentage, the X/12 pitch, the angle in degrees — plus what your slope means for drainage and roofing choice.
Use the same unit for rise and run — the ratio is what matters.
Planning estimates only — verify against local building codes and roofing-manufacturer requirements before you build.
How the slope of roof calculator works
Slope is the rise divided by the run, expressed as a percentage: a roof that climbs 6 inches over 12 inches of run has a 50% slope. The same ratio converts directly to the familiar X/12 pitch (multiply by 12) and to the angle in degrees (the arctangent).
Slope percentage is the language of drainage and codes. Most building codes demand at least 1/4:12 (≈2%) on “flat” roofs so water reaches the drains; below 2/12, only membrane or standing-seam metal systems are warranted; from 2/12 to 4/12 you are in low-slope shingle territory with upgraded underlayment.
slope % = rise/run × 100 · pitch = rise/run × 12 · angle = atan(rise/run)
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Slope of roof calculator FAQ
What is roof slope?
Roof slope is the steepness expressed as a percentage: rise ÷ run × 100. A 6/12 pitch is a 50% slope; a 12/12 pitch is a 100% slope at 45°.
Is roof slope the same as pitch?
They describe the same geometry in different units. Pitch is rise per 12 inches of run (6/12); slope is the percentage (50%). Convert with: slope % = pitch ÷ 12 × 100.
What is the minimum slope for a flat roof?
Most codes require at least 1/4:12 (≈2% slope) so water drains instead of ponding. Truly flat (0%) roofs pond and fail early — tapered insulation is used to create the fall.
How do you express roof slope as a ratio?
As rise:run in simplest terms. A 50% slope is 1:2 (1 unit of rise per 2 of run); a 100% slope is 1:1. This calculator shows the ratio automatically.
How do I measure my roof’s slope?
Hold a level horizontally against the roof surface, measure 12 inches along it, then measure straight down from the level’s end to the roof — that drop in inches is your rise over a 12-inch run.