Roof Pitch Calculator & Angle Converter

How to Use This Roof Pitch Calculator

Enter the rise (vertical distance) and run (horizontal distance) of your roof. For the standard level method, the run is 12 inches and the rise is whatever you measured, so a 6-inch rise gives a 6/12 pitch. Enter a horizontal rafter run in feet (half the building width for a gable roof) to get the actual rafter length, and a footprint area to convert flat square footage into sloped roof surface area.

The math behind each result, where rise is r and run is b:

• Pitch = r ÷ b, normalized to X/12: pitch = r × 12 ÷ b
• Angle = arctan(r ÷ b) in degrees
• Slope percentage = (r ÷ b) × 100
• Pitch multiplier = √(r² + b²) ÷ b
• Rafter length = horizontal run × multiplier
• Roof surface area = footprint area × multiplier

What Is Roof Pitch? Rise Over Run Explained

Roof pitch is the steepness of a roof, written as the vertical rise over a 12-inch horizontal run. A 4/12 roof rises 4 inches for every 12 inches it travels horizontally, a 12/12 roof rises a full foot for every foot of run, which is exactly a 45-degree angle. The 12 is fixed as a convenient standard, so a pitch is always a single number like 4, 6, or 8.

Builders describe the same slope three ways, and each trade favors a different one. Carpenters talk in pitch, 6/12, because it matches the speed square markings and rafter tables they work from every day. Engineers and designers use the angle in degrees, 26.6 degrees for that same roof. Civil engineers and road builders use slope percentage, 50%, which is why a driveway sign reads percent while a roof plan reads rise over run. They all describe the same incline, and this calculator moves between all three instantly.

Roof Pitch Chart: Angle, Slope & Multiplier

Every pitch from a nearly flat 1/12 to a full 45-degree 12/12, with the angle, slope percentage, and multiplier for each.

Notice how the multiplier grows slowly at first and then accelerates. From 2/12 to 6/12 the surface area only grows about 10%, but from 8/12 to 12/12 it jumps another 18%. That is why steep roofs cost noticeably more, you are not just paying for taller walls, you are paying for measurably more roof material per square foot of house.

How to Measure Roof Pitch Safely

Method 1: From inside the attic

This is the safest and most accurate method when you have attic access. Find any rafter and hold a 12-inch level against its underside, perfectly horizontal. Measure straight up from the 12-inch mark to the rafter, that distance in inches is your rise, and the run is 12. A 6-inch measurement means a 6/12 roof. Do this in two or three spots, rafters in old homes are rarely perfectly uniform, and average the results.

Method 2: On the roof with a level

If you must measure from outside and the pitch is gentle enough to stand on, place a 12-inch level flat on the roof surface, holding it level, and measure from the 12-inch mark straight down to the shingles. The measured rise over 12 is your pitch. Never walk a roof steeper than 6/12 without a safety harness and roof anchors, and even on low slopes, soft-soled shoes and a spotter are the minimum.

Method 3: From the ground with a phone or speed square

Stand at the gable end far enough back to see the full roof profile, then sight the roof angle with a speed square held to your eye, or open a phone angle app and lay the phone against a straight edge aligned with the roof. Read the angle in degrees and enter it into the calculator by trial, adjusting the rise until the angle matches. A protractor app and a steady hand get you within a degree, close enough for estimating and ordering.

Pitch Multiplier & Area Conversion

The single most valuable number on this page is the pitch multiplier. When you order shingles, underlayment, or gutters, the supplier sells by roof surface area, but the number you have from a floor plan or a tape measure on the ground is the footprint area. The multiplier bridges the two.

For a 1,200 sq ft ranch house with a 6/12 gable roof: 1,200 × 1.118 = 1,342 sq ft of roof surface. That is 142 sq ft of shingles and underlayment you would miss if you ordered straight off the footprint. On a steep 12/12 roof the same footprint covers 1,200 × 1.414 = 1,697 sq ft, a 41% difference that would leave you hundreds of square feet short at the lumberyard. This calculator applies that multiplier for you in the roof surface area result.

Rafter Length: The Pythagorean Formula

Rafter length is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. For any pitch, rafter length = horizontal run × pitch multiplier. A 24-foot-wide gable roof has a 12-foot horizontal run on each side, and at 6/12 each rafter runs 12 × 1.118 = 13.42 feet before the ridge cut. Add the overhang to that number for the total board length, then add the tail, the portion that extends past the wall to form the eave.

The same triangle drives the birdsmouth and ridge cuts. The birdsmouth, the notch where the rafter seats onto the top plate, is cut at the pitch angle, and the ridge cut at the top is the complementary angle. Getting the pitch right is the first step to every cut after it, which is why a quick pitch conversion beats guessing with a speed square on a windy roof.

Minimum Roof Pitch by Material

Every roofing material has a minimum pitch below which water drains too slowly and leaks through seams or fasteners. Going below the minimum, or ignoring the underlayment requirements that come with low slopes, is the fastest way to void a warranty.

Asphalt shingles are the material where this bites people most often. A shed or addition built at 3/12 looks fine for a few years, then water backs up under the shingles during a hard rain and the decking rots from the top down. The fix at 3/12 is cheap insurance, a full self-adhered membrane under the shingles, but most builders skip it because the shingles hide it.

5 Roof Pitch Mistakes That Cost You

1. Ordering material by footprint instead of surface area. The footprint number is on every floor plan, so it is the number people reach for first, and it is always too small for anything but a dead-flat roof. Multiply by the pitch multiplier before ordering shingles, underlayment, or gutter runs, or you will be short by the full difference, over 40% on a 12/12 roof.
2. Measuring the rise from the wrong point. The rise is measured straight down from the level, not along the roof surface. Measuring along the slope, or from the eave to the ridge diagonally, inflates the number and gives you a steeper pitch than you actually have. Keep the tape vertical and the level horizontal, and measure the run along the level, not the roof.
3. Shingling below the minimum pitch. Below 2/12 asphalt shingles are not rated at all, and between 2/12 and 4/12 they need full underlayment. Shingling a 2.5/12 porch roof with standard felt will leak, and the warranty will not cover it. Switch to metal or membrane on very low slopes.
4. Ignoring the multiplier when cutting rafters. The rafter is always longer than the horizontal run, and the gap grows with pitch. Cutting a 12-foot rafter for a 12/12 roof leaves you about 5 feet short, the true length is 16.97 feet. Use the multiplier, or the rise and run through the Pythagorean theorem, never the run alone.
5. Treating pitch and angle as interchangeable units. 45 is a meaningful number in both systems but means completely different things. A 45% slope is a 9.5-degree, roughly 2/12 roof, while 45 degrees is a 12/12 roof. Mixing them up is how a builder ends up framing a nearly flat roof when the plan called for a steep one. Read the unit before you cut.