Roof Pitch Calculator.

What the numbers tell you

Roof pitch describes a slope as rise:12. A 6:12 pitch means the surface rises 6 inches over 12 horizontal inches. The angle describes that same slope in degrees, while the percentage is rise divided by run, multiplied by 100. These are different ways to describe the same geometry.

Use the roof pitch calculator above with whichever measurement you have: a rise and run, a known pitch, an angle or a slope percentage. You do not need to press a calculate button. The drawing and results update as you type. The initial 6:12 result is labeled as an example. Edit a measurement or choose a common pitch to try the tool. Switching input methods carries over your current slope.

Two runs, two different jobs

For a shed or lean-to with one continuous slope, the shed roof pitch calculator uses the full run and two endpoint heights. It can also find the high-side roof height from a chosen pitch.

The horizontal run next to vertical rise is the short distance used to measure the slope. For example, measure a rise over a 12-inch level. The two numbers must use the same unit and describe the same triangle.

The roof run for rafter length is the horizontal distance across the actual roof section, from the wall toward the ridge. Open Estimate rafter length to enter this optional dimension. It scales the slope into a length. Changing or clearing this distance does not change the pitch or angle.

In rise/run mode, the diagram labels your measurement triangle using the same inches or millimeters as the inputs. In pitch, degree or percent mode, it shows the equivalent ratio over 12 horizontal units. The full-size roof run is kept separate from this drawing. Rise is shown vertically in rust; run is shown horizontally in green. Focusing an input highlights its corresponding edge.

Convert slope percent to pitch

Select Percent and enter 8.7 for an 8.7% slope. The ratio is 0.087, so the pitch is 0.087 × 12 = 1.044:12 and the angle is approximately 4.97°. The displayed pitch rounds to 1.04:12; calculations and switching input methods retain the unrounded slope. Percent and degrees are different: 100% is a 45° angle.

A metric example

Select m / mm, then enter a rise of 150 mm and a horizontal run of 300 mm. This gives 6:12, 26.57° and 50%. For a separate full roof run of 3.6 m, the optional length estimate is 4.025 m. See the run versus rafter length diagram to keep the small measuring triangle separate from the full roof.

A worked example: 6:12 over a 12-foot run

Start with a 6-inch rise and a 12-inch horizontal run. Their ratio is 0.5. With a full roof run of 12 feet, the total rise is 6 feet.

• Pitch: 0.5 × 12 = 6, written as 6:12.
• Angle: arctan(0.5) ≈ 26.57°.
• Slope: 0.5 × 100 = 50%.
• Theoretical length: √(12² + 6²) ≈ 13.416 feet.

The length displays as approximately 13 feet 5 inches, rounded to the nearest 1/16 inch. This is a geometric length before overhang and ridge adjustments, not a finished cutting specification.

Pitch and angle quick reference

For more rows, rafter factors and a printable reference, open the roof pitch chart.

These values describe geometry. They do not establish which roofing materials, structural members, or installation methods are suitable for your project.

Check before you cut

Check that your level is horizontal, your rise is vertical, and both measurements use the same reference points. Enter decimal values for fractions of an inch: for example, 6 1/2 inches becomes 6.5. In metric mode, the short rise and run use millimeters, while the full roof run uses meters.

The tool calculates length from the Pythagorean theorem. It does not account for ridge-board thickness, birdsmouth notch depth, member size, material strength, or a roof's structural loading. Match the measurements to your plans and confirm the cutting layout before using a result on site.

For a horizontal overhang allowance, continue to the rafter calculator. For a more detailed measurement walkthrough, open the roof pitch measuring guide. Copy or print your results to keep the inputs and assumptions together.

Read our calculation methods for precision and assumptions, or send a correction.

Planning the next part of your project?

• Stairs: the stair calculator draws a straight flight from floor-to-floor rise, converts inches and mm, and compares its run with the available space.
• Level railings: the baluster spacing calculator gives equal clear gaps and every center mark from one inside post face.
• Solid-wood panels: the wood movement calculator estimates width change from species, grain direction and wood moisture content.