Roof Pitch Calculator
Enter any measurement you have — get pitch, angle, slope factor and rafter lengths.
Roof area (optional)
Both planes: (span ÷ 2 + overhang) × 2 × length × slope factor. 1 square = 100 ft².
Roof elevation
Need rafter length with birdsmouth and overhang? → Rafter calculator
Checks
- Slope class: ConventionalPass
4/12–9/12 — suits most roofing materials.
- WalkabilityPass
Walkable up to 6/12 with normal care.
- Asphalt shinglesPass
Meets the 2/12 minimum (IRC R905.2.2).
- Wood shinglesPass
Meets the 3/12 minimum (IRC R905.7.2).
- Wood shakesPass
Meets the 4/12 minimum (IRC R905.8.2).
- SlatePass
Meets the 4/12 minimum (IRC R905.6.2).
- Clay / concrete tilePass
Meets the 2.5/12 minimum (IRC R905.3.2).
- Standing-seam metalPass
Meets the 1/4:12 minimum (IRC R905.10.2).
- Lapped metalPass
Meets the 3/12 minimum (IRC R905.10.2).
Confirm with local code and the manufacturer.
How to measure roof pitch
Pick the method that matches what you can reach, then choose the same mode in the calculator.
- From the attic (most accurate, no climbing): hold a level flat against the underside of a rafter, mark 12" along the level, and measure straight up to the rafter. That number is your pitch — 6" means 6 in 12. Use Rise & run.
- With a longer level: rest one end on the roof or rafter, level it, and measure the height at the free end. A 24" level with 12" at the end is 6 in 12. Use Level method and pick your level length.
- From plans or wall measurements: take the building span (outside of wall to outside of wall) and the ridge height above the wall plate. Use Span & height — the calculator halves the span for you, so don't halve it yourself.
- With a phone inclinometer app: lay the phone on the rake board or a rafter and read the angle. Use Angle.
- From the ground: compare the gable end with the pitch silhouettes under the slider. It's an estimate, good enough to know whether a roof is low, conventional or steep — measure before you order material.
How the roof pitch calculation works
Pitch is the rise of the roof for every 12 inches of horizontal run. Run is measured level, not along the roof surface; on a symmetrical gable it is half the span. The calculator converts whatever you enter into rise per 12, then derives everything else from that one number.
The angle is the arctangent of rise over run. The slope factor is the length of the roof surface per unit of run — multiply any run by it to get rafter length, or multiply the building footprint by it to get the sloped roof area. Hip and valley rafters run diagonally across the corner, so they cover more length per foot of building run than a common rafter.
Formulas
pitch (X in 12) = rise ÷ run × 12— rise and run in the same unit.angle = arctan(X ÷ 12)— degrees from horizontal. Back to pitch: X = 12 × tan(angle).percent grade = X ÷ 12 × 100.slope factor = √(1 + (X ÷ 12)²)— the rafter or area multiplier.common rafter per foot of run = √(X² + 144)inches.hip or valley per foot of run = √(X² + 288)inches.rafter length = run × slope factor— the line length from the plate to the ridge centerline, before overhang and ridge deductions.sloped roof area = (span ÷ 2 + overhang) × 2 × building length × slope factor; roofing squares = area ÷ 100 ft².
Reading the results
Pitch is how framers, roofers and codes talk about slope; degrees are for saws, solar racking and inclinometer apps; percent grade is used on some plans and in metric countries. The speed square value tells you which number to pivot to on the COMMON scale to mark plumb cuts.
Below 2 in 12 a roof counts as flat and needs a membrane. Between 2 and 4 in 12 is low slope: asphalt shingles are allowed only with extra underlayment. Most houses sit between 4 and 9 in 12. Above 9 in 12 the roof is steep — slower to work, more material, and fall protection on the job.
The material checks follow the minimum slopes in the International Residential Code (IRC section R905). Manufacturers can require more than code, and your local code may differ, so confirm before you order.
Roof pitch chart
| Pitch | Angle | Grade | Slope factor | Common per ft | Hip/valley per ft |
|---|---|---|---|---|---|
| 4.8° | 8.3% | 1.003 | 12 1/16" | 17" | |
| 9.5° | 16.7% | 1.014 | 12 3/16" | 17 1/16" | |
| 14.0° | 25.0% | 1.031 | 12 3/8" | 17 1/4" | |
| 18.4° | 33.3% | 1.054 | 12 5/8" | 17 7/16" | |
| 22.6° | 41.7% | 1.083 | 13" | 17 11/16" | |
| 26.6° | 50.0% | 1.118 | 13 7/16" | 18" | |
| 30.3° | 58.3% | 1.158 | 13 7/8" | 18 3/8" | |
| 33.7° | 66.7% | 1.202 | 14 7/16" | 18 3/4" | |
| 36.9° | 75.0% | 1.250 | 15" | 19 3/16" | |
| 39.8° | 83.3% | 1.302 | 15 5/8" | 19 11/16" | |
| 42.5° | 91.7% | 1.357 | 16 1/4" | 20 1/4" | |
| 45.0° | 100% | 1.414 | 17" | 20 13/16" | |
| 49.4° | 116.7% | 1.537 | 18 7/16" | 22" | |
| 53.1° | 133.3% | 1.667 | 20" | 23 5/16" | |
| 56.3° | 150.0% | 1.803 | 21 5/8" | 24 3/4" | |
| 63.4° | 200.0% | 2.236 | 26 13/16" | 29 3/8" |
Worked example
A garage is 28 feet wide and the ridge sits 7 feet above the wall plates, with a 1-foot overhang and a 40-foot length. The run is half the span, 14 feet, so the pitch is 7 ÷ 14 × 12 = 6 in 12, or 26.6°. The slope factor for 6 in 12 is 1.118, so the rafter from plate to ridge is 14 × 1.118 = 15' 7 13/16". With the overhang each side measures 15 × 1.118 = 16' 9 1/4" along the slope. The sloped area is 15 × 2 × 40 × 1.118 ≈ 1,342 ft², or about 13.4 squares before waste. At 6 in 12 asphalt shingles, metal and tile all meet code minimums; wood shakes and slate need 4 in 12, so they pass too.
Frequently asked questions
What is a 6/12 roof pitch in degrees?
A 6/12 pitch rises 6 inches for every 12 inches of run, which is 26.6 degrees or a 50% grade. It is one of the most common pitches on houses and still comfortable to walk on for most roofers.
How do I convert roof pitch to degrees?
Divide the rise by 12 and take the arctangent: 8/12 is arctan(0.667) = 33.7°. To go from degrees back to pitch, multiply 12 by the tangent of the angle: 30° is 12 × 0.577 = 6.9, just under 7 in 12.
What is the minimum roof pitch for shingles?
Code allows asphalt shingles down to 2 in 12, but between 2 and 4 in 12 they need two layers of underlayment. Most shingle manufacturers treat 4 in 12 as the standard minimum, so check the product's installation instructions. Flatter roofs need a membrane or standing-seam metal.
How do I find roof pitch without getting on the roof?
Measure from the attic with a level and tape — it is the safest and most accurate method. From outside, a phone inclinometer app on the rake board gives the angle, and comparing the gable to the pitch silhouettes above gives a quick estimate.
Is roof pitch the same as roof slope?
In everyday US use, yes — both mean rise per 12 inches of run. Strictly, pitch is rise divided by the full span, so a 6 in 12 slope is a 1/4 pitch. This calculator uses the everyday meaning, X in 12.
How does pitch change the amount of roofing I need?
A steeper roof has more surface over the same footprint. Multiply the footprint by the slope factor: 1.054 at 4/12, 1.118 at 6/12, 1.414 at 12/12. The Roof area card does this for you and converts it to squares.
What roof pitch is walkable?
Up to about 6 in 12 most people can walk a dry roof; from 7 to 9 in 12 roofers use roof jacks or toe boards; beyond that it's harness work. OSHA treats any roof steeper than 4 in 12 as steep for fall-protection rules on job sites, so follow your local safety requirements.