This is a quick-reference guide for drainage contractors. Use these tables for preliminary field sizing, then verify with the calculators for your actual design.
Important: These are rule-of-thumb values for common residential and light commercial conditions. Always verify with engineering calculations for permit submittals, unusual conditions, or critical applications.
Pipe Sizing Quick Reference
Assumptions: Manning’s n = 0.013 (smooth PVC/HDPE), flowing full, minimum slope shown.
Circular Pipe Capacity at Minimum Slope
| Pipe Diameter | Min Slope | Flow Capacity (cfs) | Flow Capacity (gpm) | Typical Service Area |
|---|
| 4” | 1.0% | 0.10 | 45 | Single downspout |
| 6” | 0.5% | 0.30 | 135 | 2-3 downspouts, small yard |
| 8” | 0.4% | 0.65 | 290 | Residential lot, small parking |
| 10” | 0.3% | 1.15 | 515 | 2-3 residential lots |
| 12” | 0.2% | 1.80 | 810 | Cul-de-sac, small commercial |
| 15” | 0.15% | 3.20 | 1,435 | Street drainage run |
| 18” | 0.1% | 5.10 | 2,290 | Major collector |
| 24” | 0.08% | 10.8 | 4,850 | Trunk line |
Verify with Manning’s Pipe Calculator
Quick Pipe Selection by Drainage Area
For a 10-year design storm with i = 4 in/hr and C = 0.5 (typical suburban):
| Drainage Area | Required Flow (cfs) | Minimum Pipe Size |
|---|
| 1,000 sq ft | 0.05 | 4” |
| 2,500 sq ft | 0.11 | 4” |
| 5,000 sq ft | 0.23 | 6” |
| 10,000 sq ft (1/4 acre) | 0.46 | 8” |
| 20,000 sq ft (1/2 acre) | 0.92 | 8” |
| 1 acre | 1.83 | 12” |
| 2 acres | 3.67 | 15” |
| 5 acres | 9.17 | 24” |
Calculate exact flow with Rational Method Calculator
Catch Basin Sizing
Standard Catch Basin Grate Capacities
| Grate Size | Grate Type | Capacity (cfs) at 2” ponding | Typical Application |
|---|
| 12” x 12” | Flat grate | 0.5 - 1.0 | Residential yard drain |
| 18” x 18” | Flat grate | 1.5 - 2.5 | Driveway, patio |
| 24” x 24” | Flat grate | 3.0 - 5.0 | Parking lot, commercial |
| 12” NDS | Round grate | 0.4 - 0.8 | Landscape drain |
| 6” NDS | Round grate | 0.15 - 0.3 | Planter, small area |
Catch Basin Spacing Guidelines
| Contributing Area Type | Max Spacing | Max Contributing Area per Basin |
|---|
| Paved parking lot | 150 ft | 10,000 sq ft |
| Residential driveway | One per driveway | 3,000 sq ft |
| Roadway with curb | 300-500 ft | Varies by gutter capacity |
| Flat paved area (under 1%) | 100 ft | 5,000 sq ft |
| Sloped paved area (over 2%) | 200 ft | 8,000 sq ft |
Size with Catch Basin Calculator
Slope Requirements
Minimum Pipe Slopes for Self-Cleaning Velocity
Self-cleaning velocity (2 ft/s minimum) prevents sediment buildup:
| Pipe Size | Absolute Minimum Slope | Recommended Minimum |
|---|
| 4” | 0.83% | 1.0% |
| 6” | 0.46% | 0.5% |
| 8” | 0.31% | 0.4% |
| 10” | 0.22% | 0.3% |
| 12” | 0.17% | 0.2% |
| 15” | 0.12% | 0.15% |
| 18” | 0.08% | 0.1% |
| 24” | 0.05% | 0.08% |
Surface Grading Minimums
| Surface | Minimum Slope | Recommended Slope |
|---|
| Adjacent to foundation (first 10 ft) | 2% (1/4”/ft) | 5% (5/8”/ft) |
| Lawn/landscape areas | 1% (1/8”/ft) | 2% (1/4”/ft) |
| Paved surfaces (concrete, asphalt) | 0.5% (1/16”/ft) | 1% (1/8”/ft) |
| Flat roof to drain | 1/8” per ft minimum | 1/4” per ft |
| Swale bottom | 1% | 2% |
| Gutter (roadway) | 0.3% | 0.5% |
Trench Sizing Quick Reference
French Drain Trench Dimensions
| Flow to Handle | Min Trench Width | Min Trench Depth | Pipe Size | Gravel Qty (per 10 ft) |
|---|
| Single downspout | 12” | 12” | 4” | 0.15 cu yd |
| Small yard area | 12” | 18” | 4” | 0.20 cu yd |
| Medium yard area | 18” | 18” | 6” | 0.30 cu yd |
| Large lot / high water table | 24” | 24” | 6”-8” | 0.55 cu yd |
Size with French Drain Calculator
Runoff Coefficients Quick Reference
For Rational Method (Q = CiA):
| Surface | C Value |
|---|
| Asphalt/concrete | 0.90-0.95 |
| Gravel driveway | 0.40-0.60 |
| Flat roof (no gravel) | 0.90-0.95 |
| Lawn (sandy soil, flat) | 0.10-0.20 |
| Lawn (sandy soil, steep) | 0.20-0.35 |
| Lawn (clay soil, flat) | 0.20-0.35 |
| Lawn (clay soil, steep) | 0.30-0.50 |
| Wooded area | 0.10-0.25 |
| Suburban residential (typical) | 0.35-0.55 |
| Commercial/industrial | 0.60-0.90 |
Calculate composite C with Runoff Coefficient Calculator
Detention/Retention Quick Sizing
Rule of thumb for preliminary sizing only:
| Requirement | Sizing Estimate |
|---|
| Detain 1” over impervious area | 3,630 cu ft per acre impervious |
| Standard underground chamber | ~50 cu ft storage per unit (typical 34”x16” chamber) |
| Dry well (4 ft dia x 4 ft deep) | ~37 cu ft effective storage |
| Rain garden | Size = 20-30% of contributing impervious area |