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
Circular Pipe Capacity at Minimum Slope
| Pipe Diameter | Slope | Flow Capacity, cfs (gpm) | Area Served (C = 0.5, 4 in/hr) |
|---|---|---|---|
| 4” | 1.0% | 0.19 (85 gpm) | 4,100 sq ft |
| 6” | 0.5% | 0.40 (178 gpm) | 8,600 sq ft |
| 8” | 0.4% | 0.76 (343 gpm) | 16,500 sq ft |
| 10” | 0.3% | 1.20 (539 gpm) | 0.60 acre |
| 12” | 0.2% | 1.59 (715 gpm) | 0.79 acre |
| 15” | 0.15% | 2.50 (1,123 gpm) | 1.24 acres |
| 18” | 0.12% | 3.64 (1,633 gpm) | 1.80 acres |
| 24” | 0.08% | 6.40 (2,872 gpm) | 3.17 acres |
| 30” | 0.06% | 10.0 (4,509 gpm) | 4.98 acres |
Assumptions: Manning’s equation with n = 0.013, pipe flowing full, laid at the recommended minimum slope from the slope table below. Each row runs at about 2.0 to 2.2 ft/s when full. n = 0.013 is a common design value for concrete pipe and a conservative one for smooth-wall PVC or HDPE; single-wall corrugated pipe (n of 0.018 to 0.025 in HEC-22 Table 7-1) carries only about 50 to 70% of these flows at the same slope. Capacity grows with the square root of slope, so four times the slope doubles the flow, and a pipe laid at HEC-22’s steeper 3 ft/s storm-drain slope carries about 1.4 to 1.5 times the flow shown. The area column is the approximate drainage area each pipe can serve: its capacity divided by the Rational Method flow per acre at C = 0.5 and i = 4 in/hr (2.02 cfs per acre).
Verify with Manning’s Pipe CalculatorQuick Pipe Selection by Drainage Area
| Drainage Area | Required Flow (cfs) | Minimum Pipe Size and Slope | Pipe Capacity (cfs) |
|---|---|---|---|
| 1,000 sq ft | 0.05 | 4” @ 1.0% | 0.19 |
| 2,500 sq ft | 0.12 | 4” @ 1.0% | 0.19 |
| 5,000 sq ft | 0.23 | 6” @ 0.5% | 0.40 |
| 10,000 sq ft (0.23 acre) | 0.46 | 8” @ 0.4% | 0.76 |
| 20,000 sq ft (0.46 acre) | 0.93 | 10” @ 0.3% | 1.20 |
| 1 acre (43,560 sq ft) | 2.0 | 15” @ 0.15% | 2.50 |
| 2 acres | 4.0 | 24” @ 0.08%, or 18” @ 0.2% or steeper | 6.40 or 4.70 |
| 5 acres | 10.1 | 30” @ 0.08%, or 24” @ 0.25% or steeper | 11.6 or 11.3 |
Assumptions: Rational Method, Q = CiA, with C = 0.5 (the upper end of HEC-22’s 0.30 to 0.50 range for single-family residential), i = 4 in/hr (substitute your local design intensity from NOAA Atlas 14), and A in acres at 43,560 sq ft per acre. The flows include the 1.008 unit factor (1 acre-inch per hour = 1.008 cfs) that the Rational Method Calculator applies. Pipes are n = 0.013 flowing full. The first pick in each row is the smallest size that carries the required flow at its recommended minimum slope from the slope table, except that 30” needs 0.08% rather than its 0.06% minimum to carry 5 acres; the alternative is a smaller pipe laid steeper. Flow scales directly with C and i, so at C = 0.9 (pavement) every flow is 1.8 times larger and the pipes must be re-picked.
Calculate exact flow with Rational Method CalculatorCatch Basin Sizing
Standard Catch Basin Grate Capacities
| Flat Grate Size | Clean Capacity (cfs) at 2” ponding | 50% Clogged (cfs) | Typical Application |
|---|---|---|---|
| 12” x 12” | 0.6 - 0.8 | about 0.4 | Residential yard drain |
| 18” x 18” | 0.9 - 1.2 | about 0.6 | Driveway, patio |
| 24” x 24” | 1.2 - 1.6 | about 0.8 | Parking lot, commercial |
Assumptions: grate in a sag (low point) with 2 in (0.17 ft) of ponding, sized with the weir equation of HEC-22 (3rd ed.) Eq. 4-26, Q = 3.0 x P x d^1.5, where P is the grate perimeter in feet. The low end of each clean range leaves out the side against a curb or wall (P = L + 2W); the high end counts all four sides. With 50% clogging the effective perimeter is L + W (HEC-22 Example 4-11, the same rule the Catch Basin Calculator uses), so a clogged grate against a curb keeps about two-thirds of its clean capacity, not half. Design with the clogged column. At 2 in the orifice equation (Eq. 4-27) gives more flow than the weir for grates with about 40% or more open area, so the weir controls; check a grate with less open area in the calculator. Round landscape grates (6 to 12 in) are rated by their manufacturers, usually in gpm at a stated head: use that rating, not these square-grate values.
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 |
Check each basin against the grate table: the peak flow from its area should not exceed the 50%-clogged grate capacity. For example, 10,000 sq ft of pavement at C = 0.9 and i = 4 in/hr produces about 0.83 cfs, roughly the clogged capacity of a 24” x 24” grate at 2 in of ponding. Smaller grates or higher intensities need more basins or a deeper allowable ponding depth.
Size with Catch Basin CalculatorSlope Requirements
Minimum Pipe Slopes for Self-Cleaning Velocity
| Pipe Size | Slope for 2 ft/s Full | Recommended Minimum | Slope for 3 ft/s Full (HEC-22 storm drains) |
|---|---|---|---|
| 4” | 0.84% | 1.0% | 1.9% |
| 6” | 0.49% | 0.5% | 1.1% |
| 8” | 0.33% | 0.4% | 0.75% |
| 10” | 0.25% | 0.3% | 0.56% |
| 12” | 0.19% | 0.2% | 0.44% |
| 15” | 0.14% | 0.15% | 0.32% |
| 18” | 0.11% | 0.12% | 0.26% |
| 24” | 0.077% | 0.08% | 0.17% |
| 30” | 0.057% | 0.06% | 0.13% |
Assumptions: Manning’s equation with n = 0.013, pipe flowing full. A full-flow velocity of 2 ft/s is the common self-cleansing minimum (it is the sanitary-sewer criterion in the Ten States Standards), and the recommended minimums above all reach it. For storm drains, FHWA HEC-22 recommends at least 3 ft/s flowing full, which gives about 2 ft/s at 25% depth; that takes about 2.25 times the 2 ft/s slope (the last column, which for 8 in and larger pipes matches HEC-22 3rd ed. Table 7-7). Some agencies accept 2 to 2.5 ft/s, and many set a flat minimum slope for small storm drains, so check your local standard.
Surface Grading Minimums
| Surface | Minimum Slope | Recommended Slope |
|---|---|---|
| Soil or landscape within 10 ft of a foundation | 6” of fall in the first 10 ft (5%) | 6” or more in 10 ft, measured after backfill settles |
| Paved or impervious surface within 10 ft of a foundation | 2% (1/4”/ft) away from the building | 2% or more |
| Lawn/landscape areas beyond 10 ft | 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 | 1/4” per ft (2%) for most membrane and built-up roofs; 1/8” per ft only for coal-tar built-up | 1/4” per ft or more |
| Swale bottom | 1% (2% within 10 ft of a foundation) | 2% |
| Gutter (roadway) | 0.3% | 0.5% |
Foundation grading: at least 6 in of fall in the first 10 ft from the foundation (about 5%, IRC R401.3 and IBC 1804.4); paved or impervious surfaces within 10 ft slope at least 2% away from the building. The 2% figure is for impervious surfaces (and, under IBC 1804.4, swales) next to the building, not for the soil grade. Where lot lines, walls or slopes prevent 6 in of fall within 10 ft, use swales or drains to carry water away from the structure. Roof slopes are the IBC Section 1507 minimums for built-up, modified bitumen and single-ply membrane roofs.
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” | 0.34 cu yd |
| Small yard area | 12” | 18” | 4” | 0.52 cu yd |
| Medium yard area | 18” | 18” | 6” | 0.76 cu yd |
| Large lot / high water table | 24” | 24” | 6”-8” | 1.35 - 1.41 cu yd |
Assumptions: in-place stone volume for a trench filled with gravel to the top, minus the volume of the pipe (trench width x depth x 10 ft, less the pipe’s cross-section x 10 ft), the same method the French Drain Calculator uses. Add 10 to 15% for compaction and waste, and reduce the quantity in proportion if you cap the trench with soil. About 40% of the stone volume is voids: that is the water-storage volume, not the amount of stone to order.
Size with French Drain CalculatorRunoff Coefficients Quick Reference
For Rational Method (Q = CiA):
| Surface | C Value |
|---|---|
| Asphalt/concrete pavement | 0.70-0.95 |
| Roofs | 0.75-0.95 |
| Gravel driveway (not in HEC-22 Table 3-1) | 0.40-0.60 |
| Lawn (sandy soil, flat, under 2%) | 0.05-0.10 |
| Lawn (sandy soil, average, 2-7%) | 0.10-0.15 |
| Lawn (sandy soil, steep, over 7%) | 0.15-0.20 |
| Lawn (clay soil, flat, under 2%) | 0.13-0.17 |
| Lawn (clay soil, average, 2-7%) | 0.18-0.22 |
| Lawn (clay soil, steep, over 7%) | 0.25-0.35 |
| Wooded area, flat to hilly (not in HEC-22 Table 3-1) | 0.05-0.25 |
| Residential, single-family | 0.30-0.50 |
| Residential, suburban | 0.25-0.40 |
| Commercial (neighborhood to downtown) | 0.50-0.95 |
| Industrial (light to heavy) | 0.50-0.90 |
Assumptions: ranges from HEC-22 (3rd ed.) Table 3-1, the table the Runoff Coefficient Calculator uses, except the two rows marked as not in it, which are typical values from local drainage manuals (the calculator’s woodland options are 0.05-0.15 flat and 0.10-0.25 hilly). As HEC-22 notes, the higher values in each range suit steeply sloped areas and longer return periods. Some agencies also multiply C by 1.1, 1.2 or 1.25 for 25-, 50- and 100-year storms, capped at 1.0; HEC-22 lists these frequency factors but FHWA does not endorse them.
Calculate composite C with Runoff Coefficient CalculatorDetention/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 |
| Underground chamber (34” wide x 16” tall, about 7 ft long, such as the StormTech SC-310) | about 15 cu ft inside the bare chamber; about 29 - 31 cu ft per chamber installed in stone |
| Dry well pit (4 ft dia x 4 ft deep) filled with stone | about 20 cu ft effective storage |
| Same pit with an open precast concrete chamber (4 ft outside diameter) | about 35 - 38 cu ft inside the chamber |
| Rain garden | Size = 20-30% of contributing impervious area |
Assumptions: 1 in over 1 acre is 43,560 sq ft x 1/12 ft = 3,630 cu ft. The installed chamber figure is the manufacturer’s published minimum installed storage (29.3 cu ft on the current product page, 31.0 cu ft on older spec sheets that also count 6 in of stone between rows), with 6 in of stone below and above the chambers at 40% porosity; divide the required volume by it to count chambers. The dry-well pit holds 50 cu ft gross, and stone with 40% voids stores about 20 cu ft of it. An open chamber with 3 to 4 in walls holds about 35 to 38 cu ft inside, plus 40% of the volume of any stone around it.
Related Calculators
- Manning’s Pipe Flow Calculator — Verify pipe capacity
- Rational Method Calculator — Calculate design flow
- Catch Basin Calculator — Size catch basins
- French Drain Calculator — Size French drains
- Swale Calculator — Design swales