Roof Pitch Calculator (2026) – Find Pitch, Angle & Rafter Length

Roof pitch calculators take your roof's run and rise measurements and convert them to a pitch ratio, an angle expressed in degrees, or a slope percentage. They also calculate the rafter length. The calculator will handle the rest. You don't need to calculate four different formulas. This page explains what each result means, and how you can use it to frame a roof, order materials, or discuss specs with a builder.

What Is a Roof Pitch Calculator ?

Roof pitch is the steepness or slope of a roof. It's measured in inches for every 12-inch horizontal span. For example, a 6:12 pitch rises six inches every 12 inches. Roof slope is described this way everywhere in America, regardless of whether it's a home, a garage, or a backyard shed.

Pitch and slope are often confused, but the difference is really only academic. Slope equals rise divided by run. Pitch, technically speaking, is rise divided by the full span, which is just the run doubled. Roofers, contractors, and suppliers use both terms to describe the same thing on the jobsite. This guide follows that convention.

A 4:12 pitch is about 18.4°. A pitch of 6:12 is roughly 26.6°. These two pitches are enough to cover the majority of homes in the US. The vast majority of roofs fall between 4:12 and 9:12.

How to Calculate Roof Pitch Using a Roof Pitch Calculator

You can calculate the X:12 pitch by dividing the rise by the run. Then multiply the result by 12. If your roof rises 5 inches over a 12-inch run, then you have a 5:12 pitch. This is the fastest manual method, and that's what the calculator does when you click calculate.

The formula is written below:

Pitch ratio = (Rise ÷ Run) × 12

Skip the step of multiplying by 12 if you want a percentage instead. Divide the rise by the run, then multiply by 100. A roof that rises 6 inches for every 12 inches of run has a slope of 50 percent.

Step	Action	Example
1	Measure rise in inches	5 inches
2	Measure run in inches	12 inches
3	Divide rise by run	5 ÷ 12 = 0.417
4	Multiply by 12 for the ratio	5:12
5	Multiply by 100 for the percent	41.7%

How do I measure the rise and run on my own roof?

This measurement can be taken from the roof or the attic. You can choose whichever is more convenient or safer.

Hold the level against the underside of a rafter and stretch it out 12 inches exactly from a fixed point. Measure straight down from the 12-inch mark to the top of a rafter. This distance is the rise.

From the roof surface, lay the level flush against the roof, mark off 12 inches, then measure straight down to the surface beneath that mark. The same result will be obtained by measuring the rise per 12 inches. If you don't feel comfortable getting on top of the roof, you can use a smartphone app to measure the pitch from the ground.

For the most accurate roof pitch calculator result, measure both rise and run in the same unit before entering the numbers.

How do I convert roof pitch to degrees?

You can find your angle by dividing the rise by the run and taking the arctangent of that number. This is arctan(6/12) for a pitch of 6:12. It comes out at 26.6 degrees. It's a common conversion because structural engineers and architects tend to think of pitch in degrees. At the same time, people who are actually cutting the lumber or ordering the shingles use the X:12 ratio.

Pitch	Angle	Slope %
1:12	4.8°	8.3%
2:12	9.5°	16.7%
3:12	14.0°	25%
4:12	18.4°	33.3%
5:12	22.6°	41.7%
6:12	26.6°	50%
7:12	30.3°	58.3%
8:12	33.7°	66.7%
9:12	36.9°	75%
10:12	39.8°	83.3%
12:12	45.0°	100%

The pitch of a 12:12 roof is 45 degrees. This is where the rise and run are equal. It is rare to find anything steeper than 12:12, except for A-frame cabins or decorative rooflines.

How do I calculate the length of a rafter from its pitch?

The Pythagorean theorem gives us the rafter length, as rise, run, and the rafter form a right-angled triangle. Rafter length is equal to the square root of rise squared plus run squared. For a 6:12 pitch, this works out to a multiplier of 1.118. So a 10-foot run gives you an approximate rafter length of 11.18 feet.

It's important to know that the number you get from this calculator is the length of the line, measured along the top of the rafter from the centre of the ridge outwards to the wall line. You're not done yet. Before you touch the saw, there are two more adjustments to make.

Add the overhang first. Since the overhang extends beyond the wall line, it is not included in the basic run measurement. It must be added before applying the pitch multiplier. Then subtract the thickness of the ridge beam. The rafter meets the side of the ridge board, not its centre. Therefore, subtract half the beam's thickness from the top. For a 1.5-inch ridge board, subtract 0.75 inches.

Find the pitch, angle, and rafter length using this example.

Let's do a complete calculation from start to finish using real numbers.

Imagine that a roof rises 5 inches over a 12-inch run.

Step 1: Pitch ratio. Five divided by 12, times 12, gives you 5:12.

Step 2: Angle. Arctan of 5 divided by 12 comes out to about 22.6 degrees.

Step 3: Slope percentage. Five divided by 12, times 100, equals 41.7 percent.

Step 4: Pitch multiplier. Square root of (5 squared plus 12 squared), divided by 12, equals approximately 1.083.

Step 5: Apply the multiplier to your actual run. With a 10-foot total run, that's 10 times 1.083, or 10.83 feet of rafter line length.

Step 6: Add the overhang. Say it's 1.5 feet. Ten feet plus 1.5 feet is 11.5 feet, times 1.083, which gives you 12.45 feet.

Step 7: Subtract the ridge beam. Half the thickness of a 1.5-inch beam is 0.75 inches, or 0.0625 feet. This brings the final rafter to approximately 12.39 feet.

[Diagram: A labelled triangle with rise, run and rafter. The angle is marked at the bottom, and the pitch ratio is written on the rafter's line]

What Roof Pitch is considered walkable?

In general, anything 6:12 or below is considered walkable. Most homeowners and contractors will use this line to decide whether or not a roof jack or harness is worth it.

According to OSHA roof safety guidelines, workers should use proper fall protection systems when working on steep-slope roofs or at heights where fall hazards exist.

Pitch	Walkability
Up to 6:12 (26.6°)	No special precautions are needed to walk on the surface
7:12 (30.3°)	Manageable but with caution
8:12 to 10:12 (33.7° to 39.8°)	Equipment recommended for walkable but potentially dangerous terrain
Above 10:12	Not possible to walk without scaffolding and a harness

Is it safe to walk without a harness on a roof pitch of 7:12? In theory, yes, but only for experienced climbers in dry weather. Safety guidelines recommend a harness and anchors once you pass 6:12. In wet or icy conditions, even that pitch is more dangerous.

What is the minimum roof pitch for snow to slide off?

The most common figure for the minimum roof pitch required to shed snow is between 6:12 and 7:12, or approximately 30 degrees. This is not a rule that must be followed. Roofs with a slope as low as 10 degrees can shed snow on their own, depending on the texture of the roofing surface and the amount of sun the roof receives.

It's better to go steeper than the minimum slope if you live in an area that receives a lot of snow. This will reduce the structural load on the house and the risk of ice damming at the eaves, because less snow will pile up.

What roof pitch works with my roofing material?

Each roofing material has its own minimum pitch. This is because water needs to drain before it can get underneath. Asphalt shingles can be installed as low as 2:12, but only if a waterproof underlayment is added. Standard installation instructions assume that the slope will be steeper for ordinary use.

Material	Minimum Pitch
Standing seam metal	1:12
TPO or EPDM membrane	1:12 (often used below 2:12)
Corrugated metal	3:12
Asphalt shingles	2:12 with additional underlayment
Wood shake	4:12
Clay or concrete tile	4:12
Slate	4:12

Can you shingle roofs with a 3:12 slope? Yes. A 3:12 pitch is above the minimum standard for asphalt shingles, and most manufacturers won't require additional underlayment at that slope. It's still worth checking the packaging, as brand requirements can vary.

What should I do with my calculator results?

Check your pitch for material compatibility first, as it will determine your options. If your number lands right at a material's minimum, lean toward treating it as the steeper interpretation rather than rounding down. Being a little short on slope causes a lot more headaches than being a little over.

Budget extra time and labour before calling contractors if your pitch is above 7:12. When harnesses and roof jacks are involved, most crews will charge more, and the work progresses more slowly on a steep slope. When ordering rafters, don't forget to add the overhang and subtract the ridge beam thickness. The calculator gives you a number that represents the line length, not your final cut.

What are the common mistakes people make when calculating roof pitch?

The big one is measuring the full roof span instead of the run. This doubles the rise, making your pitch appear twice as steep. The run is half the roof span, measured from the centre ridge to the outer wall. It's never the entire width.

People also make mistakes when they mix up units in the middle of a calculation, such as measuring rise in inches but run in feet without first converting. The calculator's rafter number is often misinterpreted as the final cut length, when it's actually the line length before overhang and ridge beam adjustments. Another mistake to watch out for is assuming that a single pitch applies to the entire roof. Porches, dormers, or additions are often at a different angle from the main structure and require their own separate measurement.

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