Enter rise and horizontal run
A roof slope calculator connects the measurements used by homeowners, roofers, and product instructions. Start with rise and run from a level and tape. Enter those distances in the fields above; the pitch, angle, percent slope, and category update automatically as you type. You do not need to convert your measurements before using this page.
For rise and run, enter inches in both fields. Rise is the vertical change in height. Run is the horizontal distance beneath that change, not the diagonal distance along the shingles. You can use the actual measured run; the result is normalized to a standard run of 12 inches.
Read a worked slope example
With 4 inches of rise over 12 inches of horizontal run, the roof pitch is 4:12. The same roof has an angle of 18.4 degrees and a slope of 33.33%. These results describe the same plane, so use whichever format the drawing or product instructions request.
The angle comes from the arctangent of rise divided by run. The percentage describes that ratio on a percentage scale. Keep the original measurements when calculating area or rafter length; rounding the displayed angle before another calculation adds avoidable error.
Understand the category without overreading it
This tool labels pitches below 2:12 as flat, including very low slopes. Low slope starts there and continues below 4:12. Conventional runs from that boundary through 21:12; steeper results receive the steep slope label. These are this tool's application bands, not universal roof-design categories or a walkability rating.
Avoid the measurements that change the answer
Do not mix feet in one input with inches in another, or substitute the roof's sloping edge for horizontal run. Hold the level horizontally and measure rise vertically at the marked run distance. Repeat the measurement along the same roof plane before relying on the result; a tilted level changes both reference directions.
Measure porches, dormers, and additions separately when their slopes differ. An average pitch can hide a shallow section with different roofing requirements. Use the calculated slope to locate the relevant product instructions, then check the specified underlayment and flashing details before making a material decision.