Roof Pitch Calculator

Convert rise and run into an X:12 pitch, read the slope in degrees, and get rafter length per foot of run. Angles convert back to pitch too. Everything runs in your browser.

From rise and run
Rise is how far the roof climbs; run is how far it goes across. Leave run at 12 and the pitch reads X:12 directly.

Enter a rise to read the pitch.

From angle
Convert a slope angle in degrees back to a rise per 12 pitch.

Enter an angle to read the pitch.

How to use

  1. Measure the rise: hold a level against the rafter or roof deck, 12 inches along it, and measure straight down from the level's end to the roof. That distance is the rise over a 12 inch run.
  2. Enter the rise and the run. With the run left at 12, the pitch reads X:12 directly; a longer run converts down to the per-12 number.
  3. Read the pitch, the slope angle in degrees, and the rafter length per foot of run.
  4. Use the angle card to go the other way: a slope in degrees from a set of plans or a pitch gauge becomes its X:12 pitch.

How the math works

Pitch scales your rise to a 12 inch run: rise divided by run, times 12. The angle is the arctangent of rise over run, and the conversion runs both directions, from degrees back to pitch through the tangent. Rafter length per foot of run is the slope's hypotenuse, the square root of rise squared plus run squared, scaled to one horizontal foot. Inputs are inches and results are rounded to two decimals; a flat 0:12 roof is a valid input.

The rafter figure is the top edge of a plumb-cut rafter on a simple gable. It leaves out overhangs, seat cuts, ridge thickness, and the extra length hips and valleys demand.

Roof framing is cut-to-fit work. Take a few percent extra rafter stock, about 10 percent on complex roofs, for test cuts and the board the wind gets.

Frequently asked questions

Is anything I enter uploaded or stored?

No. The calculator runs entirely in your browser tab, and your numbers never leave the page. Nothing is stored, nothing is sent to a server, and the page sets no cookies. Ad slots on the page are served by third-party networks, which follow their own privacy policies.

What does a pitch like 6:12 actually mean?

The roof climbs 6 inches for every 12 inches of horizontal run. Pitch is written as rise per 12, so 6:12 and 4.5:12 are both ordinary slopes, and 12:12 is a 45 degree roof. Flat roofs still get a small pitch such as 0.25:12 so water drains.

How do I measure pitch on an existing roof?

From inside the attic, hold a 12 inch level against the underside of a rafter, bubble centered, and measure straight down from the level's far end to the rafter. That measurement in inches is the first number of the pitch. From outside, the same trick works on the roof deck with the level held along the slope, though loose shingles make the attic the more honest measurement.

What is the rafter length per foot of run used for?

It is the hypotenuse of the slope triangle, the length of rafter material that spans one horizontal foot at your pitch. A 6:12 roof carries about 13.42 inches of rafter per foot of run, so a shed roof running 10 horizontal feet needs about 134 inches of rafter before you add any overhang. Span tables and hips and valleys change the real cut list, so treat this as the starting point.