Landscape Grading & Drainage Slope Calculator
Calculate slope percentage, grade ratio, elevation change, and estimated surface runoff flow for landscape grading and drainage design.
Lawn ≈ 0.25–0.35 | Paved ≈ 0.70–0.95 | Bare soil ≈ 0.50–0.60
Formulas Used
Slope Percentage:
Slope (%) = (Rise / Run) × 100
Slope Ratio:
Ratio = 1 : (Run / Rise) — read as "1 unit rise per X units of run"
Slope Angle:
θ = arctan(Rise / Run)
Rational Method — Peak Runoff Flow:
Q = C × i × A
Where: Q = peak flow (cfs or m³/s), C = runoff coefficient (dimensionless),
i = rainfall intensity (in/hr or m/s), A = drainage area (acres or m²)
Manning's Equation — Flow Velocity:
V = (1/n) × R2/3 × S1/2 (SI) |
V = (1.486/n) × R2/3 × S1/2 (Imperial)
Where: n = Manning's roughness coefficient, R = hydraulic radius (m or ft), S = slope (decimal)
Assumptions & References
- Manning's roughness coefficient n = 0.035 assumed for short grass / lawn swale (Chow, 1959).
- Hydraulic radius R = 0.1 m (0.328 ft) assumed for a small residential swale cross-section.
- Rational Method is valid for drainage areas up to approximately 200 acres (80 ha).
- Minimum 2% slope away from building foundations per IRC Section R401.3 and standard landscape practice.
- Minimum 1% slope for grass swales per USDA NRCS Engineering Field Handbook.
- Runoff coefficients sourced from ASCE Manual of Engineering Practice No. 36.
- Non-erosive velocity for grass-lined channels: 0.3–1.5 m/s (1–5 ft/s) per FHWA HEC-15.
- This calculator provides estimates only. Site-specific conditions, soil type, and local codes must be verified by a licensed engineer.