Free tool · Convert & check any roof slope
Roof Slope Calculator
Turn any roof measurement into every slope format: x/12 pitch, degrees, percent grade and ratio. Then see which roofing materials building codes allow at that slope. You can even measure the angle with your phone's tilt sensor.
- 7 ways to enter a slope
- Phone angle finder
- Degrees, % and x/12
- Material minimum-slope check
- Drop over any distance
By Chowdhury, FixHomeCost editor · Last reviewed October 2026
1. Enter your slope
Pick whatever you know. Every other format is worked out instantly.
Rest one end of the level on the roof or rafter, raise the other end until the bubble is centered, then measure the gap straight down at the raised end.
Lay your phone flat on a rafter or on the roof surface, along the slope. Keep it still for a second, then tap Hold.
Works on phones and tablets with a motion sensor. Your phone may ask permission. Nothing leaves your device. Check the reading against a level if accuracy matters, because phone sensors can be off by a degree or so.
Advanced options precision, level, sensor, climate
- Roof slope
- 4/12
- Angle
- –
- Percent grade
- –
- Ratio
- –
- Rise per foot / per meter
- –
- Slope factor
- –
- Slope class
- –
2. Which roofing materials work at this slope?
Minimum slopes from the International Residential Code (IRC section R905). Your local code and the manufacturer's instructions have the final say.
3. Drop over a distance
How much lower is one end? Useful for flat roofs, porch roofs, patio covers and sheds.
Show the full calculation step by step with your numbers
Every format comes from one number, the rise per unit of run. Material minimums are from IRC R905. Your local code and the manufacturer's instructions have the final say.
Four ways to write a roof slope
A roof slope is just a rise over a run, but different trades write it differently. That's why a metal roofing manual can say "minimum 3°" while a shingle bundle says "minimum 2/12." Here's what each format means, using the same 4/12 roof as the example:
- x/12 (inches per foot): 4 inches of rise for every 12 inches of run. This is the US standard for roofing. Roofers say "four twelve" or "4 in 12."
- Degrees: the angle between the roof and a level line, 18.43° here. Digital levels, angle finders and phone apps show degrees.
- Percent grade: rise divided by run, times 100. That's 33.3% for a 4/12 roof. Flat-roof and drainage specs often use percent, such as "2% minimum."
- Ratio 1:n: one unit of rise for every n units of run, 1:3 here. Common on metric plans and outside the US.
If you're planning a roof rather than converting a number, our roof pitch calculator works out rafter length, roof area, shingle bundles and cost from your house measurements.
How to convert roof slope by hand
x/12 → degrees: angle = arctan(x ÷ 12)
degrees → x/12: x = 12 × tan(angle)
x/12 → percent: % = x ÷ 12 × 100
percent → x/12: x = % × 12 ÷ 100
ratio 1:n → x/12: x = 12 ÷ n
inches per foot → mm per meter: multiply by 1,000 ÷ 12 (about 83.3)
Example 1: degrees to pitch
Your phone reads 22.6°. tan(22.6°) = 0.416, and 0.416 × 12 = 5.0. That's a 5/12 roof.
Example 2: percent to pitch
A plan calls for a 2% slope on a flat roof. 2 × 12 ÷ 100 = 0.24, so about 1/4 inch per foot (1/4:12). Over a 20-foot roof, that's a 5-inch drop from one side to the other.
Example 3: a 2-foot level reading
You hold a 24-inch level on a rafter and measure a 10-inch gap at the raised end. 10 ÷ 24 × 12 = 5/12. Using the full 24 inches gives a more accurate result than a 12-inch measurement, because a small error in the gap matters less.
Watch out: percent grade and degrees are not the same. A 100% grade is 45°, not 90°. Mixing them up is the most common slope mistake on roof plans.
Minimum roof slope by material
Every roofing material has a minimum slope. Below it, water moves too slowly, wind can drive it up under laps, and ice can back up. Installing below the minimum usually voids the manufacturer's warranty and fails inspection.
| Material | Minimum slope | Notes |
|---|---|---|
| Asphalt shingles | 2/12 | Needs extra underlayment or lap sealant below 4/12. One layer of underlayment is standard. (IRC R905.2.2) |
| Standing seam metal | 1/4:12 | Check the panel maker's minimum, often 1/2:12 to 3/12 for some profiles. (IRC R905.10.2) |
| Exposed-fastener (lapped) metal panels | 1/2:12 with lap sealant | Needs extra underlayment or lap sealant below 3/12. (IRC R905.10.2) |
| Metal shingles | 3/12 | — (IRC R905.4.2) |
| Clay or concrete tile | 2½/12 | Needs extra underlayment or lap sealant below 4/12. (IRC R905.3.2) |
| Slate | 4/12 | — (IRC R905.6.2) |
| Wood shingles | 3/12 | — (IRC R905.7.2) |
| Wood shakes | 4/12 | — (IRC R905.8.2) |
| Mineral-surfaced roll roofing | 1/12 | Best on sheds, garages and porches. (IRC R905.5.2) |
| TPO, PVC or EPDM membrane | 1/4:12 (2%) | — (IRC R905.12 / R905.13) |
| Modified bitumen | 1/4:12 (2%) | — (IRC R905.11.1) |
| Built-up roofing (tar and gravel) | 1/4:12 (2%) | Gravel tends to wash off steeper roofs. (IRC R905.9.1) |
| Spray polyurethane foam | 1/4:12 (2%) | — (IRC R905.14.1) |
Low-slope roofs below 2/12 rely on a continuous waterproof membrane rather than overlapping pieces. If your roof is in that range and leaking, see our guide to roof flashing repair, or the repair cost per square foot for membrane patches.
How much slope does a "flat" roof need?
No roof should be truly flat. Codes require at least 1/4 inch per foot (2%) on membrane roofs so water drains instead of ponding. Standing water adds weight, speeds up UV damage and finds every weak seam.
- 1/8 inch per foot (1%): allowed only for coal-tar built-up roofs. Too flat for most systems.
- 1/4 inch per foot (2%): the code minimum for TPO, EPDM, PVC, modified bitumen, built-up and spray foam roofs.
- 1/2 inch per foot (4%): better drainage, and the minimum for lapped metal panels with sealant.
Slope can come from the framing or from tapered insulation boards laid on a level deck. Use the drop over a distance tool above to see how much fall a given roof needs. A 30-foot roof at 1/4 inch per foot drops 7.5 inches.
Low slope vs. steep slope
The roofing industry splits roofs into two groups at about 3/12 to 4/12:
- Low-slope roofs (under about 3/12) use membranes or standing seam metal. Water sits on them longer, so seams and flashing matter most.
- Steep-slope roofs (4/12 and up) use shingles, tile, slate and shakes. They shed water by gravity and overlap.
Between 2/12 and 4/12 is a gray zone. Shingles and tile are allowed there, but only with extra underlayment. Steeper roofs last longer and shed snow better. Low roofs are cheaper to build and easier to walk on.
Roof slope conversion table
| Degrees | Pitch (x/12) | Percent | Ratio |
|---|---|---|---|
| 1° | 0.21/12 | 1.7% | 1:57.29 |
| 1.19° | 0.25/12 | 2.1% | 1:48.14 |
| 2° | 0.42/12 | 3.5% | 1:28.64 |
| 3° | 0.63/12 | 5.2% | 1:19.08 |
| 4° | 0.84/12 | 7.0% | 1:14.30 |
| 4.76° | 1.00/12 | 8.3% | 1:12.01 |
| 5° | 1.05/12 | 8.7% | 1:11.43 |
| 7.5° | 1.58/12 | 13.2% | 1:7.60 |
| 9.46° | 2.00/12 | 16.7% | 1:6.00 |
| 10° | 2.12/12 | 17.6% | 1:5.67 |
| 14.04° | 3.00/12 | 25.0% | 1:4.00 |
| 15° | 3.22/12 | 26.8% | 1:3.73 |
| 18.43° | 4.00/12 | 33.3% | 1:3.00 |
| 20° | 4.37/12 | 36.4% | 1:2.75 |
| 22.62° | 5.00/12 | 41.7% | 1:2.40 |
| 25° | 5.60/12 | 46.6% | 1:2.14 |
| 26.57° | 6.00/12 | 50.0% | 1:2.00 |
| 30° | 6.93/12 | 57.7% | 1:1.73 |
| 33.69° | 8.00/12 | 66.7% | 1:1.50 |
| 35° | 8.40/12 | 70.0% | 1:1.43 |
| 36.87° | 9.00/12 | 75.0% | 1:1.33 |
| 40° | 10.07/12 | 83.9% | 1:1.19 |
| 45° | 12.00/12 | 100.0% | 1:1.00 |
| 50° | 14.30/12 | 119.2% | 1:0.84 |
| 60° | 20.78/12 | 173.2% | 1:0.58 |
For a pitch-first chart with roof area multipliers, see the roof pitch chart.
Frequently asked questions
How do I convert roof slope from degrees to pitch?
Multiply the tangent of the angle by 12. For 30°, tan(30°) = 0.577, and 0.577 × 12 = 6.9, so about a 7/12 pitch. The calculator above does it instantly.
What is a 4/12 roof slope in degrees and percent?
A 4/12 roof is 18.43° and a 33.3% grade. It's a common pitch for ranch houses, and the lowest slope at which asphalt shingles need only one layer of underlayment.
What is the minimum slope for a metal roof?
Standing seam metal can go as low as 1/4:12 (about 1.2°). Lapped metal panels with exposed fasteners need 1/2:12 with sealant in the laps, or 3/12 without sealant. Metal shingles need 3/12. Always check the panel maker's instructions too.
Is a 2% slope enough for a flat roof?
Yes. 2% is about 1/4 inch per foot, the minimum the IRC requires for membrane roofs such as TPO, EPDM, PVC and modified bitumen. More slope drains faster and reduces ponding.
Can I measure roof slope with my phone?
Yes. Open the "Use my phone" tab on this page and lay the phone flat on a rafter or the roof surface. It reads the phone's tilt sensor and converts it to pitch, percent and degrees. Phone sensors can be off by about a degree, so check against a level if the result is close to a material's minimum.
What is the difference between roof slope and roof pitch?
Today most people use them to mean the same thing: rise per 12 inches of run. In older carpentry books, pitch meant rise divided by the total span, so a 6/12 slope was a "1/4 pitch." If a document uses a fraction like 1/3 or 1/4 with no "/12," it's probably using the old span-based pitch.
What slope is too steep to walk on?
Most roofers walk roofs up to about 6/12 or 7/12 without extra gear. From 8/12 they use roof jacks and harnesses, and anything 10/12 or steeper needs full fall protection. Homeowners should stay off any roof that feels steep, wet or high.
Related tools and guides
This is general information, not a professional inspection. See how we research costs.