Roof Pitch Calculator

Roof pitch calculator for rise/run, degrees, and rafter length

This roof pitch calculator is built for one clear job: turning a simple measurement into a usable roof pitch. Most people measure a roof by checking how much it rises over a known horizontal distance (the run). Once you have that, you can express the pitch as “X in 12” (or X/12), convert it to an angle in degrees, and see the slope as a percentage. Those formats show up everywhere: material specs, planning notes, rough estimates, and basic cut planning.

The default method is rise and run, because it matches how roofs are typically checked in the field. Measure the vertical rise and the horizontal run using the same units. If you only know the roof angle, switch to the angle method and enter degrees plus a reference run. The calculator will convert the angle back into rise/run and pitch-per-12 outputs so you can communicate the pitch in the format most people expect.

You also get an optional rafter-length estimate if you provide a horizontal run length for one rafter. This is useful when you want a quick sense of the diagonal length along the rafter for that run distance. It is not a full framing design tool. It does not add overhangs, ridge thickness, birdsmouth depth, heel height, or any local code requirements. It is a clean geometric result based on the pitch you entered.

Assumptions and how to use this calculator

• Rise and run must be measured in the same units (inches and inches, millimetres and millimetres, and so on).
• Pitch per 12 is computed as (rise ÷ run) × 12. If your run is already 12, your rise value is your pitch-per-12.
• Angle in degrees is computed from the slope using arctangent (atan(rise ÷ run)).
• Slope percent is computed as (rise ÷ run) × 100. This is common in drainage and grading contexts.
• Optional rafter length assumes a straight right triangle: rafter = √(run² + rise²) for the provided run length, with rise scaled by the same pitch.

Common questions

What does “6/12 pitch” mean in plain terms?

A 6/12 pitch means the roof rises 6 units for every 12 units of horizontal run. If you measure 12 inches across and the roof rises 6 inches over that distance, the pitch is 6/12. The units do not matter as long as they match for rise and run.

Why does the calculator show both pitch-per-12 and degrees?

Pitch-per-12 is common on job sites and in roofing discussions. Degrees are common in drawings, some tools, and some engineering references. They describe the same slope in different formats. Converting between them helps you communicate with whoever you are dealing with.

I do not know the run. Can I still use this?

You need some run distance to turn a rise measurement into a slope. If you can measure any horizontal distance, even a smaller one, you can use it. For example, if you measure a 300 mm run and a 150 mm rise, that is still valid because the ratio is what matters.

What is the optional rafter length actually measuring?

It is the diagonal length along the slope for the horizontal run length you entered. If you enter a 2400 mm horizontal run for one side, the calculator estimates the straight-line rafter length for that run at the calculated pitch. It does not include overhangs or extra allowances.

When might this calculator not apply?

If your roof is curved, stepped, or changes pitch, a single rise/run measurement will not describe it. Also, if you are doing structural design, you need more than geometry. Loads, spans, spacing, materials, connectors, and code rules matter. Use this calculator for pitch identification and simple geometry only.