Roof Pitch Calculator

You enter how much the roof rises over a set run, usually 12 inches, and it turns that into the four ways roofers describe a slope. It gives the pitch as a number in 12, the angle in degrees, the slope factor that stretches a flat distance into a sloped one, and the grade as a percent. Add a horizontal run and it returns the rafter length too. It all runs in your browser, so you'll see every figure update as you type.

Roof pitch is how steep the roof is, written as the rise over a 12-inch run, like 6/12, which means the roof climbs 6 inches for every 12 inches across. A low slope sits under 4/12, a common residential range is 4/12 to 9/12, and anything past 9/12 is steep. Builders favor the in-12 form because it is easy to mark on a framing square, and this tool converts it to degrees and a slope factor for you.

Take the rise divided by the run and find its arctangent, which is the angle. A 6/12 pitch is 6 divided by 12, or 0.5, and the arctangent of 0.5 is about 26.6 degrees. A 12/12 pitch is a perfect 45 degrees, since the rise equals the run. You don't have to reach for a trig table, because the calculator does the arctangent step and shows the angle alongside the pitch, so it's there at a glance.

The slope factor is the number you multiply a flat, horizontal distance by to get the true length along the slope, and it equals the rafter length over the run. For a 6/12 roof it is about 1.118, so 10 feet of horizontal run becomes 11.18 feet of roof surface. It matters because materials like shingles and underlayment cover the sloped area, not the footprint, so you'll want to order by the larger sloped figure, and that's why ignoring it leaves you short.

A rafter is the hypotenuse of the rise-and-run triangle, so its length is the run multiplied by the slope factor, before any overhang. Enter your horizontal run and the calculator returns the rafter length for that span, which is the straight-line distance from the wall to the ridge. Add a little for the eave overhang and the plumb cut, since the figure here is the bare run-to-ridge length.

Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. Type a rise and run, read the pitch, angle, slope factor, and grade, and add a span for the rafter length. Bookmark it for roofing estimates, framing, solar layout, or a permit drawing, and you'll have an accurate slope breakdown whenever you need it.