Free Engineering Tool
Oil Viscosity–Temperature Calculator
Calculate kinematic viscosity at any temperature using the Walther (ASTM D341) equation. Viscosity Index per ASTM D2270. Optimal operating range.
This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.
Viscosity Results
Walther Equation (ASTM D341)
Where ν is kinematic viscosity in mm²/s (cSt), T is absolute temperature (K), and A, B are constants determined from two reference points.
Viscosity Index (ASTM D2270)
VI compares the oil’s viscosity-temperature behavior to two reference oils: L (VI=0, naphthenic) and H (VI=100, paraffinic). Higher VI = less viscosity change with temperature.
Practical Example
Given: ν₄₀=68 cSt, ν₁₀₀=8.7 cSt
Walther: A=9.3097, B=3.6244
At 80°C (353.15 K): log log(ν+0.7)=9.3097−3.6244×log(353.15)=9.3097−9.2348=0.0749
ν = 10^(10^(0.0749))−0.7 = 14.7 cSt
VI ≈ 98 (typical mineral oil)
ISO VG Reference Table
| ISO VG | ν at 40°C (cSt) | ν at 100°C (cSt) | Typical Application |
|---|---|---|---|
| VG 10 | 10 | 2.6 | Spindle oils, hydraulics |
| VG 22 | 22 | 4.4 | Hydraulic systems |
| VG 32 | 32 | 5.4 | Hydraulic, light bearings |
| VG 46 | 46 | 6.8 | Hydraulic, general bearings |
| VG 68 | 68 | 8.7 | Heavy hydraulic, light gears |
| VG 100 | 100 | 11.4 | Circulating, light gears |
| VG 150 | 150 | 15 | Gears, bearings |
| VG 220 | 220 | 19 | Industrial gears |
| VG 320 | 320 | 24.5 | Heavy gears, worm |
| VG 460 | 460 | 31 | Heavily loaded gears |
| VG 680 | 680 | 40 | Very heavy gears |
| VG 1000 | 1000 | 52 | Open gears |
| VG 1500 | 1500 | 68 | Open gears, wire ropes |
💡 Optimal range: Most bearings and gears operate best at 13–100 cSt. Below 13 cSt boundary lubrication risk; above 100 cSt excessive churning.
The Walther equation (ASTM D341): log log(ν+0.7) = A − B × log(T). It produces a straight line on ASTM viscosity-temperature chart paper.
Very accurate for mineral oils from −20°C to 150°C. Less accurate for heavily additized synthetics, near pour point, or near flash point.
VI indicates how much viscosity changes with temperature. Higher = more stable.
- VI < 80: Low (naphthenic base)
- VI 80–100: Normal mineral oil
- VI 100–120: Good (HVI or semi-synthetic)
- VI 120–160: Very good (PAO synthetic)
- VI > 160: Excellent (multigrade, VI-improved)
For most bearings and gears: 13–100 cSt at operating temperature.
- Below 13 cSt: boundary lubrication risk, increased wear
- 13–30 cSt: hydrodynamic bearings, high-speed gears
- 30–100 cSt: general industrial gears and bearings
- Above 100 cSt: may cause churning losses, high startup torque
Kinematic viscosity: 1 cSt = 1 mm²/s exactly.
Dynamic viscosity: η = ν × ρ. Units: mPa·s = cSt × g/cm³. For typical oil at 15°C: ρ ≈ 0.87 g/cm³, so 100 cSt = 87 mPa·s.
SUS (Saybolt): roughly SUS ≈ 4.6 × cSt (for cSt > 50).
Pour point is the lowest temperature at which oil still flows. Below pour point, wax crystallization makes the oil too thick to pump. It matters for cold-start conditions, outdoor equipment, and Arctic applications.
Mineral oils: typically −15 to −30°C. PAO synthetics: −40 to −60°C.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
