A starting tank size from pump flow, with air space and cylinder volume added.
For a variable pump, use the mean flow. For a closed circuit, use the charge pump.
3–5 industrial, 1.5–2 mobile, 5–8 HFC or HFD fluid.
Empty uses 10%.
The oil that leaves the tank with every cylinder extended.
Example Result
Your result will appear here
The calculated result will appear here after you press Calculate.
The usual starting point for a hydraulic tank is a few minutes of pump flow. Enter the flow and a multiple, then add air space and the oil your cylinders draw out. The result is a first estimate, not a design: heat, fluid type and space can all move it.
Enter the pump flow. Use the mean flow for a variable pump, or the charge pump flow for a closed circuit.
Enter a multiple from the hint under the field.
Leave air space empty for 10%, or enter your own.
Add the oil your cylinders draw from the tank, then press Calculate.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
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A tank does more than store oil. It gives returning oil time to release air, drop out dirt and water, and shed some heat. A multiple of 3 means the oil spends about three minutes in the tank on average, so the multiple is really a dwell time.
Tank oil volume = pump flow × multiple
Tank size = oil volume × (1 + air space) + cylinder volume swing
For a variable pump, use the mean flow, not the maximum. Multiples are rules of thumb from industry practice. They are not limits from a standard.
| System | Multiple of pump flow per minute |
|---|---|
| Open circuit, industrial | 3–5 |
| Open circuit, fire-resistant HFC or HFD fluid | 5–8 |
| Open circuit, mobile machine | 1.5–2 |
| Closed circuit (hydrostatic), industrial | 3–5 × charge pump flow |
| Closed circuit, mobile | 1–2 × charge pump flow |
Closed-circuit systems return most oil straight to the pump, so only the charge pump flow passes through the tank. Entering the main pump flow here gives a tank far too big.
Mobile machines run smaller tanks because of weight and space, and they pay for it with less margin for air and heat. Oil that has to give up air slowly, such as a thick grade in the cold, needs more dwell time, so check the grade against the viscosity reference.
Pump flow is 40 L/min, the multiple is 4, and air space is 10%.
How the choice of multiple changes it:
| Multiple | Oil volume |
|---|---|
| 1.5 | 60 L |
| 3 | 120 L |
| 5 | 200 L |
| 8 | 320 L |
Pump flow is 10 gpm, the multiple is 3, and air space is 10%.
When a cylinder extends, the level falls by the rod volume. Two 80/56 mm cylinders with a 500 mm stroke draw down 2.46 L, which the hydraulic oil volume calculator works out for you. On a machine with long cylinders this swing can exceed the rule's margin.
Check the low level with every cylinder extended, so the pump inlet stays covered. Check the high level with every cylinder retracted, so the tank does not overflow through the breather.
The multiple says nothing about heat. Heat comes from the wasted part of the pump's power, which you can find from the hydraulic pump power calculator. A system that runs at full load all day often needs a cooler whatever its tank size, while a light intermittent one may not. A cooler can also let a designer use a smaller tank.
Shape matters too. A tall thin tank has less surface for air to escape than a low wide one of the same volume, and baffles stop return oil going straight to the suction line. Pump flow itself comes from the hydraulic pump flow calculator.
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