Calculate one casing volume, inside diameter or standing water-column height. Select the value to solve and enter the other two with their units. Gallons are US liquid gallons.

Calculates one standing-water casing volume for a straight cylinder. Water-column height is total well depth minus depth to water. This does not determine a sampling purge requirement; follow the site procedure.

Related Calculators

Purge Volume Formula

The single casing volume is the water in a straight cylindrical well casing of constant inside diameter. The calculator converts selected units exactly; the following equivalent formula uses US liquid gallons, inches and feet. Water-column height means total well depth minus depth to water, not depth below ground.

PV = pi ร— (D / 2)ยฒ ร— (DW ร— 12) / 231
  • PV = one casing volume, in US liquid gallons
  • D = well diameter, in inches
  • DW = standing water-column height, in feet
  • pi = 3.14159
  • 231 = cubic inches per US liquid gallon

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

D = 2 ร— sqrt((PV ร— 231) / (pi ร— (DW ร— 12)))
  • D = well diameter, in inches
  • PV = one casing volume, in US liquid gallons
  • DW = standing water-column height, in feet

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

DW = ((PV ร— 231) / (pi ร— (D / 2)ยฒ)) / 12
  • DW = standing water-column height, in feet
  • PV = one casing volume, in US liquid gallons
  • D = well diameter, in inches

This calculation assumes a straight cylinder with a constant inside diameter and gives one casing volume. If the approved site procedure requires multiple casing volumes, multiply by that specified number. No purge multiplier is assumed. A volume estimate does not establish that a groundwater sample is representative; low-flow methods may use drawdown and water-quality parameter stabilization instead.

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 ร— depth ft
2 in 0.1632 gal/ft PV = 0.1632 ร— depth ft
4 in 0.6528 gal/ft PV = 0.6528 ร— depth ft
6 in 1.4688 gal/ft PV = 1.4688 ร— depth ft
8 in 2.6112 gal/ft PV = 2.6112 ร— depth ft
12 in 5.8752 gal/ft PV = 5.8752 ร— depth ft
Conversion Value
1 US liquid 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 ร— (2 / 2)ยฒ ร— (50 ร— 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 ร— 8.16 = 24.48 gal

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