How to use this calculator
- Enter pump flow. Use the total return flow through the reservoir.
- Choose residence time. Use the minutes of oil residence you want before recirculation.
- Set usable fraction. Set the normal oil fill fraction, leaving headspace above.
- Read tank volume. Read usable oil volume, nominal tank volume and turnover rate.
How it works
The residence-time sizing rule is intentionally simple: usable oil volume = Q · t where Q is pump flow and t is the desired minutes of oil residence in the reservoir. The nominal tank volume then allows headspace: tank volume = usable oil volume / usable fraction
Worked example
Verified against the live calculator
A 40 L/min hydraulic unit with a 3 minute residence target needs
40 × 3 = 120 L of usable oil. At an 80% normal fill level, the nominal
reservoir is 120 ÷ 0.8 = 150 L. The calculator reports those values
and a tank turnover of about 0.267 per minute.
Frequently asked questions
How big should a hydraulic reservoir be?
A common first pass is pump flow multiplied by a residence time, often around 2–3 minutes for industrial hydraulic systems. The calculator also divides by the usable oil fraction to allow headspace above the oil.
Is three times pump flow always enough?
No. Heat load, aeration, return-line velocity, cylinder volume change and duty cycle can require a larger or smaller tank. This calculator is a screening check, not a thermal design.
What is usable oil fraction?
It is the fraction of total tank volume normally occupied by oil. A value of 0.8 means an 80% fill level and 20% headspace for expansion, foam and slosh.
What does tank turnover mean?
Tank turnover is pump flow divided by nominal tank volume. A lower turnover gives oil more time to deaerate and shed heat.
Method & assumptions
- Uses a residence-time rule, not a heat-balance calculation.
- Assumes one combined reservoir volume; split tanks, high return velocities and poor baffling need separate checks.
- Final tank design should check heat rejection, deaeration, suction head, expansion space and cylinder volume change.