How to use this calculator
- Choose line type. Select suction, return, pressure, case drain or a custom velocity target.
- Enter flow and actual ID. Use the design flow through the line and the actual inside diameter of the hose, tube or pipe.
- Set oil viscosity. Enter the kinematic viscosity at the operating temperature you want to screen.
- Read velocity margin. Compare oil velocity, target utilization, required ID and Reynolds number.
- Follow with pressure drop. Use the selected actual ID in the hydraulic hose pressure-drop calculator.
How it works
Hydraulic line velocity starts with continuity:
A = pi x D^2 / 4 v = Q / A
where Q is oil flow and D is actual inside diameter.
The calculator also reverses the same relation to show the line ID needed
for the selected velocity target:
D_req = sqrt(4Q / (pi x v_target))
Use this as the first screen before pressure-drop detail. Once a real hose or tube size is selected, carry that ID into the hydraulic hose pressure-drop calculator. If pump flow or actuator timing is still the unknown, start with the hydraulic pump flow and HP calculator or the hydraulic cylinder speed calculator.
Worked example
Verified against the live calculator
With the default 40 L/min through a 19 mm pressure
line, velocity is about 2.35 m/s. The default pressure-line
target is 4.5 m/s, so utilization is about
52%. The same flow would need about
13.7 mm actual ID to meet that target.
Reference data
The line-type targets are screening presets, not a universal standard. Use custom target velocity when a hose maker, tube supplier, project standard or pressure-drop budget gives a specific limit.
| Line type | Preset target | Caution band | Use case |
|---|---|---|---|
| Suction / inlet line | 1.0 m/s | 1.5 m/s | Pump inlet and reservoir suction runs. |
| Return line | 3.0 m/s | 4.5 m/s | Tank return lines after valves, coolers or filters. |
| Pressure line | 4.5 m/s | 7.5 m/s | Pump discharge and actuator pressure lines. |
| Case drain / pilot line | 2.0 m/s | 3.0 m/s | Low-pressure drain or pilot lines that need low back pressure. |
Source: Formula-only velocity screen with conservative preset targets; manufacturer hose/tube data and pressure-drop checks control final selection.
Frequently asked questions
How do you calculate hydraulic fluid velocity?
Use continuity. Convert flow to volume per second, calculate line area A = pi*D^2/4 from actual inside diameter, then v = Q/A.
What hydraulic line velocity should I use?
Use the preset only as a first-pass screen. Suction lines usually need the lowest velocity, return and case-drain lines sit in the middle, and pressure lines can tolerate higher velocity. Final limits depend on pressure drop, suction margin, noise, heat, hose/tube rating and manufacturer guidance.
Is this a hydraulic hose size calculator?
It back-solves actual inside diameter from flow and target velocity. After choosing a real hose, tube or pipe size, check pressure drop with the hydraulic hose pressure-drop calculator and verify manufacturer ratings.
Why enter oil viscosity?
Viscosity is used for Reynolds number. It also matters for pressure drop: cold oil can be much more viscous than hot oil, so a line that looks acceptable at operating temperature can lose much more pressure at startup.
Method & assumptions
- Uses actual inside diameter, not nominal hose dash, tube OD or pipe trade size.
- Velocity presets are first-pass screens for suction, return, pressure and case-drain lines.
- Reynolds number uses entered kinematic viscosity at the checked oil temperature.
- Does not calculate pressure drop, pump inlet NPSH, cavitation margin, hose rating, tube pressure rating, fitting losses, valve losses, heat generation, cold-start behavior or manufacturer catalog selection.