Purge Volume Calculator - Calculator Academy

Calculate purge volume from well diameter and water depth, or solve for the missing value in gallons, inches, feet, liters, and meters.

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Purge Volume Formula

The purge volume is based on the volume of water in a cylindrical well casing. The calculator uses gallons, inches, and feet as its base units.

PV = pi*(D / 2)²*(DW*12) / 231

PV = purge volume, in gallons
D = well diameter, in inches
DW = depth of water, in feet
pi = 3.14159
231 = cubic inches per gallon

To solve for well diameter, the formula is rearranged as:

D = 2*sqrt((PV*231) / (pi*(DW*12)))

D = well diameter, in inches
PV = purge volume, in gallons
DW = depth of water, in feet

To solve for depth of water, the formula is rearranged as:

DW = ((PV*231) / (pi*(D / 2)²)) / 12

DW = depth of water, in feet
PV = purge volume, in gallons
D = well diameter, in inches

This calculation assumes the well is a straight cylinder with a constant inside diameter. It gives one casing volume. If your sampling procedure requires multiple well volumes, multiply the result by the required number of volumes.

Common Well Purge Volumes by Diameter

Use this table to estimate gallons per foot of water column for common well diameters.

Well Diameter	Gallons per Foot of Water	Formula Check
1 in	0.0408 gal/ft	PV = 0.0408 × depth ft
2 in	0.1632 gal/ft	PV = 0.1632 × depth ft
4 in	0.6528 gal/ft	PV = 0.6528 × depth ft
6 in	1.4688 gal/ft	PV = 1.4688 × depth ft
8 in	2.6112 gal/ft	PV = 2.6112 × depth ft
12 in	5.8752 gal/ft	PV = 5.8752 × depth ft

Conversion	Value
1 gallon	231 cubic inches
1 liter	0.264172 gallons
1 foot	12 inches
1 meter	3.28084 feet

Purge Volume Example

For a 2 inch well with 50 feet of water:

PV = pi*(2 / 2)²*(50*12) / 231 = 8.16 gal

One casing volume is about 8.16 gallons. If you need to purge 3 casing volumes, the total purge amount is:

3*8.16 = 24.48 gal

Use the inside diameter of the well casing when possible. Using the outside diameter can overstate the purge volume.