How much storage a low-yield well needs to cover peak household demand, with credit for water held in the well casing.
What size storage tank do I need?
Minimum storage in gallons from the New York State Department of Health table. Blank cells are not listed in the fact sheet; ask a well professional.
| Well yield | 2 bed | 3 bed | 4 bed | 5 bed |
|---|---|---|---|---|
| 5 gpm or more | None | None | None | None |
| 3 to 5 gpm | 100 | 150 | 200 | 250 |
| 1 to 3 gpm | 150 | 200 | 250 | 300 |
| 0.5 to 1 gpm | 200 | 250 | – | – |
| Under 0.5 gpm | Not recommended |
Worked example
Source example
A 3-bedroom house on a well that tests at 2 gpm falls in the 1 to 3 gpm row of New York's table, so it needs at least 200 gallons of storage. If the 6-inch casing holds 40 feet of water above the pump, that's about 40 × 1.5 = 60 gallons already stored in the well, so a tank of about 140 gallons covers the rest. Penn State's ideal is a full day's use: 100 gallons per person, or 400 gallons for 4 people, refilled slowly overnight.
How it works
Peak demand for a typical household is about 5 gpm (two fixtures at 2.5 gpm), so a well that reliably yields 5 gpm or more needs no extra storage. Below that, the New York State Department of Health table sets minimum storage by yield and bedrooms. Storage can be a tank, water standing in the well casing, or both. Wells under 0.5 gpm are not recommended for household use. Penn State Extension suggests sizing for a full day, at least 2 hours of peak use, and protecting tanks from freezing.
Questions
What size storage tank do I need for a 1 gpm well?
New York's table lists 150 to 300 gallons for 1 to 3 gpm, and 200 to 250 gallons for 0.5 to 1 gpm, depending on bedrooms. Penn State's full-day rule of thumb is 100 gallons per person.
How much water is stored in my well?
A typical 6-inch well holds about 1.5 gallons per foot of casing. Multiply by the height of water above the pump when it isn't running (NY DOH).
Do I still need a pressure tank?
Yes. A storage tank refills slowly from the well, and a booster pump and pressure tank deliver water from it to the house. Size the pressure tank with the pressure tank calculator.
Sources
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