Drainage Calculator
"Quickly determine the right pipe size for your drainage project by entering your acreage and slope, or find how many acres a known pipe can serve. Supports both Single Wall and Dual Wall pipe across all standard drainage coefficients.".
| Drainage Coefficient | 1/8" | 1/4" | 3/8" | 1/2" | 3/4" | 1" |
|---|
| Drainage Coefficient | 1/8" | 1/4" | 3/8" | 1/2" | 3/4" | 1" |
|---|
| Q (CFS) | GPM | In/Hr * | Velocity (fps) |
|---|
* Drainage rate in in/hr this pipe can serve per acre of drainage area.
| Drainage Coefficient | 1/8" | 1/4" | 3/8" | 1/2" | 3/4" | 1" |
|---|
Whether you are sizing a French drain to relieve a waterlogged yard, designing a swale to carry stormwater away from a building foundation, selecting a catch basin and pipe system for a parking lot, or calculating culvert capacity under a driveway, getting your drainage design right before installation prevents property damage, erosion, foundation problems, and the expense of correcting an undersized system after the fact. A drainage calculator works from your drainage area, your local rainfall intensity, and your surface runoff coefficient to tell you the peak flow rate the system must handle and the pipe or channel size required to carry that flow without surcharging.
This guide covers the rational method formula for peak runoff, runoff coefficients by surface type, rainfall intensity data, pipe flow capacity by size and slope, swale and channel sizing, French drain design criteria, and a complete reference table for common drainage scenarios.
The Rational Method Formula
The rational method is the standard approach for calculating peak stormwater runoff from small drainage areas up to approximately 200 acres. It is used for sizing storm sewers, culverts, swales, and on-lot drainage systems:
Q = C × i × A
- Q = peak runoff rate in cubic feet per second (cfs)
- C = runoff coefficient (dimensionless, 0 to 1.0)
- i = rainfall intensity in inches per hour for the design storm duration and return period
- A = drainage area in acres
For a 0.5-acre paved parking lot (C = 0.90) during a 10-year, 1-hour storm with an intensity of 2.5 inches per hour:
- Q = 0.90 × 2.5 × 0.5 = 1.125 cfs
- Convert to gallons per minute: 1.125 × 448.8 = 505 gallons per minute
Runoff Coefficients by Surface Type
The runoff coefficient represents the fraction of rainfall that becomes surface runoff. Impervious surfaces like pavement and roofs have high coefficients. Vegetated and permeable surfaces have lower coefficients because they absorb and detain more water.
Surface Type | Runoff Coefficient (C) | Notes |
Impervious roof — flat | 0.85 – 0.95 | Slight ponding loss on flat roofs |
Impervious roof — steep | 0.90 – 0.95 | Minimal retention |
Concrete pavement | 0.80 – 0.95 | Depends on condition and slope |
Asphalt pavement | 0.70 – 0.95 | Aged asphalt absorbs slightly more |
Gravel drive or parking | 0.35 – 0.70 | Varies with compaction and condition |
Lawns — sandy soil, flat (<2%) | 0.05 – 0.10 | High infiltration capacity |
Lawns — sandy soil, steep (>7%) | 0.15 – 0.20 | Reduced infiltration time |
Lawns — clay soil, flat (<2%) | 0.13 – 0.17 | Clay limits infiltration |
Lawns — clay soil, steep (>7%) | 0.25 – 0.35 | Significant runoff on clay slopes |
Cultivated fields | 0.30 – 0.60 | Depends on slope and crop cover |
Wooded areas, flat | 0.05 – 0.25 | Good interception and infiltration |
Mixed residential | 0.25 – 0.40 | Typical suburban lot, 1/4 to 1/3 acre |
Dense urban development | 0.70 – 0.90 | High impervious coverage |
Pipe Flow Capacity by Size and Slope
Pipe flow capacity is calculated using Manning’s equation for full-flow conditions. Designing at full-flow capacity is conservative — pipes typically flow at 80 to 95 percent full under peak design conditions, which actually maximizes velocity and self-cleaning action.
Pipe Diameter (in) | Slope 0.5% | Slope 1.0% | Slope 2.0% | Slope 4.0% | Min. Self-Cleaning Velocity |
4 | 0.05 cfs | 0.07 cfs | 0.10 cfs | 0.14 cfs | 2 fps at 1.0% slope |
6 | 0.16 cfs | 0.22 cfs | 0.32 cfs | 0.45 cfs | 2 fps at 0.5% slope |
8 | 0.35 cfs | 0.50 cfs | 0.71 cfs | 1.00 cfs | 2 fps at 0.3% slope |
10 | 0.64 cfs | 0.91 cfs | 1.29 cfs | 1.82 cfs | 2 fps at 0.2% slope |
12 | 1.07 cfs | 1.51 cfs | 2.14 cfs | 3.02 cfs | Adequate at most slopes |
15 | 1.94 cfs | 2.74 cfs | 3.87 cfs | 5.48 cfs | Adequate at most slopes |
18 | 3.18 cfs | 4.49 cfs | 6.36 cfs | 9.00 cfs | Adequate at most slopes |
24 | 6.72 cfs | 9.51 cfs | 13.44 cfs | 19.01 cfs | Large storm sewer |
30 | 12.32 cfs | 17.43 cfs | 24.65 cfs | 34.85 cfs | Main trunk sewer |
Note: Flow values calculated using Manning’s equation with n = 0.013 for smooth concrete pipe and n = 0.015 for corrugated HDPE. HDPE pipe capacity is approximately 85 percent of concrete pipe at the same diameter and slope.
French Drain Design Criteria
A French drain collects subsurface groundwater and lateral seepage and conveys it to a safe discharge point. It consists of a perforated pipe in a gravel-filled trench. The pipe size, trench width, and gravel volume must all be sufficient to handle the inflow rate.
Design Parameter | Minimum Standard | Recommended | Notes |
Perforated pipe diameter | 4 inches | 6 inches | 4″ adequate for most residential |
Trench width | 12 inches | 18 – 24 inches | Wider trench increases infiltration area |
Gravel depth above pipe | 6 inches | 12 inches | Washed crushed stone #57 or #3 |
Gravel below pipe | 3 inches | 4 – 6 inches | Sets pipe at correct elevation |
Filter fabric wrapping | Full wrap of gravel | Sock on pipe plus trench liner | Prevents fines migration into gravel |
Minimum pipe slope | 0.5% | 1.0% | 6 inches fall per 100 feet minimum |
Maximum pipe run without cleanout | 100 feet | 50 – 75 feet | Cleanouts allow flushing and inspection |
Outlet protection | Outlet apron | Rodent screen + outlet headwall | Prevent backflow and pest entry |
Swale Sizing
A swale is an open channel that conveys stormwater overland. Swale capacity depends on the cross-section shape, the side slopes, the bottom width, the flow depth, the channel slope, and the type of lining (grass, concrete, rip-rap). Grass-lined swales are self-cleaning and environmentally preferred but require minimum slopes and flow velocities to prevent sediment deposition.
- Minimum slope for grass-lined swale: 1% (1 foot drop per 100 feet)
- Maximum velocity for grass lining (established turf): 3 – 4 feet per second
- Maximum velocity for rip-rap lining: 6 – 8 feet per second
- Maximum velocity for concrete-lined channel: 10 – 14 feet per second
- Minimum freeboard above design flow depth: 6 inches for swales; 12 inches for channels
Related tools and guides: earthwork calculator, gravel calculator and fill dirt calculator.
Frequently Asked Questions
How do I find the rainfall intensity for my location?
Rainfall intensity data for engineering design is published by NOAA in the Precipitation Frequency Data Server, available online. Look up your geographic coordinates or nearest weather station to find intensity-duration-frequency tables. For residential drainage, the 10-year return period storm with a 10 to 15 minute duration is the typical design event. For road culverts, the 25-year storm is standard. Critical infrastructure may require the 100-year storm event.
What size French drain do I need for my yard?
For a typical saturated residential yard of 5,000 square feet with clay soil and moderate slope, a 4-inch perforated pipe in an 18-inch wide by 24-inch deep trench is generally adequate. If groundwater is seeping from an upslope bank or the yard collects runoff from a significant area uphill, upgrade to 6-inch pipe and widen the trench to 24 inches. Always route the outlet to a storm drain inlet, dry well, or drainage swale — never to a neighbor’s property or public street without permission.
How deep should a drainage pipe be buried?
Residential drainage pipes should be buried at least 12 inches deep to protect them from surface traffic and frost heave in cold climates. In frost-affected climates, pipes that carry water year-round should be buried below the frost depth, which ranges from 12 inches in warm regions to 60 inches or more in northern climates. Storm sewer mains in roadways are typically buried 4 to 6 feet deep to provide adequate cover below the pavement structure.
What is the difference between a storm drain and a sanitary sewer?
A storm drain collects and conveys rainwater and surface runoff, typically discharging directly to a stream, lake, or outfall without treatment. A sanitary sewer collects wastewater from buildings and conveys it to a wastewater treatment plant. The two systems must never be cross-connected. Stormwater entering a sanitary sewer overwhelms treatment plant capacity and causes treatment failures and overflows. If you are connecting to an existing municipal pipe system, confirm which system you are connecting to before making any connections.
Our Latest Tools
Estimate the number of blocks, total cost, and mortar bags needed for your wall.
Estimate the volume, weight, and cost of concrete for slabs, columns, or any custom volume. Numbers update as you type.
Estimate how much concrete you need to pour a wall — including openings for doors and windows, waste allowance, bag count, and total cost.
Estimate the volume of concrete required to pour columns or piers — round, square, or rectangular — plus bag count and total cost.
Calculate the volume of concrete needed for a flight of stairs — including the optional top platform — then estimate bags and total cost.
Estimate the total cost of pouring a slab, footing, or column — including concrete volume, premix bags needed, and waste allowance.
Estimate the volume, weight, and cost of concrete needed for strip footings, pad footings, or trench fills. Numbers update as you type.
Determine exactly how much concrete you need for your floor slab, patio, or driveway. Calculate cubic yards, total bags needed, and estimated material costs instantly.
Estimate how much concrete you need for a curb run — supports rectangular or trapezoidal cross-sections, optional gutter pan, plus bags and total cost.
Estimate the number of concrete mix bags needed for a slab, footing, column, or hole — based on standard Sakrete bag yields. Numbers update as you type.