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Bladder Pressure-Tank Sizing for Residential Wells

Reference Equipment Updated Jun 19, 2026 ยท Water Treatment

Why this matters

An undersized bladder tank short-cycles the well pump. Short-cycling burns starter capacitors, overheats the motor windings, and dissolves bearings inside two years on a pump rated for 100,000 hours. An oversized tank wastes capital, increases stagnation, and on chlorinated systems eats free-chlorine residual the customer paid for. The math is straightforward and lives in WQA and AWWA design references, but the field still installs the next tank in the truck without doing the calculation. This is the calculation.

The variables

  • Pump cut-in pressure (Pon): pressure at which the pump starts. Typically 30, 40, or 50 psi.
  • Pump cut-out pressure (Poff): pressure at which the pump stops. Typically 50, 60, or 70 psi. The differential is conventionally 20 psi.
  • Drawdown volume (Vd): usable water the tank delivers between Poff and Pon. This is the spec that matters.
  • Tank acceptance volume (Va): total water the tank shell can hold at Poff. Always larger than drawdown.
  • Pre-charge pressure (Pc): the air pressure in the bladder with the tank empty. Always set to 2 psi below Pon.
  • Pump runtime per cycle: minimum 1 minute for fractional-horsepower pumps; 2 minutes for 1.5 HP and above. Manufacturer specs override.

The sizing equation

Drawdown is calculated using Boyle's Law applied to the air side of the bladder:

Vd = Va * ((Poff + 14.7) - (Pon + 14.7)) / (Poff + 14.7)

Or in the simplified form most tank manufacturers print on their data sheets:

Vd = Va * (Poff - Pon) / (Poff + 14.7)

For the canonical 30/50 setting (Pon=30, Poff=50, Pc=28):

  • Drawdown fraction = (50 - 30) / (50 + 14.7) = 0.309
  • A 36-gallon acceptance tank delivers about 11.1 gallons of drawdown
  • A 44-gallon acceptance tank delivers about 13.6 gallons
  • An 86-gallon acceptance tank delivers about 26.6 gallons

For the 40/60 setting (Pon=40, Poff=60, Pc=38):

  • Drawdown fraction = (60 - 40) / (60 + 14.7) = 0.268
  • A 36-gallon acceptance tank delivers about 9.6 gallons
  • A 44-gallon acceptance tank delivers about 11.8 gallons
  • An 86-gallon acceptance tank delivers about 23.0 gallons

For the 50/70 setting (Pon=50, Poff=70, Pc=48):

  • Drawdown fraction = (70 - 50) / (70 + 14.7) = 0.236
  • A 44-gallon acceptance tank delivers about 10.4 gallons
  • An 86-gallon acceptance tank delivers about 20.3 gallons

Raising cut-out without changing tank size reduces drawdown. Customers who upgrade to a 50/70 setting for stronger shower flow without resizing the tank are the next short-cycle service call.

Tying drawdown to pump runtime

The required drawdown is the volume that makes the pump run for at least its minimum cycle time at the design flow rate.

Vd (minimum) = Pump GPM * Minimum runtime in minutes

Examples:

  • 10 GPM pump, 1 minute minimum cycle: 10 gallons of drawdown minimum
  • 15 GPM pump, 1 minute minimum cycle: 15 gallons of drawdown minimum
  • 20 GPM pump, 2 minute minimum cycle: 40 gallons of drawdown minimum

Match the drawdown to the pump, not the other way around. A 20 GPM submersible on a 36-gallon tank at 30/50 delivers only 11 gallons of drawdown, which gives a 33-second cycle. That pump is grenading itself.

Pre-charge: the part everyone forgets

The bladder pre-charge must be set to 2 psi below the pump cut-in pressure, with the tank empty of water (drained through a hose bib with the pump breaker off and a downstream tap open).

  • 30/50 system: pre-charge to 28 psi
  • 40/60 system: pre-charge to 38 psi
  • 50/70 system: pre-charge to 48 psi

Check pre-charge annually with an automotive tire gauge on the Schrader valve at the top of the tank. A waterlogged tank (bladder failed or pre-charge lost) is the single most common cause of "my pump runs every 30 seconds" calls. Verification takes 5 minutes.

Constant-pressure (VFD) systems

Variable-frequency-drive pump controllers modulate pump speed to hold a setpoint pressure. They reduce but do not eliminate the need for a bladder tank:

  • A small (4 to 20 gallon) tank still required to absorb pressure transients and prevent pump cycling on micro-draws
  • Manufacturer spec sheet (Franklin SubDrive, Grundfos CU301, Pentair SubMonitor) governs the minimum tank size for the chosen controller
  • Pre-charge is typically set to the controller's stop pressure minus 2 psi

Field sizing decision table

Household occupants Peak fixture demand Typical pump GPM Recommended drawdown Tank acceptance at 30/50
1-2 5-7 GPM 7-10 10 gallons 34 gallons
3-4 8-12 GPM 10-15 15 gallons 48 gallons
5-6 12-16 GPM 15-20 25 gallons 81 gallons
7+ or any home with irrigation, hot tub, or two-tank water heater 16+ GPM 20+ 40 gallons 119 gallons (or two 81s in parallel)

The 81-gallon and 119-gallon WX-302 and WX-350 class tanks are stocked by every Amtrol, Wellmate, and Flexcon distributor; the column is volume class, not a single-vendor recommendation.

Installation checklist

  1. Calculate required drawdown from pump GPM and minimum runtime.
  2. Pick the tank acceptance volume that delivers that drawdown at the chosen cut-in / cut-out setting.
  3. Drain the new tank fully and set pre-charge with the Schrader valve before plumbing it in.
  4. Install a tank tee, isolation ball valve, drain valve, and a pressure gauge in plain sight.
  5. Pressurize the system, cycle the pump three times, and time one full cycle from cut-in to cut-out. Confirm it meets or exceeds the minimum runtime.
  6. Set a reminder for the homeowner: pre-charge check annually.

References

  • NSF/ANSI 61, Drinking Water System Components: Health Effects (bladder material certification)
  • ANSI/AWWA A100, Standard for Water Wells (Section 8, Storage and Pressure Equipment)
  • WQA Water Treatment Fundamentals, Chapter on Hydropneumatic Storage
  • Goulds Pumps Technical Manual, Pressure Tank Sizing
  • Franklin Electric AIM Manual, Submersible Motor Application Guide (cycle-time minimums)

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