This roof pitch calculator turns rise and run into a pitch in degrees, or works the other way. It is also a roof area calculator: give it the building size and eaves, and it works out the roof surface area and what a re-roof costs in Colorbond or tiles in your city.
Enter your numbers, or tap a common house size to start.
Your measurements
Estimated cost, Australia
$14,933 to $33,599 typical $22,399
Your numbers
- Roof pitch
- 21.8 degrees
- Slope
- 40% (1 in 2.5)
- Rafter length per metre of run
- 1.077 m
- Rafter length, wall to ridge plus eave
- 5.87 m
- Plan area including eaves
- 173.3 m²
- Roof surface area
- 186.7 m²
| Price row | Quantity | Rate | Low to high |
|---|---|---|---|
| Re-roofing Colorbond (per sqm) | 186.7 m² | $80 to $180 per m² | $14,933 to $33,599 |
Approvals and licences
Pick your city above to see the approval and licence rules in your state for this job.
Rates are the low, typical and high figures in our roofing price guide, prices verified October 2026. An estimate only: get quotes to price your job.
Type the total on your quote to see how it compares with this estimate.
This job in 15 cities: typical $20,533 in Adelaide to $26,132 in Sydney
| City | Low to high | Typical |
|---|---|---|
| Adelaide | $13,813 to $30,799 | $20,533 |
| Geelong | $13,813 to $30,799 | $20,533 |
| Hobart | $13,440 to $29,866 | $20,533 |
| Newcastle | $14,186 to $31,732 | $21,466 |
| Sunshine Coast | $14,560 to $32,666 | $21,466 |
| Brisbane | $14,933 to $33,599 | $22,399 |
| Darwin | $14,933 to $33,599 | $22,399 |
| Gold Coast | $14,560 to $32,666 | $22,399 |
| Byron Bay | $15,679 to $35,466 | $23,333 |
| Melbourne | $15,679 to $35,466 | $23,333 |
| Perth | $15,679 to $35,466 | $23,333 |
| Canberra | $16,426 to $37,332 | $24,266 |
| Townsville | $16,053 to $36,399 | $24,266 |
| Wollongong | $16,053 to $36,399 | $24,266 |
| Sydney | $17,173 to $38,265 | $26,132 |
Each city is priced from the rows in its own guide, cheapest typical figure first.
Roof area calculator: from the floor plan to the roof surface
A roof is bigger than the floor it covers because it slopes. To get the roof area, take the building's length and width plus the eaves on each side, then multiply by the pitch factor (1 divided by the cosine of the pitch). The calculator does this for a gable or hip roof of one pitch.
| House | Roof surface at 21.8° | Colorbond re-roof |
|---|---|---|
| Small house, 12 m × 8 m | 123.7 m² | $9,892 to $22,258 |
| Family home, 15 m × 10 m | 186.7 m² | $14,933 to $33,599 |
| Large home, 20 m × 12 m | 290.4 m² | $23,230 to $52,268 |
The roof surfaces above use the calculator's default 450 mm eaves. Change the pitch or the eaves in the calculator and the area and cost update.
How the calculation works
Pitch. The pitch in degrees is the inverse tangent of rise divided by run. The example the calculator opens with is a rise of 400 mm over a run of 1000 mm, which is 21.8 degrees, a slope of 40% (1 in 2.5).
Roof area. A roof with one pitch throughout has a surface area equal to its plan area divided by the cosine of the pitch. The plan area is the building length and width with the eave overhang added on every side. For a 15 m × 10 m building with 450 mm eaves the plan area is 173.3 m² and the roof surface is 186.7 m².
Cost is the roof surface area multiplied by the re-roofing row per square metre for the material you pick. New gutters (per metre) and removing an asbestos roof first each add their own row from the same guide. Valleys, dormers and roofs with more than one pitch need measuring section by section.
Measuring with a phone. On a phone, tap "Measure the pitch with your phone" and lay it flat along a rafter in the roof space, or along a sloping surface you can reach safely, and hold it still. The calculator reads the phone's tilt sensor. Do not climb onto the roof to do it.
Roof pitch table
Common pitches with the rise for every metre of run, the slope and the number to multiply a plan area by to get the roof area.
| Pitch | Rise per 1,000 mm run | Slope | Roof area multiplier |
|---|---|---|---|
| 5 degrees | 87 mm | 8.7% (1 in 11.43) | 1.004 |
| 10 degrees | 176 mm | 17.6% (1 in 5.67) | 1.015 |
| 15 degrees | 268 mm | 26.8% (1 in 3.73) | 1.035 |
| 20 degrees | 364 mm | 36.4% (1 in 2.75) | 1.064 |
| 22.5 degrees | 414 mm | 41.4% (1 in 2.41) | 1.082 |
| 25 degrees | 466 mm | 46.6% (1 in 2.14) | 1.103 |
| 30 degrees | 577 mm | 57.7% (1 in 1.73) | 1.155 |
| 35 degrees | 700 mm | 70% (1 in 1.43) | 1.221 |
| 40 degrees | 839 mm | 83.9% (1 in 1.19) | 1.305 |
| 45 degrees | 1,000 mm | 100% (1 in 1) | 1.414 |
Roofing rates the calculator uses
These are the national figures from our roofing price guide. Pick a city in the calculator and it switches to the same rows from that city's guide.
| Price row | Low | Typical | High |
|---|---|---|---|
| Re-roofing Colorbond (per sqm) | $80 | $120 | $180 per m² |
| Re-roofing tiles, incl. strip-off of old roof (per sqm) | $100 | $150 | $220 per m² |
| Gutter replacement (per m) | $40 | $55 | $110 per m |
| Asbestos roof removal (add-on to re-roof) | $5,000 | $10,000 | $16,000 per job |
Roofing prices by city
SydneyMelbourneBrisbanePerthAdelaideGold CoastCanberraHobartDarwinNewcastleGeelongSunshine CoastTownsvilleWollongongByron Bay
Frequently asked questions
How do I calculate roof pitch in degrees?
Divide the rise by the run and take the inverse tangent. A rise of 400 mm over a run of 1000 mm is 21.8 degrees.
How do I work out roof area from the floor plan?
Take the plan area of the roof, including the eaves, and divide it by the cosine of the pitch. A 15 m × 10 m building with 450 mm eaves has a plan area of 173.3 m²; at 21.8 degrees the roof surface is 186.7 m².
Does it work for a hip roof as well as a gable roof?
Yes, as long as every face has the same pitch. The surface area of a roof with one pitch is its plan area divided by the cosine of the pitch, whatever the shape. Roofs with more than one pitch need each section worked out separately.
How much does it cost to re-roof per square metre?
Our national roofing guide puts re-roofing in Colorbond at $80 to $180 per square metre, with $120 typical, and re-roofing in tiles at $100 to $220 per square metre. Choose your city in the calculator to use that city's figures.
How long is a rafter for a given pitch?
Divide the run by the cosine of the pitch. At 21.8 degrees each metre of run needs 1.077 m of rafter. For the example building the rafter from the wall to the ridge, plus the eave, is 5.87 m.
How do I calculate roof area?
Add the eaves to the building's length and width, multiply them for the plan area, then multiply by the pitch factor. For a 15 m × 10 m house with 450 mm eaves at the default pitch, that is 186.7 m² of roof.
An estimate is a starting point. Get quotes from local roofing businesses to price your job.
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