Roof Pitch Calculator

What is a Roof Pitch?

A Roof's pitch is the slope created by the rafter. It can be measured in two ways
1) As the angle the rafter makes with the horizontal
2) The proportion between the rise and the run of the roof.

Roof pitch is usually expressed as a ratio between rise and run in the form of rise / 12. For example, a pitch of 1:12 means that for every twelve yards of building length, the rise will be equal to one yard.

Types of Roof Pitch

Roofs are classified by their pitch—or steepness. Here's a breakdown of the most common categories:

Flat Roofs
These aren't completely flat—they have a slight slope to allow water to drain off. Flat roofs typically range from 0.5:12 to 2:12 in pitch (about 4%–17%).

Low-Pitched Roofs
Any roof with a pitch under 4:12 (or 33%) is considered low-pitched. These require special waterproof materials and are harder to maintain.

Standard/Moderate Pitched Roofs
The most common roof pitch falls between 4:12 and 9:12 (33%–75%). These are easy to build, safe to walk on, and work well with most roofing materials.

Steep or High-Pitched Roofs
Roofs with a pitch greater than 9:12 are considered steep. Some go as high as 21:12 (about 175%). These roofs need extra fasteners and safety precautions during installation.

How to Calculate Roof Pitch

You can figure out your roof pitch using basic math from right triangles. It only takes a couple of measurements—rise and run—to calculate everything you need.

Use these formulas to get started:

rafter² = rise² + run²   
pitch = rise / run   
pitch = tan(angle)   

If you want to convert pitch into the standard x:12 format, just multiply your slope (rise/run) by 12. For example, a pitch of 0.25 turns into 3:12.

You can enter any of these values—rise, run, pitch percentage, or angle—into the calculator, and it will fill in the rest for you automatically.

Roof Slope Calculator – Example of Use

Before jumping into your roof pitch calculations, it's a good idea to have an angle converter on hand in case you need to switch between degrees and radians. Once that's ready, here's how to calculate your roof pitch step-by-step:

First, measure the run—this is the horizontal distance from the outer wall to the center ridge of your roof. Let's say the run is 6 meters.

Next, measure the rise—this is how tall the roof rises vertically over that same span. In this case, we'll use 1.5 meters.

To figure out the length of the rafter, plug these numbers into the Pythagorean theorem:

Step 1

rafter² = rise² + run²
1.5² + 6² = 2.25 + 36 = 38.25
rafter = √38.25 = 6.18 meters

Now calculate the roof slope by dividing rise by run:

Step 2

pitch = rise / run = 1.5 / 6 = 0.25 or 25%

To convert that slope into an angle, take the arctangent of the pitch:

Step 3

angle = arctan(0.25) = 14°

Finally, to express the pitch as a ratio (x:12), multiply the slope by 12:

Step 4

x = 0.25 × 12 = 3
So the roof pitch is 3:12. That's the same as 25% or 14 degrees.

If you've figured out your pitch and you're ready to move on, you may want to check out our Roof Shingle Calculator to estimate how much material you'll need.