How to use this roof pitch calculator
Measure on the rafter or with a level and tape—not from a ground photo if you need to order materials. Use the same unit for rise and run (inches is the US default on the labels).
- Enter rise: the vertical change over the horizontal run you measured.
- Enter run: the horizontal change. A 12-inch run is the usual US convention so the pitch reads as inches of rise per foot of run.
- Read pitch as X:12, angle in degrees, slope percent, area/rafter factor, and rafter length for that same-unit run.
How this calculation works
- Pitch (US) = (rise/run) × 12, written as that value : 12. Example: rise 6 and run 12 → 6.0:12.
- Angle in degrees = arctan(rise/run).
- Slope percent = (rise/run) × 100. A 6:12 roof is a 50% slope—not “6%.”
- Area / rafter factor = √(1 + (rise/run)²). Multiply plan (footprint) area by this factor for sloped surface area.
- Rafter length per same-unit run = √(rise² + run²). If run is 12 in, that length is rafter inches per 12 in of run.
pitch = (rise/run)×12 : 12; degrees = arctan(rise/run); factor = √(1 + (rise/run)²)
What roof pitch means
Pitch describes how steep the roof is. In US practice, “6:12” (sometimes written 6/12) means the roof rises 6 inches for every 12 inches of horizontal run. Steeper pitches shed water faster and use more surface material for the same footprint.
Pitch is not the same as slope percent. The same 6:12 example is 50% slope and about 26.6°, with an area factor near 1.118.
Rise and run
Rise is vertical; run is horizontal. Keep both fields in the same unit. The ratio cancels, so feet and feet work the same as inches and inches—the labels prefer inches because that matches how most US tapes are used on rafters.
How pitch affects roof area
Roof pitch is one of the inputs that can affect the roof area used in a cost estimate. The area factor from this page is the same √(1 + (rise/run)²) multiplier used in the roof area calculator and in the footprint-and-pitch path of the roof cost calculator. A steeper pitch raises surface square footage—and therefore material and installed-cost planning ranges—without changing the building’s floor plan.
Worked example
Using the form defaults: rise 6 in, run 12 in (a common 6:12 roof).
- Pitch = (6/12) × 12 = 6.0 → written 6.0:12.
- Angle = arctan(6/12) ≈ 26.6°.
- Slope = (6/12) × 100 = 50.0%.
- Area factor = √(1 + 0.5²) = √1.25 ≈ 1.1180.
- Rafter length per 12 in of run = √(6² + 12²) ≈ 13.42 in.
Result: 6.0:12 pitch, 26.6°, factor 1.1180, 50.0% slope, 13.42 rafter length per run—the same numbers this calculator returns for those inputs.
Common mistakes
- Swapping rise and run, which invents an impossible or wildly steep pitch.
- Mixing units (rise in inches, run in feet) without converting.
- Treating 6:12 as “6% slope.” It is 50% slope.
- Guessing pitch from a curb photo when you need a material order. Measure on the rafter when accuracy matters.
- Forgetting that pitch changes roof surface area used later in the roof area and roof cost calculators.
What to do next
- Use the factor in the roof area calculator rather than guessing living-area square footage.
- If you only know degrees (from a drawing), convert: rise/run = tan(angle).
- Confirm whether “flat” roofs in your market still have a 1/4:12 or 1/2:12 drainage slope.
Next, convert footprint × this pitch factor into square footage with the roof area calculator, then estimate an installed range with the roof cost calculator or order asphalt with the shingle calculator. Browse all tools on the roofing calculators hub.
FAQ
- How do I calculate roof pitch?
- Measure rise over run on the rafter, then pitch X:12 = (rise/run) × 12. Enter those two numbers above to also see degrees, slope percent, and the area factor.
- Is 6/12 the same as 6:12?
- Yes. Both mean 6 inches of rise for every 12 inches of run. Avoid writing 6/12 if it could be read as a fraction of a foot.
- What does a 6:12 roof pitch mean?
- The roof rises 6 inches for every 12 inches of horizontal run. On this calculator that is also about 26.6°, a 50% slope, and an area factor of about 1.118.
- Can I enter rise and run in feet?
- Yes, as long as both fields use the same unit. The ratios cancel. The labels say inches because that is how most US tapes are used on rafters.
- How does roof pitch affect roof area?
- Steeper pitch increases the area factor √(1 + (rise/run)²), so the same footprint has more roof surface to cover. That larger surface feeds area, shingle, and cost takeoffs.
- Is pitch the same as slope percent?
- No. Pitch is usually X:12. Slope percent is (rise/run) × 100. A 6:12 pitch is a 50% slope.
Sources
- Pitch X:12 = (rise/run)×12; angle = arctan(rise/run); factor = √(1 + (rise/run)²)
Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.