Trade & construction Australia
Roof Pitch Calculator
Roof pitch is quoted three ways: as an angle in degrees, as a rise over run, or as a percentage grade. This calculator converts between them, and gives the rafter length and the factor that turns a plan area into a roof area.
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Horizontal distance from the wall plate to the ridge.
Pitch
—Degrees
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Rise over run
—
Percent grade
—
Rafter length
—
Roof area for the plan area
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The working
How do I calculate roof pitch?
Roof pitch in degrees is the angle whose tangent is the rise divided by the run. A 1 in 4 pitch (rise 1 for every 4 of run) is 14.04 degrees, or a 25% grade. To get the roof area, divide the plan area by the cosine of the pitch angle: at 22.5 degrees the roof is about 8% bigger than the area it covers on the ground.
Pitch and materials
Each roofing material has a minimum pitch below which water can be driven back under it, and a maximum for fixing and safety. Metal sheeting can run at very low pitches, concrete and terracotta tiles need more, and flat roofs use membranes. Check the manufacturer's data sheet for the profile you choose.
A steeper roof sheds water and leaves faster but costs more, and costs more again to build safely.
Measuring a pitch
On an existing roof, measure rise over a set run with a level and tape, or use a digital angle gauge. From the ground, a pitch can be estimated from a photo taken square-on to the gable, but access for measurement should be safe and legal; do not climb a roof without suitable safety equipment.
To estimate the insulation for the roof space, use the insulation calculator.
How this is calculated
The rule, step by step, so you can check any figure by hand.
tan(angle) = rise ÷ runpercent grade = rise ÷ run × 100rafter length = half span ÷ cos(angle)roof area = plan area ÷ cos(angle)
The three forms are the same slope. A 22.5 degree roof is 41.4% grade and about 5 in 12, and a 15 degree roof is 26.8%, so a figure quoted in one unit can be compared with another once converted. Metal roofing manufacturers publish minimum pitches for their profiles, often a few degrees for corrugated and more for tiles, and the pitch you use must meet them.
The area factor is why a roof costs more than the floor beneath it. A 30 degree roof has a surface 15% larger than its plan, and a 40 degree roof 30% larger, before allowing for eaves, hips and valleys.
A worked example
A roof with a half span of 4 m, a pitch of 22.5 degrees and a plan area of 120 m².
- Rise over run0.414 (about 1 in 2.41)
- Percent grade41.4%
- Rafter length, 4 ÷ cos 22.5°4.33 m
- Roof area, 120 ÷ cos 22.5°129.9 m²
A 41% grade, rafters 4.33 m long and a roof about 8% larger than its plan.
Add allowance for overhangs, hips, valleys and waste when ordering sheets or tiles.
Rafter length and overhang
The rafter length in the calculator is from the wall plate to the ridge along the slope. Add the overhang along the slope to get the full length of material, and allow for the ridge and cutting.
What it leaves out
Hips, valleys, dormers and complex roofs need to be worked out one plane at a time. A roof plan from the designer is the right basis for ordering.
What this calculator does not do
- Simple gable or skillion; other forms by plane
- Overhangs, hips and valleys excluded
- Minimum pitch limits are product specific
- Not for structural sizing
Common questions
What is a 1 in 4 pitch in degrees?
14.04 degrees, or a 25% grade.
What is a 22.5 degree roof as a ratio?
About 1 in 2.4, or roughly 5 in 12 in the imperial convention.
How do I convert percent to degrees?
Take the arctangent of the percent divided by 100. 25% is about 14 degrees.
How much bigger is a roof than the floor plan?
By 1 ÷ cos(pitch). At 22.5 degrees that is about 8%, at 30 degrees about 15%.
What is the minimum roof pitch?
It depends on the material and profile. Check the manufacturer's data sheet.
Where these numbers come from
- National Construction Code (ABCB) The code that sets minimum building requirements in Australia
- Your Home — Australian Government guide to sustainable homes Independent guidance on insulation, materials and design
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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