How do I calculate roof pitch?

Hold a level against the underside of a rafter, mark 12 in along it, and measure vertically from that mark up to the rafter. That reading is the rise per 12 in of run, so a 6 in reading is a 6:12 pitch. Using a 24 in level and marking 12 in halves the effect of any error in the vertical measurement.

What is a 6:12 pitch in degrees?

26.57 degrees. The conversion is the arctangent of the rise over 12, so arctan(6/12) = 26.57. A 4:12 is 18.43 degrees, an 8:12 is 33.69 and a 12:12 is exactly 45.

What is a 6:12 pitch as a percentage?

50%. Percent slope is rise over run, so 6 over 12 is 50%. Note that this is not halfway to vertical: 50% slope is 26.57 degrees. Percent reaches 100% at 45 degrees and keeps going above it, so a 24:12 roof is a 200% slope.

What does quarter pitch mean?

A 6:12 roof. The old form measures rise against the full span rather than the run, and the run is half the span, so a rise of a quarter of the span is 6 in per 12 in of run. By the same logic a half pitch is 12:12, not 6:12, which is where the confusion usually starts.

How long is a rafter for a 28 foot span at 6:12?

15.65 ft as a line length each side. Half the span is a 14 ft run, and the slope factor for 6:12 is 1.1180, so 14 x 1.1180 = 15.65 ft from the outside of the plate to the ridge centreline. Add the overhang at the same factor and take off half the ridge thickness.

What is the lowest pitch for a shingle roof?

Two units vertical in 12 horizontal, per IRC R905.2.2, and between 2:12 and 4:12 the underlayment must be doubled. Below 2:12 asphalt shingles are not permitted at all and the roof needs a different covering.

My measured pitch is 5.9:12. What is it really?

Almost certainly a 6:12. Roofs are cut in whole rises because that is how framing squares and rafter tables are marked, and a measurement is moved by sag, sheathing thickness and where the level sat. A reading within a tenth or two of a whole number is that number.

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Roof Pitch Calculator

Trigonometry, plus the IRC R905 thresholds the same pitch trips

Pitch is written four different ways and they are all the same angle. Rise per 12 is what a framing square is marked in, degrees is what a phone reports, percent is what a civil drawing uses, and rise over span is what an older builder means by a quarter pitch. Convert between them here, get the rafter length, and see the three points where the code changes its mind about your roof.

Roof pitch, in whichever form you have it

What do you have

Rise (per 12, or measured)

Level length you measured over

Angle

deg

Percent slope

Run to work out a rise over (0 to skip)

Span for a common rafter (0 to skip)

Rafter lengths are line lengths along the slope, plate to ridge centreline. Overhang and the ridge deduction are not included.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

One angle, four ways of writing it

Worked example

A rise of 12 in measured over a 24 in level, on a house spanning 28 ft.

Pitch. 12 in over 24 in is 6 in over 12, so 6:12.
Angle. arctan(6/12) = 26.57 degrees.
Percent. 6/12 = 50%. Note that 50% slope is not 45 degrees, which catches people out.
Slope factor. √(144+36)/12 = 1.1180, so 11.8% more roof than footprint.
Common rafter. Half of 28 ft is a 14 ft run, times 1.1180 = 15.65 ft, with the ridge 7 ft above the plate.

The percent figure is the one that trips people. A 50% slope sounds like halfway to vertical and it is 26.57 degrees, nowhere near it. Percent is rise over run, so 100% is 45 degrees and there is no upper bound: a 24:12 roof is 200%. Degrees and percent are not interchangeable and a civil drawing marked 2% is a drainage fall, not a roof.

The formula

One angle, four notations, and the conversions between them:

θ = arctan(r/12) percent = 100 r/12 factor = √(144 + r²)/12

r: rise in inches per 12 in of run. The framing form
θ: the angle from horizontal, in degrees
percent: rise over run as a percentage. 100% is 45 degrees, not vertical
factor: slope length over horizontal length. Multiplies footprint into roof area
rise over span: the old form. A 6:12 is a quarter pitch, because the rise is a quarter of the span

The fourth notation is the one that causes arguments. A quarter pitch is 6:12, not 3:12, because it is measured against the whole span rather than the run, and the run is half the span. When somebody old says half pitch they mean 12:12.

Four names for one angle

Roof slope is written in at least four ways and each trade has its favourite. A framing carpenter says 6:12 because that is how a framing square is marked and how a rafter is cut. A phone inclinometer says 26.57 degrees. A civil engineer says 50%. And a builder over seventy says quarter pitch.

They are all the same roof. The conversions are elementary trigonometry and this page does them in both directions, so a pitch measured with a phone comes out as something you can mark on a square.

The one that causes real confusion is percent, because it looks like a fraction of vertical and is not. Percent slope is rise over run, so it passes 100% at 45 degrees and keeps going. A 24:12 roof is a 200% slope.

And rise over span is the oldest and the most misread. It is measured against the full span rather than the run, and since the run is half the span, a quarter pitch is 6:12 and a half pitch is 12:12.

Measuring a pitch you cannot climb on

From inside the attic, hold a level against the underside of a rafter, mark 12 in along it, and measure straight up from that mark to the rafter. That reading is the rise per 12, and it is the most reliable method because nothing is in the way.

Use a longer level if you have one. A 24 in level with a 12 in mark halves the effect of any error in the vertical reading, which is why it is the standard site method. Measure over 12 in on a short torpedo level and a sixteenth of an inch of error is a sixteenth of a pitch.

From outside, a level held against the rake board works the same way. A phone inclinometer laid on a rafter or a straight board is fine and usually within a few tenths of a degree, which is far better than the roof was built to.

Do not measure on the shingles if you can avoid it. They are not flat, and a course butt under the level will read a pitch that is not there.

Why your measurement is not a whole number, and why it probably is one

Roofs are designed and cut in whole rises: 4:12, 6:12, 8:12. Framing squares are marked that way and rafter tables are written that way.

What you measure will not be whole. Sag over a long span, sheathing thickness, a level bridging two rafters at slightly different heights, and the shingles themselves all move the reading by a tenth or two.

So if the calculator reports 5.9:12, the roof is a 6:12. The calculator says so, because a measured pitch landing near a whole number almost always is that number, and cutting a rafter to 5.9 would be an odd thing to have done deliberately.

The exception is a roof that was framed to a span rather than a pitch, or one that has been altered. Then the odd number is real and it matters, because a new rafter has to match what is there rather than what the square says.

Rafter length, and what this page leaves off it

The rafter length here is the line length: the distance along the slope from the outside of the wall plate to the centreline of the ridge. It is the run times the slope factor, which is why the two numbers appear together.

Two adjustments are not included and both matter on the saw.

The overhang adds to it, at the same slope factor. A 24 in horizontal overhang on a 6:12 adds 26.8 in of rafter, not 24.

Half the ridge thickness comes off it. A rafter runs to the ridge centreline in the arithmetic and stops at the face of the ridge board in reality, so a 1 1/2 in ridge takes 3/4 in off each rafter, measured horizontally and then converted at the slope factor.

Birdsmouth depth, plumb cuts and any drop for a bearing detail are cutting questions this page does not touch.

Frequently asked questions

How do I calculate roof pitch?: Hold a level against the underside of a rafter, mark 12 in along it, and measure vertically from that mark up to the rafter. That reading is the rise per 12 in of run, so a 6 in reading is a 6:12 pitch. Using a 24 in level and marking 12 in halves the effect of any error in the vertical measurement.
What is a 6:12 pitch in degrees?: 26.57 degrees. The conversion is the arctangent of the rise over 12, so arctan(6/12) = 26.57. A 4:12 is 18.43 degrees, an 8:12 is 33.69 and a 12:12 is exactly 45.
What is a 6:12 pitch as a percentage?: 50%. Percent slope is rise over run, so 6 over 12 is 50%. Note that this is not halfway to vertical: 50% slope is 26.57 degrees. Percent reaches 100% at 45 degrees and keeps going above it, so a 24:12 roof is a 200% slope.
What does quarter pitch mean?: A 6:12 roof. The old form measures rise against the full span rather than the run, and the run is half the span, so a rise of a quarter of the span is 6 in per 12 in of run. By the same logic a half pitch is 12:12, not 6:12, which is where the confusion usually starts.
How long is a rafter for a 28 foot span at 6:12?: 15.65 ft as a line length each side. Half the span is a 14 ft run, and the slope factor for 6:12 is 1.1180, so 14 x 1.1180 = 15.65 ft from the outside of the plate to the ridge centreline. Add the overhang at the same factor and take off half the ridge thickness.
What is the lowest pitch for a shingle roof?: Two units vertical in 12 horizontal, per IRC R905.2.2, and between 2:12 and 4:12 the underlayment must be doubled. Below 2:12 asphalt shingles are not permitted at all and the roof needs a different covering.
My measured pitch is 5.9:12. What is it really?: Almost certainly a 6:12. Roofs are cut in whole rises because that is how framing squares and rafter tables are marked, and a measurement is moved by sag, sheathing thickness and where the level sat. A reading within a tenth or two of a whole number is that number.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 9 Roof AssembliesSeattle Department of Construction and Inspections. Prints IRC R905.2.2, which sets the 2:12 minimum for asphalt shingles and the doubled underlayment between 2:12 and 4:12, and R905.2.6, which hands attachment to the manufacturer above 21:12.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Conversions are exact, measurements are not. Rafter lengths are line lengths from the plate to the ridge centreline, with no overhang added and no ridge deduction taken. The code thresholds shown are from IRC Chapter 9 as printed in the 2018 Seattle Residential Code, read 8 September 2026, and your jurisdiction may have amended them.