Roof pitch calculator

Common questions

How can I calculate my roof pitch?

Measure how far the roof rises over a fixed horizontal run, then divide. Hold a level out horizontally from the roof surface, measure 12 inches along it, and measure straight down to the roof — that drop is the rise. A 4-inch drop over 12 inches is a 4:12 pitch. In formula form, pitch = rise ÷ run, usually written as the rise per 12 of run.

What is a 4/12 roof pitch formula?

A 4/12 pitch means 4 units of rise for every 12 units of run. From that: the angle is arctan(4 ÷ 12) = 18.4°, the grade is 4 ÷ 12 = 33.3%, and the slope factor is √(4² + 12²) ÷ 12 ≈ 1.054, so a rafter covering 12 ft of run is about 12.6 ft long.

What does a 4/12 pitch roof look like?

A 4/12 roof rises 4 inches for every foot of horizontal run, an angle of about 18.4°. It reads as a clear, gentle slope — common on houses, still walkable for most people — steeper than a low-slope porch but far shallower than a steep gable.

What is the golden rule for roof pitch?

As a general rule of thumb, the lower the pitch the more the roof relies on its covering and underlayment to stay watertight, so steeper sheds water and snow better. Most asphalt-shingle roofs are 4:12 or steeper, and very low slopes need special membranes. The actual minimum slope for a given material is set by the manufacturer and your local building code — confirm those for your project.

Does this work as a roof slope or rafter calculator?

Yes. Pitch and slope are the same measurement (rise over run), so this doubles as a roof slope calculator — it shows the slope as a ratio, an angle in degrees and a grade percentage. It also returns the rafter length for your rise and run, so it works as a rafter and rafter-length calculator too.

Want the full walk-through? Read the roof pitch guide →