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Formulas, threads, fluids and conversions.
The engineering reference our own team uses — core hydraulic formulas, thread identification, fluid selection and a working unit converter.
Fourteen formulas that cover most of the work.
Cylinder force
F = P · A · η
- F = force (N)
- P = pressure (Pa)
- A = piston area (m²)
- η = efficiency
210 bar × 78.5 cm² → ≈ 165 kN (adjusted for efficiency)
Cylinder speed
v = Q / A
- Q = flow (m³/s)
- A = area (m²)
40 l/min into 78.5 cm² → ≈ 0.085 m/s
Cylinder volume
V = A · stroke
- stroke = length (m)
78.5 cm² × 500 mm → ≈ 3.93 L
Pump flow
Q = Vd · n · ηv
- Vd = displacement (m³/rev)
- n = speed (rev/s)
- ηv = vol. efficiency
32 cc × 1450 rpm × 0.92 → ≈ 42.7 l/min
Pump displacement
Vd = Q / (n · ηv)
- Q = flow
- n = speed
Solve required displacement for a target flow
Hydraulic power
P = p · Q
- p = pressure (Pa)
- Q = flow (m³/s)
210 bar × 40 l/min → ≈ 14 kW
Pump torque
T = (Vd · Δp) / (2π · ηm)
- Δp = pressure diff.
- ηm = mech. eff.
Input torque at the pump shaft
Hydraulic motor speed
n = (Q · ηv) / Vd
- Q = flow
- Vd = displacement
Output speed of a hydraulic motor
Hydraulic motor torque
T = (Vd · Δp · ηm) / (2π)
- Δp = pressure diff.
Output torque of a hydraulic motor
Pipe velocity
v = Q / (π/4 · d²)
- d = internal diameter (m)
40 l/min through Ø19 mm → ≈ 2.4 m/s
Pressure drop (Darcy)
Δp = f · (L/d) · (ρ·v²/2)
- f = friction factor
- L = length
- ρ = density
Straight-run line loss
Accumulator gas law
P₁V₁ = P₂V₂ (isothermal)
- P = pressure
- V = gas volume
Boyle’s law for accumulator sizing
Heat generation
Q_heat = P_in − P_out
- Power lost to heat
Losses convert to heat in the oil
Convert pressure, flow, force, power and more.
Pick a quantity and a source unit, type a value, and every equivalent updates instantly. Useful when a datasheet is in psi and your drawing is in bar.
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Know the thread before you fit it.
Always verify before assembly. Thread forms can appear similar but are not interchangeable. Mismatched threads may appear to engage and then fail under pressure. Confirm with thread gauges and manufacturer data, and never pressurise a connection you have not positively identified.
| Thread | Form | Sealing method | Typical use |
|---|---|---|---|
| BSPP (G) | Parallel | Bonded seal / O-ring on face | Common on ports and adaptors |
| BSPT (R) | Tapered | Thread interference + sealant | Older / low-medium pressure |
| NPT | Tapered | Thread interference + sealant | North American standard |
| JIC 37° | Parallel | 37° flare cone seat | Very common hydraulic connection |
| ORFS | Parallel | O-ring on flat face | High-pressure, low-leakage |
| SAE ORB | Parallel | O-ring boss | Preferred port connection |
| Metric parallel | Parallel | O-ring / bonded seal | DIN / ISO ports |
| Metric tapered | Tapered | Thread + sealant | Some European equipment |
| DIN 2353 (L/S) | Parallel | Cutting-ring / 24° cone | Light & heavy tube series |
| SAE Flange 61 | Flange | O-ring under clamp | To ~ 350 bar |
| SAE Flange 62 | Flange | O-ring under clamp | High pressure |
Choosing and protecting the fluid.
ISO viscosity grades
ISO VG 32, 46 and 68 are the most common hydraulic grades; the number is the nominal viscosity in cSt at 40 °C.
Viscosity–temperature
Viscosity falls as temperature rises. The viscosity index (VI) describes how stable a fluid is across the operating range.
Mineral oil
General-purpose hydraulic fluid for most industrial and mobile systems.
Biodegradable fluids
Ester-based fluids for environmentally sensitive applications.
Fire-resistant fluids
HFA/HFB/HFC/HFD groups for high fire-risk environments.
Water glycol (HFC)
Fire-resistant fluid — check seal and material compatibility.
Marine EALs
Environmentally Acceptable Lubricants for vessels operating in regulated waters.
Contamination
Target an ISO 4406 cleanliness code appropriate to the most sensitive component.
Working through a calculation?
Run the numbers in our calculator, or send us the application and we will size it with you.