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How to calculate roof pitch
This roof pitch calculator finds the angle in degrees, rise-to-run ratio and geometric slope length for one roof slope · Choose the method that matches the measurements you already have
Which calculation method should you use?
Distance + height · Enter the horizontal run and the vertical height difference between the two ends
Distance + pitch · Enter the horizontal run and the rise per 12 units · For a 4:12 pitch, enter 4
Distance + angle · Enter the horizontal run and the known roof angle in degrees
Distance + wall heights · Enter the horizontal run and the heights of both supporting walls or beams · Measure the top of each support from the same level reference · The tool subtracts the lower height from the higher height
Run is a level horizontal measurement, not a measurement along the roof · For a symmetrical gable roof, use half the full span · For a lean-to roof, use the horizontal distance between the two support points · Select metric or imperial and follow the units beside each input
Formulas and result meanings
A 4:12 pitch means 4 units of vertical rise for every 12 horizontal units · It is a ratio, not 4 degrees · The slope length joins your measured points and excludes overhangs, birdsmouth cuts, ridge details and connection allowances
When pitch is known: rise = run × pitch rise ÷ 12
When angle is known: rise = run × tan(angle)
Rise = high support height − low support height
Pitch per 12 = 12 × rise ÷ horizontal run
Roof angle = arctan(rise ÷ run)
Slope length = √(run² + rise²)
Worked examples in imperial and metric
An 8 ft horizontal run with supports at 7 ft and 9 ft has a 2 ft rise, a 3:12 pitch, a 14.04° angle and an 8.25 ft geometric slope length
A 2.4 m run with supports at 2.1 m and 2.7 m has a 0.6 m rise, the same 3:12 pitch and 14.04° angle, and a 2.47 m slope length
Roof pitch to angle
| Pitch | Angle | Rise over 1 m |
|---|---|---|
| 1:12 | 4.76° | 83 mm |
| 2:12 | 9.46° | 167 mm |
| 3:12 | 14.04° | 250 mm |
| 4:12 | 18.43° | 333 mm |
| 6:12 | 26.57° | 500 mm |
| 8:12 | 33.69° | 667 mm |
| 12:12 | 45.00° | 1000 mm |
Metal roofing and minimum pitch
This tool calculates geometry for a single slope, including a metal-clad roof, but does not select its minimum allowable pitch · Check the exact roofing profile, laps, fixings and manufacturer instructions
For metric roof pitch calculations, use the same unit for rise and run · The pitch ratio does not change with the measurement system
A little help
What does a 4:12 roof pitch mean?
The roof rises 4 units for every 12 units of horizontal run – the ratio is the same whether you measure in inches or centimetres
Is the run the full width of the roof?
For a symmetrical gable roof, the run is half the horizontal span – for a single-slope roof, use the horizontal distance between its low and high points
Is slope length my finished rafter cut length?
No, this is the straight geometric distance between the endpoints you measured, before overhangs, ridge details, seat cuts or other construction allowances