How Much Water Does a Storage Tank Hold?

Does a 20 gallon tank hold 20 gallons of water? Actually, no. Storage tanks are labeled according to their “shell” size. But their actual water storage capacity is usually about 30% of that. So a 20 gallon tank really holds about 6 gallons of water.

For example:

Tank Size Gallons of Water
20 6
30 8
44 13
50 18
60 24

The rest of that space, about 70%, holds air. Why air? Because as the water enters into your tank, the air compresses.  When you turn your faucet on, that air then pushes the water back out of your tank and up to your kitchen. The air gives your water pressure. It also helps to protect your pump to cycle normally.

How Much Water in Water Storage TankWhat tank size do you need?

  • A properly sized tank should hold as much water as your well pump can pump in one minute.
  • Example: If your pump can deliver 8 gallons a minute, your tank size should be a “30” gallon tank because a 30 gallon tank really hold about 8 gallons of water. (Remember, the rest of the space holds air). See the chart above.
  • Generally speaking, most residential homes in CT have pumps that deliver between 5-10 gallons of water. So the tank “shell” size should be between 30-50 gallons.

Note: Water storage fluctuates between manufacturers AND pressure settings. The higher you set your pressure switch, the fewer gallons of water will be stored.