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Expansion-Tank Preliminary Sizing Calculator

Estimate diaphragm-tank volume from system volume, entered fluid expansion, and initial and maximum absolute pressure relationships.

Why it matters: Required tank volume depends on fluid expansion and the usable acceptance fraction between initial and maximum pressures.

User inputs

Enter field measurements

6 inputs
Select I-P or SI units. Unit-dependent entries are converted when the selection changes.
gal or LDocumented total system fluid volume.
%Volume expansion from the selected cold to hot condition based on the actual fluid.
psig or kPa gCold initial gauge pressure at the tank connection.
psig or kPa gMaximum allowable operating gauge pressure at the tank connection.
psi or kPaLocal absolute atmospheric pressure used to convert gauge pressure to absolute.

Calculated result

Engineering evaluation

Preliminary sizing complete
Preliminary tank volume (Vtank)74.7 gal
Fluid expansion volume (Vexp)
30 gal
Tank acceptance fraction (Fa)
0.402 ratio
Initial absolute pressure (P1a)
44.7 psia
Maximum absolute pressure (P2a)
74.7 psia

Engineering interpretation

The entered assumptions produce 30 gal expansion volume and 40.16% acceptance, requiring approximately 74.7 gal preliminary tank volume.

Important limitations

  • Final selection must address tank type, precharge, connection location, fill pressure, minimum/maximum temperatures, glycol properties, relief setting, equipment ratings, reserve, and manufacturer sizing.

Engineering knowledge center

Understand the calculation—not just the answer

2026-08-07 · Free Library
Governing relationships
P1a = P1g+Patm
P2a = P2g+Patm
Vexp = Vsys(e/100)
Fa = 1−P1a/P2a
Vtank = Vexp/Fa
Current substitution

P1a = 30+14.7 = 44.7; P2a = 60+14.7 = 74.7; Vexp = 1000×3/100 = 30 gal; Vtank = 74.7 gal.

Calculation sequence

Step-by-step method

  1. Convert initial and maximum gauge pressures to absolute pressure.
  2. Calculate expanded fluid volume from system volume and entered expansion percentage.
  3. Calculate tank acceptance fraction and divide expansion volume by that fraction.

Equation legend

Inputs and calculated quantities

Vsys
System fluid volume — gal or L
e
Entered fluid expansion — %
P1g
Initial tank pressure — psig or kPa g
P2g
Maximum tank pressure — psig or kPa g
Patm
Atmospheric pressure — psi or kPa
Vtank
Preliminary tank volume — gal
Vexp
Fluid expansion volume — gal
Fa
Tank acceptance fraction — ratio
P1a
Initial absolute pressure — psia
P2a
Maximum absolute pressure — psia

Continue the evaluation

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Engineering calculation disclaimer

These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.

Read the full use and limitation statement