Roof Pitch Calculator
Measure the rise over a foot of run and this gives you the pitch as X:12, the angle in degrees, the slope as a percentage, and the pitch multiplier you need before ordering any roofing material. Enter a span and it works out the rafter length too.
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How roof pitch is measured
Roof pitch is written as a ratio of rise to run — how many inches the roof climbs for every 12 inches it travels horizontally. A 6:12 roof rises 6 inches per foot. The run is always 12 by convention, which is why pitch is spoken as "six twelve" rather than as a fraction.
The field method needs a 2-foot level and a tape. Hold the level dead horizontal against the roof surface or a rafter, mark 12 inches along it from the end touching the roof, then measure straight down from that mark to the roof. That vertical distance is the rise. If you can only reach a rafter in the attic, measure the same way against its underside — the pitch is identical.
Formula: pitch = rise ÷ run × 12, and angle = arctan(rise ÷ run)
Pitch, angle, slope and multiplier are four names for one thing
Every trade describes the same steepness differently, and material is ordered in whichever one the supplier uses:
| Pitch | Angle | Slope | Multiplier | Typical use |
|---|---|---|---|---|
| 1:12 | 4.8° | 8.3% | 1.003 | Membrane only — porches, additions |
| 2:12 | 9.5° | 16.7% | 1.014 | Minimum for shingles, doubled underlayment |
| 3:12 | 14.0° | 25.0% | 1.031 | Low-slope metal, doubled underlayment |
| 4:12 | 18.4° | 33.3% | 1.054 | Standard shingle installation starts here |
| 5:12 | 22.6° | 41.7% | 1.083 | Very common on ranch houses |
| 6:12 | 26.6° | 50.0% | 1.118 | The most common residential pitch |
| 7:12 | 30.3° | 58.3% | 1.158 | Edge of comfortable walking |
| 8:12 | 33.7° | 66.7% | 1.202 | Needs roof jacks or a harness |
| 9:12 | 36.9° | 75.0% | 1.250 | Steep slope, surcharge territory |
| 10:12 | 39.8° | 83.3% | 1.302 | Steep slope |
| 12:12 | 45.0° | 100.0% | 1.414 | A-frames, dormers, some Tudor styles |
Note that slope percentage and pitch are not the same number even though people use the words interchangeably. A 6:12 roof is a 50% slope, not a 6% one.
The multiplier is the number that costs you money
The footprint of a house is not the area of its roof. A 40 × 30 ft rectangle is 1,200 square feet on the ground, but at 6:12 the two sloping planes add up to 1,200 × 1.118 = 1,342 square feet of roof — an extra 142 square feet, or nearly a square and a half of shingles. Order to the footprint and you come up short by that much.
Formula: multiplier = √(rise² + run²) ÷ run, which for a 12-inch run is √(1 + (rise ÷ 12)²)
The same multiplier converts a horizontal run into a rafter length, because a rafter is the hypotenuse of exactly that triangle. For a gable, the run is half the span: a 30 ft span at 6:12 has a 15 ft run, and the common rafter is 15 × 1.118 = 16.77 ft, or about 16 ft 9 in, before the overhang is added and the birdsmouth is cut.
Pitch decides what you are allowed to put on the roof
- Under 2:12 — asphalt shingles are not rated. Water moves too slowly and wind drives it back under the tabs. Use a membrane system.
- 2:12 to 4:12 — shingles are permitted with a doubled underlayment. Most manufacturers make the warranty conditional on it.
- 4:12 and above — standard shingle installation, single underlayment layer.
- Above 6:12 — walkability ends for most people. Roof jacks, staging or a harness become part of the job, and labor is the line item that moves, not material.
- Above 9:12 — steep slope. Expect a surcharge and a slower schedule, and remember the multiplier is now adding 25% or more to the material order.
Frequently asked questions
What is roof pitch?
Roof pitch is how steep the roof is, written as the number of inches it rises for every 12 inches of horizontal run. A 6:12 pitch rises 6 inches per foot. It is the same quantity as the roof angle and as the slope percentage, just expressed in the units roofers use.
How do I measure roof pitch with a level?
Hold a 2-foot level horizontally against the roof surface or the underside of a rafter, mark 12 inches along it from the end that touches the roof, and measure vertically from that mark down to the roof. That measurement in inches is the rise, and the pitch is that number over 12. Doing it from a rafter in the attic gives the same answer without getting on the roof.
What is a 6/12 roof pitch in degrees?
A 6:12 pitch is 26.6 degrees from horizontal, which is a 50% slope. It is the most common residential roof pitch in the US and it carries a pitch multiplier of 1.118, meaning the roof surface is about 12% larger than the footprint underneath it.
What is the minimum roof pitch for shingles?
Asphalt shingles are rated down to 2:12, and between 2:12 and 4:12 they require a doubled underlayment for the manufacturer warranty to hold. Below 2:12 shingles are not an approved covering at all — that range needs a membrane roof such as EPDM, TPO or modified bitumen.
What roof pitch is walkable?
Up to about 6:12 most roofers will walk the roof in ordinary boots. From 7:12 the surface gets away from you and roof jacks, staging or a harness become necessary, which is why quotes step up at that point. Above 9:12 nearly every contractor charges a steep-slope surcharge.
How do I convert roof pitch to a multiplier?
Divide the rise by 12, square it, add 1, and take the square root. For a 6:12 roof that is √(1 + 0.25) = 1.118. Multiply the flat footprint area by that figure to get the true roof area before ordering shingles, metal panels or underlayment.