Roof Pitch Calculator
Roof pitch is the steepness of your roof, written as rise over a 12-inch run — 6/12 means the roof climbs 6 inches for every 12 inches of horizontal distance. This roof pitch calculator converts any rise-and-run measurement into the x/12 pitch, the angle in degrees, the slope as a percentage, and the area multiplier roofers use to convert plan area into true sloped area. Pitch drives material quantities, walkability, and which roofing products are allowed, so getting the number right matters before you order.
Vertical rise, e.g. measured over a 12 in level in the attic.
Horizontal run the rise was measured over.
Roof pitch
6/12
- Roof angle
- 26.6°
- Slope
- 50%
- Area multiplierMultiply plan area by this for true slope area
- 1.118×
How it was calculated
- Pitch = (6.00 ÷ 12.00) × 12 = 6.00/12
- Angle = arctan(6.00 ÷ 12.00) = 26.6°
- Slope = 0.5000 × 100 = 50.0%
- Multiplier = √(1 + (6.00/12)²) = 1.118
- Pitch category: Flat < 2/12 · Low slope 2–4/12 · Conventional 4–9/12 · Steep > 9/12 —
- this roof is conventional.
How it works
Pitch is rise divided by run, scaled to a 12-inch run: pitch = (rise ÷ run) × 12. A 6-inch rise over a 12-inch run gives exactly 6/12, and the formula works for any measured pair — 4 inches over 16 inches is also a 3/12 pitch.
The angle in degrees is the arctangent of the rise/run ratio: angle = arctan(rise ÷ run). Slope percent is the same ratio × 100, so a 6/12 pitch is a 50% slope at 26.6°.
The area multiplier converts a flat plan measurement into the true sloped surface: multiplier = √(1 + (pitch/12)²). Multiply your footprint area by it to get the area shingles must actually cover.
Pitch categories follow convention: under 2/12 is flat or nearly flat, 2–4/12 is low slope, 4–9/12 is the conventional residential range, and anything above 9/12 is steep — walkable only with staging and fall protection.
Worked examples
A 6-inch rise over a 12-inch run
Measure 6 in of rise over a 12 in level: pitch = (6 ÷ 12) × 12 = 6/12. The angle is arctan(0.5) = 26.6°, the slope is 50%, and the area multiplier is √(1 + 0.25) = 1.118 — so a 1,000 ft² footprint is really 1,118 ft² of roof. This is a conventional, comfortable-to-walk pitch.
A 3-inch rise over a 12-inch run
Measure 3 in of rise over 12 in of run: pitch = (3 ÷ 12) × 12 = 3/12. The angle is arctan(0.25) = 14.0°, the slope is 25%, and the multiplier is √(1 + 0.0625) = 1.031. At 3/12 this is a low-slope roof — shingles are allowed but need upgraded underlayment per manufacturer specs.
How to measure
- The easiest method: hold a 12 in level against the underside of a rafter in the attic and measure straight down from the level’s end to the rafter — that drop is your rise.
- No attic access? At the gable-end eave, hold the level horizontal against the fascia and measure down to the roof deck line.
- Measure on the underside of the rafter rather than the top of the roof so shingle thickness does not skew the reading.
- Check both sides of the roof — additions and sagging rafters can give a different pitch than the original framing.
- Use a 24 in level and divide the drop by 2 if the rafter bay is cramped; the ratio is what matters, not the level length.
- Never guess pitch from the ground by eye — a 4/12 and a 6/12 look similar from the street but differ by 12% in material.
Assumptions & limitations
Planning estimate
- Calculations assume a straight rafter line; sagging or wavy old roofs will read slightly differently along their length.
- The area multiplier converts plan area to slope area geometrically — it does not add waste, overlaps, or starter courses.
- Pitch category thresholds (flat < 2/12, low 2–4/12, conventional 4–9/12, steep > 9/12) are conventional labels, a rule of thumb used across the trade.
- Minimum pitches for specific roofing products come from manufacturers and building codes — this tool reports geometry only.
- This is a planning estimate only — not a substitute for code compliance or professional design.
Frequently asked questions
It means the roof rises 6 inches vertically for every 12 inches of horizontal run. The first number is always the rise; the run is always normalized to 12. So 6/12, 12/24, and 3/6 all describe the same steepness — this calculator normalizes any measurement to the x/12 form.
Most roofers consider up to 6/12 or 7/12 walkable in dry conditions with proper footwear; 8/12 and above usually needs roof jacks, staging, or harnesses. Pitch is only part of the story — surface material, moisture, and temperature matter just as much for safety.
Manufacturers generally allow shingles down to 2/12 with special low-slope application (double underlayment and sealed laps), and standard application from 4/12 up. Below 2/12 you are in membrane, metal, or built-up roofing territory. Always confirm with the shingle manufacturer’s specs.
Three reasons: steeper roofs have more surface area over the same footprint (the multiplier), they are slower and riskier to work on (labor premium), and very steep or very low pitches may require different materials or underlayment. A 12/12 roof can cost 30–50% more per square than a 4/12 on labor alone.
From inside the attic, hold a level against a rafter and measure the drop — no ladder needed. From the ground, you can measure a gable-end rake board’s rise over a known run, though it is less precise. Smartphone inclinometer apps work in a pinch but verify with a level before ordering material.
They describe the same steepness in different units. Pitch is the x/12 ratio (6/12), slope is the percentage (50%), and angle is degrees (26.6°). This calculator reports all three so you can use whichever your contractor or material supplier asks for.