Roof Pitch Calculator

This roof pitch calculator turns two measurements, rise and run, into every way roof slope is written: the roofer’s x/12 pitch, the angle in degrees, the slope as a percent, and the roof-area multiplier that converts a flat footprint into real square footage. Enter the rise and run in inches. Leave the run at 12 to read pitch directly, or enter any run you actually measured.

A 6-inch rise over 12 inches of run, the default, comes out to a 6/12 pitch, 26.6°, 50% slope, and a 1.118 multiplier.

Roof pitch formulas

Every number the calculator returns comes from the same right triangle of rise and run:

• Pitch (x/12) = (rise ÷ run) × 12
• Angle in degrees = arctan(rise ÷ run)
• Slope percent = (rise ÷ run) × 100
• Roof-area multiplier = √(rise² + run²) ÷ run

To convert an angle you already know into pitch, reverse it: pitch = 12 × tan(angle). A 30° roof is 12 × 0.577 = 6.93/12, so it is just under 7/12. A 35° roof is 12 × 0.700 = 8.40/12.

Worked examples

All results match what the roof pitch calculator shows.

Example 1: the classic 6/12

Rise 6 in, run 12 in.

• Pitch: (6 ÷ 12) × 12 = 6.0-in-12
• Angle: arctan(0.5) = 26.6°
• Slope: 0.5 × 100 = 50.0%
• Multiplier: √(36 + 144) ÷ 12 = 1.118

Example 2: measured with a 16-inch level

You hold a 16-inch level flat against a rafter and the gap at its end is 4 inches. Enter rise 4, run 16.

• Pitch: (4 ÷ 16) × 12 = 3.0-in-12
• Angle: 14.0°
• Slope: 25.0%
• Multiplier: 1.031

That is a low-slope roof. Asphalt shingles are allowed at 3/12, but only with the double underlayment the code requires below 4/12.

Example 3: an odd measurement from the attic

A rafter rises 7 inches over a 9½-inch level. Enter rise 7, run 9.5.

• Pitch: (7 ÷ 9.5) × 12 = 8.8-in-12
• Angle: 36.4°
• Slope: 73.7%
• Multiplier: 1.242

Roofs are framed to whole or half-inch pitches, so this is almost certainly a 9/12 roof with some measuring error. Re-measure with a longer level before ordering material.

Roof pitch chart: 1/12 to 12/12

Pitch	Angle	Slope %	Roof-area multiplier
1/12	4.8°	8.3%	1.003
2/12	9.5°	16.7%	1.014
3/12	14.0°	25.0%	1.031
4/12	18.4°	33.3%	1.054
5/12	22.6°	41.7%	1.083
6/12	26.6°	50.0%	1.118
7/12	30.3°	58.3%	1.158
8/12	33.7°	66.7%	1.202
9/12	36.9°	75.0%	1.250
10/12	39.8°	83.3%	1.302
11/12	42.5°	91.7%	1.357
12/12	45.0°	100.0%	1.414

Each multiplier is also the rafter length per foot of run, which is how the rafter calculator turns run into board length.

What is a 6/12 pitch?

A 6/12 pitch climbs 6 inches for every foot you move toward the ridge. On a house 28 feet wide, each side has a 14-foot run, so the ridge sits 14 × 6 ÷ 12 = 7 feet above the top of the walls. The roof surface is 11.8% larger than the footprint, which is why you cannot order shingles from floor-plan square footage.

6/12 sits in the middle of the conventional range. It sheds water and light snow, takes every common roofing material, and is workable for roofers with standard fall protection.

Which roofing materials suit which pitch

The International Residential Code sets minimum slopes for each roof covering. Manufacturers may require more, so check the product’s installation instructions too.

Roofing material	IRC minimum pitch	Notes	IRC section
Built-up, modified bitumen, single-ply (EPDM, TPO, PVC)	¼/12	The usual choice for flat and low-slope roofs	R905.9–R905.13
Standing-seam metal	¼/12	Lapped, non-soldered metal panels need 3/12 (½/12 with applied sealant)	R905.10.2
Mineral-surfaced roll roofing	1/12	Low cost; sheds, porches, outbuildings	R905.5.2
Asphalt shingles	2/12	Two layers of underlayment required from 2/12 up to 4/12	R905.2.2
Clay and concrete tile	2½/12	Two layers of underlayment required below 4/12	R905.3.2
Metal shingles	3/12		R905.4.2
Wood shingles and shakes	3/12		R905.7, R905.8
Slate	4/12		R905.6.2

Once you know the pitch, size the material with the roofing shingle calculator or the metal roofing calculator, and the underlayment layers with the roofing underlayment calculator.

Finding pitch from the ridge height

You do not need a 12-inch measurement to use the roof pitch calculator. Any rise and run in the same units works, which is useful when you can measure the whole roof but not a single rafter.

Say a shed roof climbs 5 feet over a 24-foot run. Convert both to inches: rise 60, run 288. The calculator returns 2.5-in-12, 11.8°, 20.8% slope, and a 1.021 multiplier. At 2½/12 that roof is too shallow for slate or wood shingles but fine for tile with double underlayment, asphalt shingles with double underlayment, or standing-seam metal.

Roof slope categories

• Flat and low-slope (under 4/12): OSHA treats roofs of 4/12 or less as low-slope for fall-protection rules. These roofs need membranes, roll roofing, metal, or shingles with extra underlayment.
• Conventional (4/12 to about 9/12): the range most US homes fall into. All common materials work.
• Steep (over about 9/12): sheds snow fast but needs roof jacks or staging for installers. OSHA defines any roof steeper than 4/12 as a “steep roof” for fall protection, so crews treat even a 6/12 roof with care.

How to measure roof pitch

From the attic (safest). Hold a level horizontally against the bottom edge of a rafter. Mark 12 inches along the level from where it touches the rafter. Measure straight up from that mark to the rafter. That distance is the rise per 12. If your level is longer or shorter than 12 inches, enter whatever length you used as the run.

From a ladder at the gable end. Hold the level against the rake board and measure the same way. Stay off the roof itself.

From the ground. A smartphone angle app placed on the rake board or a fascia return gives the angle. Convert with pitch = 12 × tan(angle), or enter rise = tan(angle) × 12 and run = 12.

From plans. Elevation drawings show a small triangle with the pitch, such as “6 / 12”, next to each roof slope.

Using pitch to estimate roof area

Multiply the flat footprint by the multiplier. Measure the footprint including overhangs, because shingles cover the eaves too.

A 50 × 30 ft footprint is 1,500 sq ft. At 6/12 that is 1,500 × 1.118 = 1,677 sq ft of roof, or 16.8 squares (one square = 100 sq ft). Add waste for hips, valleys, and starter courses on top of that. The roof area calculator does this in one step, and the hip roof calculator handles four-sided roofs.

Common roof pitch mistakes

• Measuring along the slope. The 12 inches must be level. Measuring 12 inches down the rafter gives a lower number and the wrong pitch.
• Reading pitch off the shingle surface. Sagging decks and layered reroofs can throw the reading off. Rafters are more reliable.
• Confusing pitch and percent. A “50% roof” is 6/12, not 5/12.
• Ordering from the floor plan. Footprint area ignores slope. On a 12/12 roof that underestimates material by 41.4%.
• Going below the material’s minimum. A 2/12 roof with standard single-layer underlayment and shingles will leak and void the warranty.

Minimum slopes and underlayment rules vary with local amendments and manufacturer instructions. Confirm the roofing system for your pitch with your local building department or roofing inspector before you buy materials.