Well Volume Calculator: Gallons of Water in a Well
The water in a well is a column. Its height is the well's total depth minus the depth to the water, and its width is the casing. A 6-inch casing holds 1.47 gallons for every foot of water (Kansas State Extension), so a well with 100 feet of water in it holds about 147 gallons.
Enter your well's depth, static water level and casing size to get the volume in gallons, liters and cubic feet. You need this number to dose a well for shock chlorination, and it tells you how much stored water a slow well has to draw on.
Worked example
A 200-foot drilled well with water standing 50 feet below the surface, in 6-inch casing
- Well depth:
- 200 ft
- Depth to water:
- 50 ft
- Casing:
- 6 inches (1.47 gal/ft)
About 220 gallons
150 feet of water × 1.47 gallons per foot.
Well volume chart: gallons per foot by casing size
A 6-inch well casing holds 1.47 gallons per foot of water, so a well with 100 feet of water in it holds about 147 gallons (Kansas State Extension, Penn State Extension).
| Casing inside diameter | Gallons per foot | 50 ft of water | 100 ft of water | 200 ft of water |
|---|---|---|---|---|
| 2 inches | 0.16 | 8 gal | 16 gal | 33 gal |
| 4 inches | 0.65 | 33 gal | 65 gal | 131 gal |
| 5 inches | 1.02 | 51 gal | 102 gal | 204 gal |
| 6 inches | 1.47 | 73 gal | 147 gal | 294 gal |
| 8 inches | 2.61 | 131 gal | 261 gal | 522 gal |
| 10 inches | 4.08 | 204 gal | 408 gal | 816 gal |
| 12 inches | 5.88 | 294 gal | 588 gal | 1,175 gal |
| 24 inches | 23.5 | 1,175 gal | 2,350 gal | 4,700 gal |
| 36 inches | 52.9 | 2,644 gal | 5,288 gal | 10,575 gal |
Gallons per foot = diameter in inches, squared, × 0.0408 (the volume of a one-foot cylinder at 7.48 gallons per cubic foot). The per-foot figures match Table 1 in Kansas State Extension's MF911 and the tables in UGA Extension's C 858-4. The 50, 100 and 200-foot columns are our arithmetic, worked from the unrounded per-foot figure.
How to calculate the volume of water in a well
Kansas State Extension gives the method: multiply the volume per foot of the casing by the depth of water, which is the well depth minus the depth to water. The volume per foot is the volume of a cylinder one foot tall, converted at 7.48 gallons per cubic foot (Nebraska Extension). With the diameter in inches, that works out to diameter squared × 0.0408.
The per-foot figures match the published tables: 0.65 gallons per foot for 4-inch casing, 1.02 for 5-inch, 1.47 for 6-inch, 2.61 for 8-inch and 4.08 for 10-inch (Kansas State Extension, UGA Extension). Bored wells hold far more: 23.5 gallons per foot at 24 inches across and 52.9 at 36 inches (UGA Extension).
Where the inputs come from: Kansas State Extension says a well log, where one is on file with the county or state water office, lists the well depth and the depth to water at the time the well was drilled. Use the inside diameter of the casing. Because the log records the water level on one day, the result is an estimate.
What the number is for: it is the starting point for a shock chlorination dose, and it is the storage a slow well draws on. Penn State Extension notes that a typical 6-inch well with 100 feet of water in the borehole stores 147 gallons, and that a 10-inch well would store 408 gallons, which may be enough to serve a single-family home even when the well's yield is very low.
Estimates only — independent local providers quote their own pricing. Last reviewed: November 13, 2026.
Sources
- Kansas State Research and Extension: Shock Chlorination for Private Water Systems (MF911), Table 1, volume of water in wells
- UGA Extension: Disinfecting Your Well Water: Shock Chlorination (C 858-4), storage per foot of water
- Nebraska Extension: Drinking Water Treatment: Shock Chlorination (G1761), Table I
- Penn State Extension: Using Low-Yielding Wells (borehole storage)
Frequently Asked Questions
How many gallons of water are in a well?
Multiply the feet of water in the well by the gallons each foot of casing holds. The feet of water is the well's depth minus the depth to water. A 6-inch casing holds 1.47 gallons per foot, so a 200-foot well with water at 50 feet holds 150 × 1.47, or about 220 gallons.
How many gallons per foot does a 6-inch well casing hold?
1.47 gallons per foot. Kansas State Extension, UGA Extension and Nebraska Extension all publish that figure. A 4-inch casing holds 0.65 gallons per foot and an 8-inch casing 2.61.
How do I calculate well volume for shock chlorination?
Kansas State Extension's method is the volume per foot of the casing times the depth of water, where depth of water is the well depth minus the depth to water. The bleach dose also has to cover the water in the house plumbing. Our shock chlorination calculator does both steps.
Does a wider well hold more water?
Yes, much more, because volume rises with the square of the diameter. Penn State Extension's example: a 6-inch well with 100 feet of water stores 147 gallons, and a 10-inch well with the same water column stores 408 gallons.
How much water is in a 4-inch well?
A 4-inch casing holds 0.65 gallons per foot of water (Kansas State Extension, UGA Extension). A 4-inch well with 100 feet of water in it holds about 65 gallons.
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