Roof Pitch Calculator — Pitch, Angle, Slope & Rafter Length
A roof slope has four names and they are all the same number: the pitch ratio a builder quotes, the degree angle an inclinometer reads, the percentage a spec sheet lists, and the multiplier that turns run into rafter. Enter whichever one you have and the other three follow, along with the rafter length for your actual building — half the span, plus the overhang, less half the ridge board. The section drawing is at the true angle, and the reference table is generated from the same math the calculator runs.
Private by design. Pitch, angle and rafter length are computed in your browser — nothing about your building is sent to a server.
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Roof pitch chart — angle, slope and rafter multiplier
Every common pitch from 1/12 to 12/12, with the degree angle, the slope percentage and the rafter multiplier. The multiplier column is the useful one: multiply any horizontal run by it to get the sloped length above that run, and multiply any plan area by it to get the real roof area. Every cell is computed from angle = arctan(rise ÷ 12) and multiplier = sqrt(rise² + 144) ÷ 12 — the same functions the calculator calls, so the chart cannot drift from the tool.
| Pitch | Ratio | Angle | Slope % | Rafter multiplier |
|---|---|---|---|---|
| 1/12 | 1:12 | 4.76° | 8.3% | 1.0035 |
| 2/12 | 1:6 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 1:4 | 14.04° | 25.0% | 1.0308 |
| 4/12 | 1:3 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 5:12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 1:2 | 26.57° | 50.0% | 1.1180 |
| 7/12 | 7:12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 2:3 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 3:4 | 36.87° | 75.0% | 1.2500 |
| 10/12 | 5:6 | 39.81° | 83.3% | 1.3017 |
| 11/12 | 11:12 | 42.51° | 91.7% | 1.3566 |
| 12/12 | 1:1 | 45.00° | 100.0% | 1.4142 |
- A 12/12 pitch is 45° exactly, and its multiplier is √2 = 1.4142. That is the easiest sanity check on any roof pitch table: if 12/12 does not read 45°, the table is quoting rise over span rather than rise over run.
- Run is half the span on a symmetric gable. On a shed or lean-to roof the run is the full span, because there is no ridge in the middle — the calculator halves what you type, so enter twice the run for a shed roof.
- Overhang is a horizontal dimension on the plan. Add it to the run and then apply the multiplier; adding it to the finished rafter length gives a short soffit, by a factor of the multiplier.
- Walkability is a judgement, not a code line, but the practical breaks are around 4/12 (comfortable), 7/12 (jacks and harness start earning their keep) and 10/12 (nothing but the equipment holds you).
How to calculate roof pitch and rafter length
- Measure rise over a 12" run. Hold a level out from the roof surface, mark 12 inches along it, and measure straight down from that mark to the roof. That vertical distance in inches is the rise: 6 inches means a 6/12 pitch. If you only have a phone inclinometer, switch the calculator to degree mode instead.
- Read the four equivalents. Pitch, angle, slope percentage and rafter multiplier update together. The multiplier is the one that does real work — it converts any horizontal run into the sloped length above it, and it is also the roof-area factor for ordering material.
- Enter the building span, not the run. On a gable the ridge sits over the middle, so each rafter only covers half the span. Type the full outside-to-outside width and the calculator halves it for you — this is the single most common way rafter length gets doubled by mistake.
- Add overhang and deduct the ridge. Overhang is measured horizontally on the plan, so it is added to the run before the multiplier applies, not to the finished length. Half the ridge board thickness comes off, because the run is measured to the ridge centre line but the rafter stops at its face.
Frequently Asked Questions
- What is a 6/12 roof pitch in degrees?
- 26.57°. The conversion is angle = arctan(rise ÷ 12), so 4/12 is 18.43°, 6/12 is 26.57°, 8/12 is 33.69° and 12/12 is exactly 45°. The full table under the calculator is generated from that same formula, so it cannot drift from the tool.
- How do I find rafter length from roof pitch?
- Rafter length = horizontal run × the pitch multiplier, where the multiplier is sqrt(rise² + 144) ÷ 12. For a 6/12 pitch the multiplier is 1.1180, so a 12 ft run needs a 13.416 ft rafter. Remember the run is half the span on a gable, and add the overhang to the run before multiplying — not to the answer.
- What is the difference between roof pitch, slope and angle?
- Pitch is quoted as rise per 12 of run ("6/12"). Slope is the same thing as a percentage (6/12 = 50%). Angle is the degrees from horizontal (6/12 = 26.57°). Some trades also use "pitch" to mean rise over the full span rather than over the run, which halves the number — if a figure looks half of what you expect, that is usually why.
- What is the minimum roof pitch for asphalt shingles?
- 2/12 is the floor, and between 2/12 and 4/12 the IRC (R905.2.2) requires two layers of underlayment cemented together. Below 2/12 asphalt shingles are not permitted at all — that range needs a membrane: TPO, EPDM, modified bitumen or standing seam metal. The calculator flags which band you are in.
- How much extra roofing material does a steep roof need?
- Exactly the pitch multiplier. A 4/12 roof is 5.4% more surface than its footprint, a 6/12 is 11.8% more, and a 12/12 is 41.4% more. Multiply the plan area you are covering by the multiplier before converting to squares, or you will order short on every steep roof.
- Why deduct half the ridge board thickness?
- Because the run is measured from the outside of the wall to the centre line of the ridge, but the rafter stops at the ridge board's face. On a 1.5" ridge that is 0.75" off the run — small, but it is the difference between the ridge sitting where it was drawn and the whole roof creeping wide.
Use Cases
- Convert a measured rise and run into pitch, degrees and slope percentage
- Get rafter length for a gable including overhang and ridge deduction
- Check whether a slope is legal for asphalt shingles before ordering
- Work out how much extra roof area a steep pitch adds over the footprint
- Match a phone inclinometer reading to a standard pitch ratio
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