Well Volume Calculator: Purge Volumes for Groundwater Sampling
Casing diameter, depth to water, total depth. Out comes the well volume, the 3x or 5x purge total, and the drum count for the truck.
Purge volume decides how long you pump; the drum count decides what you load. A 10-well quarterly event on 2 inch casings with 20 foot water columns runs about 98 gallons at 3x, which is 2 drums. Step the same site up to a 5x protocol and it becomes 163 gallons and 4 drums, a different truck load entirely.
Related calculators
Well volume per foot by casing diameter
These factors come straight from the cylinder formula, and the third column is the number crews actually plan with: a 3x purge on a typical 50 foot water column.
| Casing diameter | Gallons per foot | 3x purge, 50 ft column |
|---|---|---|
| 1 inch | 0.041 | 6.1 gal |
| 2 inch | 0.163 | 24.5 gal |
| 4 inch | 0.653 | 97.9 gal |
| 6 inch | 1.469 | 220.3 gal |
3x vs 5x purge, and where low-flow sampling fits
The volume-purge tradition removes 3 to 5 casing volumes so the sample comes from the formation instead of the stagnant column. Three volumes is the standard floor; crews step up toward five when field parameters have not stabilized, when the well has sat unsampled for years, or when the project plan simply says so. EPA Region 4's field SOP, the reference many state UST programs borrow, frames purging around parameter stabilization rather than a fixed count, with the multiplier as the working envelope.
Purge protocols are set by the sampling plan, not by the crew.
Low-flow purging replaces the multiplier entirely: pump at 0.1 to 0.5 L/min at screen depth, watch parameters, sample when stable. Less IDW, faster events, better metals data. It is not automatic everywhere; deep water tables, silty wells, and programs whose SOPs predate the technique keep the 3x method alive, and switching methods mid-monitoring-history can make trend data harder to defend.
What IDW is, and why purge water goes in drums
Investigation derived waste is everything an investigation generates that might be contaminated: purge water, drill cuttings, decon rinse, used PPE. Purge water from monitoring wells at a UST or remediation site is presumed contaminated until analytical results say otherwise, so it leaves the wellhead in labeled 55 gallon drums and waits on site for characterization. Clean results can open cheaper paths (some programs allow permitted discharge or ground surface application); dirty results route the drums to licensed disposal. Either way, hauling and disposal is contractor work with manifests attached, part of what a remediation contractor prices into a monitoring program.
Where purge math sits in a sampling event
The sequence is short: measure depth to water with an interface probe, compute the water column and purge target (this page), purge while logging field parameters, sample when stable, drum the water, label, and log. The math step is thirty seconds with the calculator and the step most often botched on paper, because a transposed depth-to-water reading silently doubles or halves every downstream number. The full craft of well construction, development, and sampling technique lives in the monitoring wells guide; this tool just makes the field arithmetic instant. For the drilling side of the program, see environmental drilling and the sampling context in the Phase 2 ESA guide.
Frequently Asked Questions
How do you calculate well volume?
Water column times gallons per foot. Subtract depth to water from total well depth to get the standing water column in feet, then multiply by the casing factor: 0.041 gal/ft for 1 inch, 0.163 for 2 inch, 0.653 for 4 inch, 1.469 for 6 inch. The factor is just the cylinder formula, pi times radius squared times 12 inches, divided by 231 cubic inches per gallon. A 2 inch well with water at 12 feet and a bottom at 35 feet holds 23 feet of water, which is 3.75 gallons. Measure both depths from the same reference point, almost always the surveyed notch on the top of casing.
How many gallons per foot in a 2 inch well?
0.163 gallons per foot of water column, the most-used number in field sampling because 2 inch PVC is the default monitoring well casing in UST and remediation work. Handy multiples for mental math: 10 feet of column is 1.6 gallons, and a typical 20 to 25 foot column holds 3 to 4. A 3x purge on a standard well is usually a 10 to 12 gallon job. The factor assumes clean open casing; a silted well bottom shortens the true water column below what the depth math says.
How much water do you purge before sampling?
Under the traditional approach, 3 to 5 casing volumes, pumping until field parameters (pH, conductivity, temperature, turbidity) stabilize, with 3x as the common floor and 5x when parameters refuse to settle. The alternative is low-flow purging at 0.1 to 0.5 liters per minute, which samples the screen zone directly and often removes under 2 gallons total. Which one you use is not a preference call on regulated sites: the sampling plan or state program SOP names the method, and the lab data gets defended against that document.
What is low flow sampling?
A purge technique that pumps very slowly (typically 0.1 to 0.5 L/min) with the intake set in the well screen, drawing fresh formation water while barely disturbing the standing column above. Field parameters are monitored until stable, then the sample is taken. The payoffs are big: minutes of pumping instead of an hour, a fraction of the IDW to drum and dispose, and less turbidity bias in metals results. The costs are equipment (a low-flow pump and flow-through cell) and time-per-well setup, which is why quick quarterly events on shallow wells often stay with the 3x volume method.
Field planning tool, not a substitute for your project's sampling and analysis plan. Purge protocol, stabilization criteria, and IDW handling for regulatory submittals come from the governing SOP and your state program, and this calculator's numbers do not belong in a report unverified. Casing factors assume nominal inside diameters and a clean well bottom.
Per-foot casing factors are computed from the cylinder formula on this page; the 50-usable-gallon drum convention was compiled by USTContractors.com from field sampling practice. Last verified July 2026.
Authoritative references:
Building out a monitoring program? The directory lists environmental drilling and remediation contractors by state, or request quotes for well installation and sampling work.
Spot an error? Report it and we will check the math against the sources.