Calculate roof pitch as an angle, ratio, slope multiplier and rafter length — enter the rise and run, or the pitch, to find the roof angle and true roof area factor. Works in any unit (US inches-per-12, or metric).
Roof pitch is written as rise:run — in the US, run is almost always 12 (so "6:12" means 6" of rise per 12" of run); in Australia it's usually expressed as a degree angle instead.
Enter rise and run below (or an angle) to convert between all three, plus find the slope multiplier for roof area and the exact rafter length.
Enter rise and run (e.g. 4 and 12 for a 4:12 pitch), or type an angle to convert back. US convention: run = 12 (inches per foot of span). Any consistent unit works.
Enter the real horizontal distance from wall to ridge (in metres or feet — your choice, output matches) to also calculate the exact rafter length.
| Item | Value |
|---|
Roof pitch describes how steep a roof is. It is written as a ratio of vertical rise to horizontal run — a "4:12" pitch rises 4 units for every 12 units across.
Pitch can also be expressed as an angle in degrees, and as a slope multiplier used to turn a roof’s flat footprint into its true, larger surface area.
| Pitch | Angle | Area multiplier |
|---|---|---|
| 3:12 | 14.0° | ×1.031 |
| 4:12 | 18.4° | ×1.054 |
| 6:12 | 26.6° | ×1.118 |
| 9:12 | 36.9° | ×1.250 |
| 12:12 | 45.0° | ×1.414 |
Say your roof is a 4:12 pitch and the building footprint under it is 100 m². The calculator gives a slope multiplier of about 1.054 for a 4:12 pitch.
Multiply the footprint by the multiplier: 100 × 1.054 = 105.4 m² of actual roof surface. That true area — not the flat footprint — is what you use to order roofing materials like shingles, tiles or metal sheeting.
Roof pitch can be expressed three ways, and it helps to understand how they relate.
The traditional builder’s notation is a ratio of rise to run, like 4:12, meaning the roof rises 4 units for every 12 units it runs horizontally.
The same slope can be stated as an angle in degrees — a 4:12 pitch is about 18.4° — which is useful for cutting timber and setting out.
It can also be given as a percentage grade, found by dividing rise by run, so 4:12 is a 33% grade.
Beyond describing steepness, pitch has a practical effect on materials through the slope multiplier: because a sloped surface is longer than its flat shadow on the ground, a steeper roof has proportionally more area to cover.
This is why two houses with identical footprints can need very different amounts of roofing if their pitches differ — and why estimating from the footprint alone, without applying the pitch, leads to under-ordering.
Rafter length is the actual, physical length of timber needed to span from the wall plate to the ridge along the slope — a common question for anyone framing or re-roofing.
It follows directly from the pitch: the rafter is the hypotenuse of a right triangle formed by the rise and the horizontal run, so it's found with the same slope multiplier used for roof area.
| Australia | United States | |
|---|---|---|
| Common notation | Degrees (e.g. 22.5°) | Rise:run out of 12 (e.g. 5:12) |
| Typical roof pitch | 15°–25° for tiled roofs | 4:12 to 6:12 (18°–27°) for shingle roofs |
| Steep pitch example | 35°+ (Colorbond, some heritage styles) | 9:12 to 12:12 (37°–45°), common on traditional/Victorian homes |
Both notations describe the same physical slope — this calculator converts freely between them, so it works equally well as a "roof pitch calculator" for a US rise:run job and an Australian degree-based one.
How do I calculate roof pitch?
Divide the vertical rise by the horizontal run and take the arctangent to get the angle. For example, a roof that rises 4 units over a 12-unit run is a 4:12 pitch, which is 18.4 degrees.
Enter the rise and run above and the calculator gives the angle, ratio, area multiplier and rafter length.
What does a 4:12 or 6:12 roof pitch mean?
A "4:12" pitch (the standard US notation) means the roof rises 4 inches vertically for every 12 inches of horizontal run — about 18.4 degrees. A "6:12" pitch rises 6 in 12, or 26.6 degrees. Australian trades usually just quote the equivalent degree figure instead.
What is a roof pitch multiplier?
The slope multiplier converts a roof's flat footprint area into its actual sloped surface area. A 4:12 pitch has a multiplier of about 1.054, so the true roof area is 5.4% larger than the footprint. It is essential for estimating roofing materials like shingles or tiles.
How do I find rafter length from roof pitch?
Multiply the actual horizontal run (wall to ridge, not the "12" in the pitch notation) by the slope multiplier: √(rise²+run²)÷run.
Enter your actual run in the field above and the calculator adds the rafter length automatically, in both decimal and feet-and-inches form.
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run. A 6:12 pitch is arctan(6/12) = 26.6 degrees. You can also enter an angle in the calculator above to convert it back to a rise-and-run ratio.
Why does pitch matter for ordering materials?
A steeper roof has more surface area than its footprint suggests. Using the pitch multiplier gives the true area, so you order enough shingles, tiles or sheeting instead of coming up short.