Roof Pitch Calculator

What roof pitch means

Roof pitch describes how steep a roof is, written as the rise over a 12-inch run: a 6/12 pitch rises 6 inches for every 12 inches of horizontal run. That single number drives almost everything else in a roofing job — the angle, the rafter length, how much material you need, and whether a given roofing product is even allowed. Asphalt shingles, for example, generally need at least a 2/12 pitch, and low-slope roofs need different underlayment and detailing. Getting the pitch wrong means ordering the wrong amount of material or choosing a product that will leak.

How the numbers work

The angle is the arctangent of rise over run, so a 6/12 pitch is about 26.57 degrees. The slope percentage is simply rise divided by run times 100 — 6/12 is a 50% slope. Rafter length comes from Pythagoras: the square root of rise squared plus run squared. The area multiplier is the rafter length divided by the run, and it is the number estimators use most: multiply the flat footprint of the roof by the multiplier to get the actual sloped area, which is what you order shingles for. A 6/12 roof needs about 12% more material than its footprint suggests; a 12/12 roof needs about 41% more.

How to measure it

The easiest accurate method is from inside the attic: hold a level horizontally from a rafter, measure 12 inches along the level, then measure vertically down to the rafter — that vertical distance is the rise. From outside, a pitch gauge or a smartphone level app on the roof surface gives a usable reading, but be careful on the roof itself. Enter the rise and run here and the calculator returns the pitch, angle, slope and rafter length together.

Frequently asked questions

What is a 6/12 roof pitch in degrees?

A 6/12 pitch is 26.57 degrees. The angle is the arctangent of rise over run: arctan(6/12) = 26.57°. Use the calculator above for any other pitch.

What is the roof area multiplier for?

It converts the flat footprint area of a roof into the actual sloped surface area. Multiply your footprint (length × width) by the multiplier to estimate how much roofing material to order. A 6/12 roof has a multiplier of about 1.118.

What pitch is too low for shingles?

Most asphalt shingle manufacturers require at least a 2/12 pitch, and below 4/12 they usually require special underlayment. Below 2/12 you need a low-slope system such as a membrane roof, not shingles.