Roof Pitch Calculator

What roof pitch means

Roof pitch is how much a roof rises for every 12 inches it runs horizontally. A 6/12 pitch (said "six in twelve") climbs 6 inches for every foot you move in from the wall toward the ridge. It's written as a fraction, x/12, because American framing squares, roofing tools and building codes all use a 12-inch run.

The same steepness can be described three other ways: as an angle (6/12 is 26.57°), as a percent grade (6/12 is 50%), or as a ratio (1:2). Roofers and shingle makers almost always use x/12. Engineers, metal roofing manuals and some plans use degrees or percent. If you have a number in one of those formats and need the others, our roof slope calculator converts between all of them and checks which roofing materials are allowed at your slope.

This calculator goes the other way: it starts from what you can measure on a real house and turns it into pitch, rafter length, roof area and materials.

How to measure roof pitch (3 ways)

1. From the attic (safest)

1. Mark a point exactly 12 inches along a level on the underside of a rafter. Hold a torpedo level against the rafter so its end touches the rafter.
2. Hold the level perfectly level (bubble centered) with one end touching the bottom edge of the rafter.
3. At the 12-inch mark on the level, measure straight up to the rafter. That measurement in inches is your rise. 6 inches means a 6/12 pitch.

2. On the roof or a ladder at the eave

1. Rest one end of a 2-foot level on the shingles at the roof edge, with the level running up the slope.
2. Lift the low end until the bubble is centered, then measure from the shingles to the bottom of the level at the 12-inch mark.
3. Use the "Rise & run" tab with a 12-inch run. If you measured at 24 inches, enter 24 as the run.

Only do this if the roof is dry, low and walkable. At 8/12 and steeper, stay on the ladder or use the attic or ground method.

3. From the ground, using span and height

1. Measure the width of the house from wall to wall. That's the span.
2. Measure the height of the ridge above the top of the walls. You can count siding courses on a gable end (most lap siding shows 4 to 8 inches per course) or use a laser measure.
3. Enter both on the "Span & height" tab. For a gable or hip roof, the run is half the span. For a shed roof, the run is the full span.

Phone apps and a digital angle finder also work. Lay the phone on a rafter or the roof surface, read the angle, and enter it on the "Angle" tab. Our slope calculator can read your phone's tilt sensor directly.

Roof pitch chart: degrees, percent and multiplier

Use the multiplier to turn a house footprint into real roof area. A 2,000 sq ft footprint with a 6/12 roof has about 2,000 × 1.118 = 2,236 sq ft of roof surface, before overhangs and waste.

The formulas behind the calculator

A roof slope is a right triangle. The run is the bottom side, the rise is the upright side, and the rafter is the sloped side.

Pitch = rise ÷ run × 12
Angle = arctan(rise ÷ run)
Grade % = rise ÷ run × 100
Multiplier = √(1 + (rise ÷ run)²)
Rafter length = run × multiplier
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × multiplier

Worked example: a 28-foot-wide gable house with a ridge 7 feet above the walls has a run of 14 feet. 7 ÷ 14 = 0.5, so the pitch is 0.5 × 12 = 6/12. The multiplier is √(1 + 0.25) = 1.118. With a 1-foot overhang, each common rafter is (14 + 1) × 1.118 = 16.8 feet long. A 50 × 28 foot house then has (52 × 30) × 1.118 = 1,744 sq ft of roof, or about 17.4 squares before waste.

The area formula works for gable, hip and shed roofs alike, as long as every slope has the same pitch. Hip roofs need more waste, because every hip and corner means cut shingles.

How pitch changes what a roof costs

Pitch affects cost in two ways. A steeper roof has more surface to cover for the same house, and it's slower and more dangerous to work on.

• Up to 7/12: walkable for roofers. No steep-roof charge in most markets.
• 8/12 to 9/12: roof jacks and harnesses are usually needed. Expect about 10% more.
• 10/12 to 12/12: slow, fully harnessed work. Expect 20% or more on top.
• Steeper than 12/12: mansard-like slopes. Often 30% or more, and fewer crews will bid.
• Below 4/12: shingles need extra underlayment, and below 2/12 they aren't allowed at all. A membrane or standing seam roof is the usual answer.

The calculator applies these steep-roof adjustments to the installed price ranges we use in our roof replacement cost estimator. For patch jobs instead of a new roof, see roof repair cost per square foot.

Common roof pitches and where you'll see them

• 1/4:12 to 1/12: "flat" roofs on porches, additions and commercial buildings. They still need this slight fall to drain.
• 3/12 to 4/12: ranch houses, sheds, patio covers and many metal-roofed buildings.
• 5/12 to 7/12: the most common range for American houses. A good balance of drainage, attic space and cost.
• 8/12 to 9/12: colonials, Cape Cods and homes in snowy areas.
• 10/12 to 12/12: Tudors, Victorians and A-frames. A 12/12 roof is a 45° angle.

Steeper roofs shed snow and water faster and leave room for an attic room. Lower roofs cost less to build and repair, but they depend more on underlayment and flashing to stay dry.

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