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Roof Pitch Calculator

Rise and run to roof pitch, angle in degrees, slope percent, and rafter length.

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Roof pitch

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A 4/12 pitch is the walkable line

Below roughly 4/12, most roofers can walk a roof safely without ropes; above it, expect a steeper labor rate and mandatory fall protection on most jobs. This is also the rough dividing line building codes use for shingle underlayment requirements.

Roof pitch is traditionally expressed as rise over a 12 in run — a "6/12" roof rises 6 in for every 12 in it runs horizontally. Divide your actual rise by your actual run, then multiply by 12 to express it the standard way. The same numbers give you the angle in degrees (arctangent of rise over run) and the slope as a percentage.

Rafter length uses half the building's span as the horizontal run for a common rafter, then adds any overhang before applying the Pythagorean theorem with the pitch. This gives you the rafter's total length along its top edge; actual lumber length should add a bit for the ridge and tail cuts.

For a shed roof with only one slope, run isn't halved — use the building's full span as the run instead.

Where these numbers come from

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

What's the difference between pitch and slope?

Pitch (X/12) and slope (as a percent) measure the same steepness, just in different units. A 6/12 pitch is a 50% slope. Angle in degrees is the same measurement again, just expressed the way a speed square or protractor reads it.

How do I measure rise and run on an existing roof?

Hold a level horizontally against the underside of the rafter for exactly 12 in, then measure straight down to the rafter at that 12 in mark — that vertical measurement is your rise per 12 in of run.

Does this account for hip and valley rafters?

No, this covers common rafters running straight up the slope. Hip and valley rafters run at 45 degrees to the common rafters and need a different length multiplier; a framing square or rafter table handles that separately.

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