Quick answer: gallons = π × (casing diameter ÷ 2)² × water column depth × 7.48. The critical detail is the water column — total well depth minus the depth to the static water level, not the total drilled depth. Getting that wrong is the most common well-dosing error there is.
Before you start: measuring a static water level and disinfecting a well are jobs with real safety and water-quality consequences. This calculator gives you the volume; follow your state or local health department's written procedure for the disinfection itself, and have the water tested afterwards.
Gallons per foot by casing size
| Casing diameter | US gal per ft | Gal per 100 ft | Liters per m |
|---|---|---|---|
| 2 in | 0.163 | 16.3 | 2.03 |
| 3 in | 0.367 | 36.7 | 4.56 |
| 4 in | 0.653 | 65.3 | 8.11 |
| 5 in | 1.020 | 102.0 | 12.67 |
| 6 in | 1.469 | 146.9 | 18.24 |
| 8 in | 2.611 | 261.1 | 32.43 |
| 10 in | 4.080 | 408.0 | 50.67 |
| 12 in | 5.875 | 587.5 | 72.97 |
| 24 in (bored well) | 23.50 | 2,350 | 291.9 |
| 36 in (dug well) | 52.88 | 5,288 | 656.7 |
Six inches is the most common modern drilled-well casing in North America. Older dug and bored wells are far wider, which is why a shallow 36-inch dug well can hold more water per foot than a deep drilled one — and why it needs a much larger disinfection dose.
Worked example
A typical drilled well: 6-inch casing, 300 ft total depth, static water level at 150 ft.
- Water column = 300 − 150 = 150 ft (not 300)
- Per foot for 6-inch casing: 1.469 gal
- Casing volume = 150 × 1.469 ≈ 220 gallons
- Add the pressure tank (say 20 gal) and plumbing (say 10 gal) → about 250 gallons of system volume
That last step is the one most people skip. Shock chlorination has to disinfect the whole system, not just the hole in the ground — the pipe volume calculator covers the distribution side.
Shock chlorination dosing
Typical guidance targets roughly 100 ppm of free chlorine held in the system for 12 to 24 hours. Using unscented household bleach at 5–6%:
| System volume | 5–6% bleach (approx.) |
|---|---|
| 50 gallons | 1.5 cups |
| 100 gallons | 3 cups |
| 250 gallons | 7.5 cups (≈ 1.9 quarts) |
| 500 gallons | 15 cups (≈ 3.75 quarts) |
| 1,000 gallons | 30 cups (≈ 1.9 gallons) |
Practical notes that matter as much as the arithmetic: use plain unscented bleach with no additives; circulate by running a hose back down the well until you smell chlorine; open every tap in the house until chlorine reaches each one, then shut them and let it stand; and afterwards, discharge the chlorinated water somewhere it will not kill vegetation or reach a septic field or waterway. Test the water once the chlorine has cleared.
Common mistakes & pro tips
- Using total well depth instead of the water column. The single biggest error, and it typically doubles the dose.
- Forgetting the pressure tank and plumbing. Those volumes need disinfecting too.
- Using scented or "splash-less" bleach. Additives do not belong in a drinking water system.
- Sampling without purging. Water standing in a casing is not representative of the aquifer. Purge three to five casing volumes first.
- Pro tip — measuring the static level. A weighted line with chalk, or an electronic sounder, gives the depth to water. Take the reading before running the pump, not after.
- Pro tip — recovery tells you yield. Drawdown and recovery rate say more about a well's usable yield than casing volume does. A 220-gallon casing that refills slowly is not a 220-gallon supply.
Frequently asked questions
How do I calculate the water volume in my well?
Measure the casing's inside diameter and the depth of the water column — total well depth minus the depth to the static water level. Then volume = π × (diameter ÷ 2)² × water column. A 6-inch well with 150 feet of standing water holds about 220 gallons.
How many gallons per foot is a 6 inch well?
About 1.47 gallons per foot of water column. The common casing figures are 0.65 gal/ft for a 4-inch well, 1.47 for 6-inch, 2.61 for 8-inch and 4.08 for 10-inch. Multiply by the water column depth, not the total well depth.
How much bleach do I need to shock chlorinate my well?
A widely used guideline is about 3 cups of unscented household bleach (5–6%) per 100 gallons of well water to reach roughly 100 ppm. For a 6-inch well with 150 ft of water — around 220 gallons — that is about 6.5 cups. Always follow your local health department's procedure, and remember to include the water in your pressure tank and plumbing.
What is the difference between well depth and water column?
Well depth is the total drilled depth. The water column is only the flooded portion — total depth minus the static water level. A 300-foot well with water standing at 150 feet has a 150-foot column, so half the depth holds no water at all. Using total depth doubles your dose.
How long should I run the pump to purge a well?
Standard sampling practice is to remove three to five casing volumes before collecting a sample, so that you are testing aquifer water rather than water that has been sitting in the casing. For a 220-gallon well at 10 gallons per minute, three volumes takes about 66 minutes.