Roof Gradient Calculator
Roof slope is described in several ways — 30 degrees, 7 in 12, 58% or 1 in 1.7 all mean roughly the same thing. Enter the gradient in any form, or the rise and run you measured, and the calculator converts it to every other form, works out the rafter length and true roof area, and compares it with typical minimum pitches for common roof coverings.
Roof gradient calculator
On a phone, lay the long edge along a rafter or roof photo’s slope line and press the button (your browser may ask for permission to use motion sensors).
Roof Pitch Reference Pack
Printable roofing reference set: roof pitch conversion chart, slope factor and roof area worksheet, minimum pitch reference for common coverings and a roof survey checklist.
- Pitch chart (PDF/XLSX)
- Roof area (XLSX)
- Minimum pitches (PDF)
- Survey checklist (PDF, DOCX)
Formats: PDF, XLSX, DOCX. Instant download after payment (link valid 72 hours, up to 5 downloads). AI-assisted: the templates were drafted with AI help and reviewed and laid out by Kedop.
$3.00 USD, one-time
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Ways to express a roof gradient
| Form | Meaning | Example (same roof) |
|---|---|---|
| Degrees | Angle between the roof and horizontal | 30.26° |
| Pitch x:12 | Inches of rise per 12 inches of run (US and Canada) | 7:12 |
| Rise over run | Any vertical rise divided by horizontal run | 7 / 12 |
| Percentage | Rise ÷ run × 100 | 58.3% |
| Ratio 1 in n | Run needed for one unit of rise (common for flat roofs and drains) | 1 in 1.71 |
All of these describe the same thing: the tangent of the roof angle. The calculator converts using tan and arctan, so results are exact rather than read from a rounded table.
Conversion formulas
- Degrees = arctan(rise ÷ run).
- Pitch in 12 = tan(angle) × 12.
- Percentage = tan(angle) × 100.
- Ratio 1 in n: n = run ÷ rise = 1 ÷ tan(angle).
- Slope factor = √(1 + (rise ÷ run)²) — multiply a horizontal run or plan area by it to get the sloping length or true area.
Common pitches
| Pitch (x:12) | Degrees | Percent | Slope factor |
|---|---|---|---|
| 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 |
| 12:12 | 45.0° | 100.0% | 1.414 |
| 18:12 | 56.3° | 150.0% | 1.803 |
How to use the calculator
- Choose what you know: rise and run, degrees, x in 12, percentage or a 1 in n ratio.
- Enter the value — rise and run can be in any unit as long as both are the same.
- Optionally enter the plan area of the roof slope to get its true area, or a horizontal run to get the rafter length.
- Choose your roof covering to compare the pitch with its typical minimum.
- On a phone, try the tilt button to take an approximate angle reading.
Worked example
A roofer measures 7 inches of rise over a 12-inch level. The calculator gives 30.26°, a 58.3% gradient, 1 in 1.71 and a slope factor of 1.1577. A rear slope covering a plan area of 600 sq ft therefore has a true area of about 695 sq ft, which is the figure to use when ordering tiles or shingles (plus waste). At 30° the pitch is comfortably above the typical minimums for interlocking concrete tiles (12.5°), slate (around 20°) and asphalt shingles (4:12, about 18.4°).
Minimum pitch for roof coverings
| Covering | Typical minimum | Notes |
|---|---|---|
| Plain clay or concrete tiles | 22.5° | Many plain tiles need 35° or more; check the product |
| Interlocking concrete tiles | 12.5° | Typical range 12.5–22.5° depending on the tile and exposure |
| Natural slate | 20° | Depends on slate size, lap and exposure; often 20–25° |
| Fibre cement slates | 20° | Typical manufacturer minimum |
| Asphalt shingles | 18.4° | 4:12 standard; 2:12–4:12 only with special underlayment (US codes) |
| Metal standing seam | 1.5° | Some systems as low as about 1°; check the system |
| Corrugated metal sheet | 5° | Varies with profile and end laps |
| Flat roof membrane (EPDM, felt, single-ply) | 1.4° | Aim for a finished fall of at least 1:80 (design about 1:40) |
These are typical values to help with early planning. The real minimum depends on the specific product, the headlap, the site’s exposure to wind-driven rain, and local building regulations — always use the manufacturer’s specification and ask a roofer or building control officer when in doubt.
Measuring a roof safely
You rarely need to climb on a roof to find its pitch. In the loft, hold a spirit level horizontally against the underside of a rafter, measure 12 inches (or 300 mm) along the level and then measure straight up to the rafter — that is the rise. From outside, an angle-finder app or the phone tilt reading on a photo of the gable end gives a close estimate. Working at height is dangerous; leave roof access to professionals with the right equipment.
Why pitch matters
Pitch affects which coverings you can use, how quickly water and snow run off, how much loft space you get, wind uplift, and the quantity of materials. Steeper roofs need more covering per square metre of floor, which is why the slope factor matters when estimating. In some areas, planning rules also limit ridge height, which indirectly caps the pitch on a wide building.
Frequently asked questions
How do I convert roof pitch to degrees?
Divide the rise by the run and take the arctangent: 6:12 is arctan(0.5) = 26.57°.
What is a 4:12 roof pitch in degrees?
About 18.43°.
What is a 30 degree roof in pitch?
About 6.93:12, or roughly 7 in 12.
What is the gradient of a 45 degree roof?
100%, or 12:12.
What is the minimum fall for a flat roof?
Guidance commonly asks for a finished fall of at least 1:80, designed at about 1:40.
How accurate is the phone tilt reading?
Usually within a degree or two; use a level and tape for precise work.