Practical guide
How to Calculate Roof Pitch in Degrees
Convert measured rise and horizontal run into degrees, percentage slope and rise-per-12 notation without confusing the units.
Updated 16 October 2026
Reviewed by the 99Tools team. Editorial and corrections policy.
Identify the two measurements
Roof slope needs vertical rise and horizontal run between the same measurement points on a plane. A sloping tape measurement is a different triangle side and cannot be entered as run. Use suitable drawings or measurements available from an accessible location. Where the geometry or access is uncertain, arrange a competent measured survey. You do not need to climb onto a roof to use the calculator.
Use inverse tangent for degrees
Divide vertical rise by horizontal run and take the inverse tangent. If the result is in radians, multiply by 180/π for degrees. The Roof Pitch Calculator handles independent rise and run units, so 1200 mm rise and 4 m run are converted before division. That example has a rise/run gradient of 0.3 and an angle of about 16.699244°. These are geometric measurements, not recommended construction dimensions.
What does 6:12 mean?
A 6:12 slope rises 6 units for 12 horizontal units, giving a gradient of 0.5. Multiply by 100 for 50% slope, or use atan(0.5) for about 26.565051°. The matching units need not be inches. Six units of rise over twelve units of horizontal distance is the same gradient in metres or millimetres. The ratio is not 6 degrees, and 50% is not 50 degrees.
Work out a 5 degree pitch
For a known angle, rise is horizontal run × tangent of the angle. Over a hypothetical 4 m run at 5°, the rise is about 0.349955 m. The percentage slope is about 8.748866%. The geometric sloping side is about 4.015279 m. Enter a known angle in degrees to perform the conversion, but check the exact covering and system before assuming such a pitch is suitable.
Run is not always half the span
For a symmetrical gable, the horizontal run from one eave to the ridge can be half the building span. That relationship cannot be assumed for an offset ridge, unequal planes or a different roof shape. A mono-pitch roof has one main sloping plane; use the actual horizontal run of that plane. Do not halve an already measured run again or use the full span automatically for one gable side.
Overhang and exported geometry
The additional-overhang input is horizontal and uses the run unit. It extends the calculated line at the same pitch without altering the initial rise/run angle. It is not a sloping allowance or a timber cutting specification. Copy the result, download CSV or choose Print / save PDF in a supported browser. The export records geometry, not structural member sizes, connection details, covering suitability or certified survey accuracy.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
Frequently asked questions
What formula gives roof pitch in degrees?
atan(vertical rise ÷ horizontal run) × 180/π, after matching measurement units.
Is 100% roof slope 100 degrees?
No. A 100% slope has equal rise and run, so its angle is 45°.
Can I use the sloping roof length as run?
No. This formula requires horizontal run. A sloping length is a different triangle side.
Does the PDF result certify the roof?
No. Browser print-to-PDF records the calculated geometry only.