What is Slope?
Slope (also called grade, pitch, or gradient) is the measure of steepness or incline of a surface. It's expressed as rise over run (vertical change / horizontal distance), as a percentage, or as an angle in degrees. For example, a 2% slope means 2 feet of rise for every 100 feet of run. Slope is critical in construction for roofs (water drainage), roads (safe driving), ramps (accessibility), drainage pipes (gravity flow), and landscaping. Understanding slope helps ensure proper water runoff, structural stability, and code compliance.
Slope Calculation Formulas
1. Slope % = (Rise / Run) × 100
2. Angle = arctan(Rise / Run) = arctan(Slope %/100)
3. Ratio = Rise : Run (simplified)
4. Slope Length = √(Rise² + Run²)
Common Slopes
Low roof pitch: 2/12 to 4/12 (9.5° to 18.4°)
Standard roof: 4/12 to 8/12 (18.4° to 33.7°)
Steep roof: 8/12+ (33.7°+)
ADA ramp: 1:12 max (8.33%, 4.76°)
Driveway: 10-15% max (5.7° to 8.5°)
Road grade: 6-10% typical (3.4° to 5.7°)
Drainage: 1-2% minimum (0.57° to 1.15°)
Slope Format Examples
Percentage: 25% (25 units rise per 100 units run)
Ratio: 1:4 (1 unit rise per 4 units run = 25%)
Angle: 14.04° (same as 25% or 1:4)
Fraction: 3/12 roof pitch (3" rise per 12" run = 25%)
Common Applications
- Roofing: Pitch affects water drainage, snow load, and material selection
- Ramps: ADA requires max 1:12 (8.33%) for wheelchair accessibility
- Roads: Grade affects vehicle performance, drainage, and safety
- Drainage: Minimum 1-2% slope needed for gravity flow in pipes
- Landscaping: Proper slope prevents water pooling and erosion
- Stairs: Rise/run ratio determines comfort and code compliance
Tips for Slope Calculations
- Minimum roof pitch for shingles: 2/12 (16.7%), for metal: 1/12 (8.3%)
- Maximum driveway slope: 15% (8.5°) for safe vehicle access
- Drainage pipes: 1/4" per foot (2%) minimum for wastewater
- ADA ramps: 1:12 max (8.33%) with landings every 30 feet
- Road grades above 10% are considered steep and may need warnings
- Consider snow/ice in cold climates - limit slopes for safety
- Steeper slopes require more structural support and materials
Building Code Requirements
Building codes specify slope requirements for safety and functionality. Roofs need minimum slope for water drainage (varies by material). Ramps must meet ADA guidelines: maximum 1:12 slope (8.33%), with level landings every 30 feet and at top/bottom. Driveways typically limited to 15% maximum grade. Plumbing drainage requires 1/4 inch per foot (2%) minimum slope for proper flow. Roads may have maximum grades of 6-10% depending on terrain and vehicle types. Always verify local building codes as requirements vary by jurisdiction and application.
Frequently Asked Questions
You enter a rise (vertical) and a run (horizontal) in feet, metres, inches or centimetres, and the calculator returns the slope in four equivalent forms: percent grade, angle in degrees, ratio as 1:N, and the slope length (the sloped distance you actually travel, the hypotenuse). It also shows rise, run and length converted to both metres and feet. It is built for estimators, roofers, plumbers, civil/MEP engineers and accessibility designers who need a quick, code-aware reading rather than coordinate geometry.
Use the 'What to Calculate' menu to pick which value is unknown. Slope (from Rise & Run) takes both measurements and gives you percent, angle, ratio and length — the everyday case. Rise (from Slope & Run) takes a known grade and a horizontal distance and returns the vertical rise, for example how much a 30 m drain drops at 2%. Run (from Slope & Rise) takes a grade and a target rise and returns the horizontal run needed, for example how long a 1:12 ramp must be to climb a given step height.
All three describe the same steepness. Slope = rise / run; percent grade = (rise / run) × 100; angle = arctan(rise / run); and the ratio is 1:N where N = run / rise = 100 / percent. Handy reference points: 1% = 0.57° = 1:100; 2% = 1.15° = 1:50; 8.33% = 4.76° = 1:12 (the ADA ramp maximum); 25% = 14.04° = 1:4; 100% = 45° = 1:1. The percent and degree scales diverge once slopes get steep — 100% is only 45°, not 90° — so never treat percent as if it were degrees.
Carpenters express roof pitch as rise-on-run over a 12-inch run, such as 6:12. To check it here, enter the rise (6) in inches and the run (12) in inches, or use 6 and 12 in any matching unit. A 6:12 pitch returns 50% grade, 26.57° and a 1:2 ratio. Common roofs: 4:12 = 33.3% = 18.43°, 8:12 = 66.7% = 33.69°, 12:12 = 100% = 45°. Pick the Roof preset to also check the result against the minimum pitch for asphalt shingle or metal panel.
Choose an application from the 'Application / Standard' menu before you press Calculate. The tool compares your computed grade to the governing limit for that use and shows a PASS, FAIL or CAUTION badge plus the limit text. ADA ramps and driveways are maximum limits (steeper fails); roof minimums and drainage pipe fall are minimum limits (too flat fails). CAUTION appears when you are within 10% of the limit. The presets are a quick design-review aid, not a substitute for the local code of record.
The ADA running-slope maximum is 1:12, which equals 8.33% or 4.76°. Steeper than that fails. For long ramps a gentler 1:16 to 1:20 is preferred, and level landings are required at the top, bottom and every 30 ft (9 m) of horizontal run. Cross-slope (across the ramp width) should stay under 2%. Use the Run (from Slope & Rise) mode with 8.33% and your step height to size the minimum ramp length, then pick the ADA Ramp preset to confirm the verdict.
Sanitary and waste lines rely on gravity, so they need a minimum fall, not a maximum. A common rule of thumb is 1/4 inch per foot, which is 2% (1:50), and many codes accept 1% (1:100) on larger-diameter sanitary lines. Too flat and solids settle; too steep and liquid outruns the solids. Enter your rise and run, or use Rise (from Slope & Run) with 2% over the pipe length, then pick the Drainage Pipe preset to confirm the fall meets the minimum.
The slope length is the diagonal distance measured along the sloped surface, equal to √(rise² + run²) — the hypotenuse of the right triangle formed by rise and run. It is longer than the horizontal run and is what you actually walk, roof or cable. Estimators use it for ramp surface area, roof underlayment, conveyor and cable runs, and rope over a sloped trench. The calculator reports run and slope length separately so material take-offs stay accurate when the surface is not flat.
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