Divide your roof’s rise by its run, take the arctangent, and you have the pitch in degrees. The roof pitch calculator above does both steps the moment you enter two measurements. The maths is rarely where it goes wrong. If you run the tape out to the eave tip instead of the wall line, the roof looks flatter than it is.
This page shows how to calculate roof pitch for an Australian house or shed, how to read the results in degrees, percent and ratio, and one catch the pitch number alone won’t show you: how long your rafter is.
How to calculate roof pitch from rise and run
Pitch comes from a right triangle. The rise is the vertical height, the run is the horizontal distance, and the rafter is the sloping side. Four formulas cover the lot:
- Ratio: rise ÷ run
- Angle in degrees: arctan(rise ÷ run)
- Slope as a percentage: (rise ÷ run) × 100
- Rafter length: √(rise² + run²)
Omni Calculator and SkyCiv both build their tools on exactly this triangle, so any answer you get from them should match the one above.
Here’s a worked example. Say your house is 9.6 metres wide, so the run to the ridge is 4800 mm. The ridge sits 2000 mm above the top of the wall.
Rise ÷ run is 0.4167, which is 41.7%. The arctangent of 0.4167 is 22.6 degrees, and the rafter is √(2000² + 4800²) = 5200 mm.
That’s a 5, 12, 13 triangle, so the rafter lands on a round number. Real roofs rarely cooperate like that.
Every “find the pitch of a roof” calculator, this one included, runs the same four formulas. The only difference is how many of the answers it shows you.
Measuring rise and run without guessing
For a gable roof, ShedBlog’s guide measures the rise from the top edge of the wall to the peak. The run is the horizontal distance from the wall to a point directly under the ridge centre. Measure to the wall line, not the eave tip, because the overhang adds distance that isn’t part of the triangle. Pick millimetres or metres in the calculator to match your tape, and use the same unit for both numbers.
The safest way to get both numbers is from inside the roof space, so you stay off the slope. Measure the same edge of the rafter at both ends, and keep the tape taut. A tape that sags across five metres adds length the roof doesn’t have.
If you can’t get to the ridge, use a level. Fielders notes that Australian trades often quote pitch as rise over a one metre run. Hold a level horizontal against a rafter or a straight board laid on the slope, mark 1000 mm along it, then measure the vertical gap to the roof. Type 1000 as the run and the gap as the rise.
A pitch gauge or digital level skips the tape altogether. ShedBlog describes centring the bubble and reading the angle in degrees. A phone inclinometer app reads angles the same way, so check it against a spirit level before you trust it.
Whichever tool you used, switch the roof pitch calculator to angle mode and enter the reading. Add a run if you want the rise and rafter length too. If you leave the run blank, you still get the percentage, ratio and multiplier.
What the roof pitch calculator gives you back
The calculator returns seven core results, and each one answers a different question.
Pitch angle is the roof angle in degrees. Australian specs commonly use it, and Wikipedia notes that metric countries use either degrees or a “1 in x” slope. Slope is the same triangle as a percentage, and ratio is the “1 in x” version. A 22.6 degree roof is a 41.7% slope and a 1 in 2.4 ratio.
Rise per metre of run is the one to carry onto site. For the example roof it’s 417 mm, so a level and a tape are enough to check it.
Imperial figures matter if your plans or products come from overseas. Our example works out at 5 in 12. If you came here after a 6/12 roof pitch, that’s 26.6 degrees (see the questions below).
The pitch multiplier is the number people skip until the roofing quote arrives. It’s √(1 + (rise ÷ run)²), according to Omni Calculator, and it converts the footprint to the actual surface. At 22.6 degrees it’s 1.083, so a roof covering 100 m² of plan area is about 108.3 m² of sheeting or tiles. Your roof is bigger than your floor plan, which is why the quote looks the way it does.
Rafter length is the common rafter from the wall line to the ridge, so the tool doubles as a basic rafter calculator. If you searched for a roof angle calculator or a roof slope calculator, you’re in the right place. Angle, slope and pitch are three ways to describe one triangle.
One wrinkle on terms: Wikipedia says strict pitch means rise over the full span, while slope means rise over the run. This tool uses rise over run, like the calculators named above.
Pitches you’ll see on Australian roofs
Built Simple’s reference table puts most Australian roofs between 15 and 25 degrees, with 30 degrees counted as steep or loft-friendly. Here’s what four common angles look like in the numbers you’d measure on site.
| Pitch | Rise per metre of run | Imperial | Pitch multiplier |
|---|---|---|---|
| 15° | 268 mm | 3.2 in 12 | 1.035 |
| 22.5° | 414 mm | 5.0 in 12 | 1.082 |
| 25° | 466 mm | 5.6 in 12 | 1.103 |
| 30° | 577 mm | 6.9 in 12 | 1.155 |
The multiplier column shows why steeper roofs cost more to cover. Going from 15 to 30 degrees adds roughly 12% to the surface area for the same footprint. Built Simple also notes that steeper pitches shed water faster but cost more in materials and framing.
Treat any list of typical roof pitches as a starting point. A roof pitch calculator gives you the angle, but your roofing material sets the real floor, and the last section explains why length matters as much as angle.
Shed and skillion roofs
A skillion has one slope and no ridge, so there’s nothing to measure to in the middle. ShedBlog treats the run as the full span of the shed in that case. The rise is the height difference between the high wall and the low wall.
That makes a shed roof pitch calculator job simple. A 3 metre span with a 262 mm drop across it is 5 degrees. The span goes in as the run, and the height difference goes in as the rise.
Why a long rafter needs a steeper roof
A pitch that works on a 3 metre carport slope doesn’t automatically work on a 7 metre house slope. Water collects as it runs down, so the longer the slope, the more water reaches the bottom edge.
CSR Roofing’s technical sheet turns this into a guide for contoured tiles. A rafter shouldn’t exceed 4.5 m at a 15 degree pitch. Every extra 0.5 m adds about one degree, so 6 m needs 18 degrees and 8 m needs 22. It adds that AS 2050 states long rafters may need sarking.
Tile makers set their own floors on top of that. Monier says most of its concrete tiles go down to 15 degrees, while its Terracotta Nouveau tile goes to 12 degrees with sarking. The company adds that rafter length, wind speed, terrain category and profile all feed into the decision. Bristile’s NSW brochure lists 15 degrees with sarking and 20 degrees without for several concrete profiles.
Metal works the same way, only lower. Lysaght’s brochure gives minimum slopes of 5 degrees for Custom Orb, 2 degrees for Trimdek and 1 degree for Klip-Lok 700. Its advisory bulletin on maximum roof lengths for drainage then ties the usable length to slope and rainfall.
The National Construction Code points the same direction: Part 7.2 of the 2022 Housing Provisions requires metal roofing to meet the minimum pitch for its profile, and tells designers to weigh drainage capacity against sheet length. Which edition of the code applies depends on your state, so your certifier can confirm.
The calculator’s rafter length stops at the wall line, but rain keeps running to the gutter. Add the eave overhang measured along the slope, which is roughly the horizontal overhang times the pitch multiplier. A 450 mm overhang on the 22.6 degree example adds about 490 mm, so 5.2 m becomes about 5.7 m.
Run your numbers before you order. If the rafter plus overhang tops 4.5 m and your pitch sits near 15 degrees, call your tile or sheeting supplier and get the minimum pitch for that length in writing before you pay a deposit.
Roof pitch questions, answered
What is a 6/12 roof pitch in degrees?
About 26.6 degrees. The rise is half the run, so the slope is 50% and the rafter is 1.118 times the run. Australian plans rarely use the x in 12 style, but imported products and some designs do.
Does this calculator work for a hip roof?
Yes for the main roof faces, as long as you measure the common rafter, the one that runs straight up the slope at right angles to the wall. Hip rafters run diagonally, so they’re longer and sit at a lower angle, and this tool doesn’t give you their length.
Which is steeper, 10 degrees or 10 percent?
10 degrees. A 10 percent slope is only 5.7 degrees, while 10 degrees is a 17.6 percent slope. Check the unit before you compare any two numbers.
Can the calculator tell me which roofing I can use?
No. It gives pitch, rise, rafter length and the multiplier, and nothing about materials. Minimum pitch depends on the profile and the slope length, so get the figure from the manufacturer for your rafter length.
How accurate do my measurements need to be?
Aim for the longest run you can measure. On a 1 metre run, a 20 mm slip in the rise shifts a 22.5 degree roof by about 1 degree, and on a 4 metre run the same slip moves it by about a quarter of a degree.
Sources
- Australian Building Codes Board, NCC 2022 Housing Provisions, Part 7.2 Sheet roofing
- Wikipedia, Roof pitch
- Built Simple, Roof Pitch Calculator
- Omni Calculator, Roof Pitch Calculator
- SkyCiv, Roof Pitch Calculator
- ShedBlog, How Do I Calculate The Roof Pitch Of My Shed Or House?
- Fielders, roof pitch guidance
- CSR Roofing, Pitch and rafter requirements technical sheet
- Monier, roofing information and FAQs
- Bristile Roofing, NSW concrete roof tile brochure
- Lysaght, Key Account Brochure and Product Advisory Bulletin PAB03 (maximum roof lengths for drainage)