How to calculate roof pitch
Roof pitch is the relationship between a roof's vertical rise and its horizontal run. This roof pitch calculator uses the same right-triangle maths used to describe a roof slope, then presents the result in the formats people commonly need for planning and quoting.
slope percentage = slope × 100
angle = arctan(rise ÷ run)
rafter length = √(rise² + run²)
- Measure the vertical rise and horizontal run over the same section.
- Enter both figures in millimetres or metres, using the same unit.
- Read the roof pitch in degrees, percentage, 1:12 ratio and rafter length.
For example, a 450 mm rise over a 2,400 mm run gives a 10.6° angle, 18.8% slope and approximately 2.3:12 pitch. The calculator also shows the pitch multiplier, which can help turn a horizontal plan area into a rough sloping surface area.
Roof pitch conversions at a glance
The table below shows approximate conversions between angle, slope percentage and the 1:12 format. Use the live calculator for your exact measurements.
| Angle | Slope | Approx. pitch | Common description |
|---|---|---|---|
| 5° | 8.7% | 1.1:12 | Very low angle |
| 10° | 17.6% | 2.1:12 | Low angle |
| 15° | 26.8% | 3.2:12 | Moderate angle |
| 20° | 36.4% | 4.4:12 | Steeper angle |
| 25° | 46.6% | 5.6:12 | High angle |
| 30° | 57.7% | 6.9:12 | High angle |
How to find the pitch of a roof safely
You do not need to stand on a roof to use a roof pitch calculator. If the roof space is safely accessible, measure from a level surface to the underside of a rafter or use a known roof plan. From the ground, a gable end or a clear side elevation may provide a safer reference.
- Keep the rise and run measurements on the same plane.
- Check whether your run is the horizontal distance, not the sloping rafter length.
- Record the unit beside every measurement before entering it.
- Stop if access is unstable, the roof space is unsafe or electrical and other hazards are present.
This roof slope calculator can describe geometry, but it cannot confirm waterproofing, structural capacity, wind fixing, drainage, product minimums or building compliance. A roof inspection is still needed before repair, restoration, replacement or construction decisions.
Roof pitch planning for Mornington Peninsula homes
For homes around Mornington, Mount Martha, Mount Eliza, Rosebud, Dromana, Sorrento and nearby suburbs, roof pitch is one part of a wider assessment. Roof material, age, access, flashings, valleys, drainage and the condition of the supporting structure can all change the right next step.
Use this roof pitch estimator to prepare a useful starting point, then compare it with the roof plan or an on-site measurement. If the result is part of a repair, restoration or re-roofing decision, the roof repairs team and roof restoration guide can help you understand the next questions to ask.
Roof pitch calculator FAQs
What is roof pitch?
Roof pitch describes how quickly a roof rises compared with its horizontal run. It can be shown as an angle in degrees, a slope percentage or a ratio such as 3:12.
How do I calculate roof pitch?
Measure the vertical rise and horizontal run over the same roof section. Divide rise by run for the slope, multiply by 100 for the percentage, and use arctan(rise ÷ run) for the angle in degrees.
Is roof pitch the same as roof slope?
People often use roof pitch and roof slope to mean the same thing. In practice, pitch may be written as a rise-to-run ratio while slope is often shown as a percentage or angle, so check which format a plan or product guide requires.
What is a normal roof pitch in Australia?
There is no single normal roof pitch for every Australian home. The suitable pitch depends on the roof design, material, drainage, manufacturer requirements and applicable building requirements.
Can this roof pitch calculator tell me if a roof is compliant?
No. This roof pitch calculator is a planning aid. It does not inspect the roof, confirm structural capacity, select a compliant product or replace advice from a qualified building or roofing professional.
Need the roof checked on site?
Share your measurements with the Mornington Peninsula Roofing team and ask about the right next step for repairs, restoration or re-roofing.