Drainage Rate Calculator
Gallons (US)
Liters
Cubic Feet
Seconds
Minutes
Hours
Use this for soil drainage tests (how fast water drops in a test hole).
Inches
Millimeters
Centimeters
Minutes
Hours
Calculated Drainage Rate:
0.00 Unit
Understanding Drainage Rate Calculation
Drainage rate determines how quickly liquid moves through a specific system or medium. Whether you are an engineer sizing a storm drain, a landscaper assessing soil permeability for a garden, or a homeowner conducting a "perc test" for a septic system, accurate calculation is vital to prevent flooding and ensure structural integrity.
Two Main Types of Drainage Calculations
This calculator handles the two most common scenarios found in hydrology and civil engineering:
- Volume Flow Rate (Q): Used for pipes, tanks, and pumps. It measures the volume of water moved over a set period (e.g., Gallons per Minute).
- Infiltration/Percolation Rate (I): Used for soil testing. It measures the vertical distance water drops in a hole over time (e.g., Inches per Hour).
Formula 1: Volume Flow Rate
The basic formula for calculation volume discharge is:
Q = V / t
Where:
- Q = Flow Rate (Discharge)
- V = Volume of liquid (Gallons, Liters, etc.)
- t = Time elapsed
Example: If a 50-gallon sump pit drains completely in 5 minutes, the drainage rate is 10 GPM (Gallons Per Minute).
Formula 2: Soil Percolation Rate
When testing soil for a septic leach field or rain garden, you measure how fast the water level drops in a test hole:
I = D / t
Where:
- I = Infiltration Rate
- D = Drop in water level (Inches or mm)
- t = Time elapsed
Standardizing this to Inches per Hour (in/hr) is standard practice for analyzing soil types.
Interpreting Soil Drainage Results
| Drainage Class | Inches per Hour (approx) | Implication |
|---|---|---|
| Excessively Drained | > 6 in/hr | Sand/Gravel. Water drains too fast for some plants. |
| Well Drained | 0.6 – 6 in/hr | Loam. Ideal for most agriculture and gardens. |
| Poorly Drained | < 0.6 in/hr | Clay/Silt. High risk of pooling and root rot. |
Why Accurate Drainage Calculation Matters
1. Preventing Foundation Damage: If drainage rates around a home are too slow, hydrostatic pressure builds up against the foundation, leading to cracks and basement leaks.
2. Septic System Viability: A "perc test" determines if the soil can absorb effluent. If the rate is too slow (high clay content), a standard septic field will fail, requiring an expensive mound system.
3. Stormwater Management: For paved surfaces like driveways and parking lots, calculating the flow rate ensures that drains and gutters are sized large enough to handle peak rainfall intensity without overflowing.