Hydropneumatic Tank Drawdown Calculator

Drawdown is the water a pressure tank actually delivers between pump cut-out and cut-in. Enter your tank and switch settings to size it with Boyle’s law.

Tank total volume (gal)

Precharge pressure (psig)Measured with the tank empty of water. Typically 2 psi below cut-in.

Cut-in pressure (psig)Pump starts.

Cut-out pressure (psig)Pump stops.

Pump flow (gpm) — optionalFill in to estimate the drawdown (pump-off) time between cycles.

Tank Drawdown

Item	Value

How the calculation works

Boyle’s law on the air cushion (absolute pressures, psia = psig + 14.7)
P₀ = precharge, P₁ = cut-in, P₂ = cut-out (all absolute)
Drawdown V_d = V_tank × P₀ × (1/P₁ − 1/P₂)
When precharge equals cut-in (P₀ = P₁), this simplifies to V_d = V_tank × (1 − P₁/P₂)
Drawdown time ≈ drawdown (gal) ÷ pump flow (gpm)

Common switch setting	Cut-in / cut-out (psig)	Typical drawdown (% of tank)
20/40	20 / 40	~30%
30/50	30 / 50	~27%
40/60	40 / 60	~25%

Notes & limitations

Boyle’s law (P₁V₁ = P₂V₂ at constant temperature) is the standard basis for hydropneumatic tank drawdown sizing (AWWA M22; tank manufacturer engineering data).

Pressures must be absolute in the formula — this tool adds 14.7 psi for you. Never mix gauge and absolute pressures.
Set precharge with the tank drained of water, about 2 psi below cut-in. Precharge above cut-in wastes usable drawdown; precharge far below cut-in risks the bladder/diaphragm over-extending.
Drawdown controls pump cycling: short drawdown (or a waterlogged tank with lost air) causes rapid cycling and motor wear. Size tanks for an acceptable minimum pump run time per the motor manufacturer’s starts-per-hour limit.
Bladder/diaphragm and plain steel (air-over-water) tanks follow the same Boyle’s-law math, but air-over-water tanks lose air over time and need an air-volume control.

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