How to Calculate Roof Pitch (Multiplier Chart + Simple Method)

To calculate roof pitch, measure the rise over a 12 inch level run. Hold a level flat against the roof or a rafter, mark 12 inches along it, then measure straight down from that mark to the roof surface. That vertical drop in inches is your rise. A 6 inch drop is a 6/12 pitch. From there, the roof pitch multiplier is the square root of (1 + (rise/12) squared), and that number is what converts your flat footprint into the real sloped area you have to cover.

That is the whole job. Two measurements and one formula. Everything below is detail, safety, and the chart you can just read off instead of doing the math.

What does 6/12 pitch mean?

Pitch is written as rise over run, and the run is always 12 inches. So 6/12 means the roof climbs 6 inches vertically for every 12 inches it travels horizontally. A 4/12 roof climbs 4 inches over that same 12 inches. A 12/12 roof climbs a full 12 inches, which is a 45 degree angle.

Roofers say it out loud as "six twelve" or "a six pitch." Nobody uses degrees on a job site. If someone asks what your pitch is, they want the rise number.

The reason the run is locked at 12 is convenience. It makes every roof comparable, and it makes the arithmetic easy, because rise divided by 12 is the only variable in the formula.

How do you measure roof pitch with a level?

You need a level (a 2 foot one is ideal), a tape measure, and a pencil. Here is the sequence.

• Step 1. Mark 12 inches from one end of your level with the pencil or a piece of tape.
• Step 2. Set that end of the level against the roof surface, or against the bottom edge of a rafter.
• Step 3. Bring the level up or down until the bubble is centered. It must be truly horizontal or the number is garbage.
• Step 4. Hold it steady. Measure straight down from your 12 inch mark to the roof surface below.
• Step 5. Read the tape. That distance in inches is your rise. Six inches means 6/12.

Round to the nearest whole inch. Roofs are built to whole pitches. If you get 5.75, you have a 6/12 with a bit of sag or a bumpy shingle under your level.

Measure in two or three spots on the same plane. If the numbers disagree by more than an inch, something is off, either your technique or the framing.

How do you measure roof pitch from the attic?

Go in the attic instead of on the roof. It is the same measurement, and it is far safer. You are standing on joists in a warm dry space instead of balancing on granules that act like ball bearings under your boots.

Find an exposed rafter. Hold the level horizontally against the underside or the side of the rafter, level the bubble, mark your 12 inches, and measure straight up or down to the rafter line. Same rise, same pitch. The rafter runs parallel to the roof plane, so the number matches.

Bring a headlamp, step only on the joists, and watch for nail points coming through the sheathing above you. That is the extent of the risk, and it beats a ladder every time.

Is measuring roof pitch from a ladder dangerous?

Yes. Working from a ladder or standing on a slope is where people get hurt, and a pitch measurement is not worth a fall. If you are going to do it from outside, do it from a ladder set at the eave, keep both feet on the rungs, keep your belt buckle between the rails, and measure the rake edge or the first foot of roof you can safely touch. Do not lean. Do not climb onto the deck to get a better spot.

If the roof is steep, wet, mossy, frosted, or more than one story up, stop. Use the attic, or hire someone. A licensed roofer arrives with fall protection and does this in two minutes.

What is the roof pitch multiplier?

The roof pitch multiplier is the number you multiply your flat footprint by to get the true sloped roof area. The formula is the square root of (1 + (rise/12) squared).

Why it exists is worth understanding, because it is the single biggest mistake people make ordering material. Your footprint is flat. It is what the house covers on the ground, measured as if the roof were a lid. But your roof is not flat. It tilts, so the actual surface is longer than the footprint underneath it. A flat measurement always under-counts your material. Always.

Think of a ramp over a step. The step is 12 inches deep. The ramp laid over it is longer than 12 inches, because it travels diagonally. Same idea, scaled up to a whole roof.

Here is the full chart, so you never have to run the square root yourself.

• 3/12 = 1.031
• 4/12 = 1.054
• 5/12 = 1.083
• 6/12 = 1.118
• 7/12 = 1.158
• 8/12 = 1.202
• 9/12 = 1.250
• 10/12 = 1.302
• 12/12 = 1.414

Our Roofing Calculator applies this for you. Enter length, width and pitch, and it returns true roof area and squares without any chart-reading.

How do you use the multiplier? (worked example)

Take a 2,000 sq ft footprint on a 6/12 roof.

2,000 x 1.118 = 2,236 sq ft of actual roof. That is 22.4 squares. A square is 100 sq ft, and standard shingles come 3 bundles per square, which is how you get from area to a shopping list. The full walkthrough is in how many bundles of shingles do I need.

Now watch what pitch alone does to that same house. At 4/12, that 2,000 sq ft footprint is 2,108 sq ft of roof. At 12/12, it is 2,828 sq ft. Same footprint. Same walls. Over 700 sq ft of difference, purely from slope.

That is why "how big is your house" is not the same question as "how big is your roof." Skip the multiplier on a steep roof and you will be short by several squares.

Getting the footprint itself right comes first, and that is covered in how to measure a roof for shingles. Pitch is step two.

What counts as a low, conventional, or steep roof?

Roofs sort into three rough categories, and the category changes how the job gets done.

• Low slope: under about 4/12. Shallow. Water drains slowly, so underlayment and flashing details matter more.
• Conventional: about 4/12 to 9/12. The normal range for most houses. Walkable for a pro with proper footwear.
• Steep: above about 9/12. Not walkable. Needs staging, harnesses, and roof jacks.

Steep roofs cost more to work on for a simple reason: everything takes longer and the safety requirements are real. They also eat more material, as that 12/12 number above shows.

Roofing is governed by local code. Minimum pitch for a given material, underlayment requirements, and ice barrier rules all vary by where you live. Check with your building department before you buy anything, and if the roof is steep or high, hire a licensed roofer.

Bottom line

Pitch is rise over a 12 inch run, measured with a level and a tape. Do it from the attic against a rafter if you can, because it is safer than a ladder. Then multiply your flat footprint by the pitch multiplier to get real roof area, because a flat number always under-counts. On a 2,000 sq ft footprint, the gap between 4/12 and 12/12 is over 700 sq ft of shingles.

Get your pitch, then run the numbers in the Roofing Calculator. It does the multiplier and hands you the squares.