How to Size a Well Pump & Pressure Tank

Determine how much water your household needs. Use a rule of thumb that each fixture using water requires a pump supply capacity of 1 gallon per minute. This is your peak water requirement. A typical household requires a capacity of about 10 gallons per minute. Check to see if the well can supply the calculated amount. Have a well testing professional perform a well test if necessary.

Select the well pump to supply the calculated amount of capacity or the supply capacity of the well, whichever is less. Calculate the minimum pressure tank drawdown by multiplying the capacity of the pump (in gallons per minute) by the minimum amount of time the pump must run (in minutes), according to the manufacturer's specifications. The drawdown is the amount of water the tank can supply to the household, and it is less than the total volume of the tank. A 10 gallon per minute pump, with a minimum run time of 2 minutes, will give a minimum pressure tank drawdown of 20 gallons.

Adjust the minimum tank drawdown calculated to reflect how long the tank must supply the household at peak water use. If the well capacity is adequate, and the pump can supply the peak use, the minimum tank drawdown size calculated in the previous step is enough. If the well capacity is lower than the peak water use, subtract the well capacity in gallons per minute from the peak water use. Multiply the result by the number of minutes the system should supply the household with peak water to get the tank drawdown required.

Check the calculated tank drawdown against the pressure tank manufacturer's specifications. The total tank volume is greater than the tank drawdown by the amount of water left in the tank at the tank low pressure setting. Consult the tank manufacturer's documentation to find the total tank volume for the calculated tank drawdown.