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Extension > Crop production > Soil and water > Online drainage calculator

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Online drainage calculator

These calculators are based on corrugated polyethylene pipe. Calculations for concrete and clay pipe can be approximated by using the smooth pipe option.

Calculate pipe diameter

Pipe Diameter (inches)

Use this calculator to compute the required pipe size for a given area and % grade.

Smooth Interior Pipe?

Area Drained:

*%Grade is the feet of fall per 100 feet of length.

Drainage Coefficient (inches/day)
1/4" 3/8" 1/2" 3/4" 1"
Required Pipe Diameter (inches): *
Full pipe flow (cu ft/sec):
Velocity (ft/sec): **

*Use commercially available size equal to or larger than size shown.

**A minimum velocity of 1.5 ft/sec should be used when fine sands and silts are present —
otherwise a velocity of 0.5 ft/sec is sufficient.

Calculate area drained

Area Drained (Acres)

Use this calculator to compute the area that can be drained by a pipe of a given size and grade.

Smooth Interior Pipe?

* %Grade is the feet of fall per 100 feet of length.
Drainage Coefficient (inches/day)
1/4" 3/8" 1/2" 3/4" 1"
Area Drained (acres):
Full pipe flow (cu ft/sec):
Velocity (ft/sec):

Flowrate calculator/converter

Flowrate Calculator

Use this calculator to obtain flowrates for a given area, for various drainage coefficients.
Drainage Coefficient (inches/day)
Flowrate: 1/4" 3/8" 1/2" 3/4" 1"
Acre-inches per day:
Cubic feet per second (cfs):
Gallons per minute (gpm):
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