What is the Walther equation?

The Walther (ASTM D341) equation: log log(ν + 0.7) = A − B × log(T), where T is in Kelvin. It produces a straight line on ASTM viscosity-temperature paper. Accurate for mineral oils from -20°C to 150°C.

What is Viscosity Index (VI)?

VI indicates how much viscosity changes with temperature. Higher VI = less change. Mineral oils: VI ≈ 80-100, synthetic PAO: VI ≈ 120-160, multigrade engine oils: VI > 150.

What is ISO VG classification?

ISO VG classifies industrial lubricants by kinematic viscosity at 40°C. VG 32 means ν₄₀ ≈ 32 cSt. Range: VG 2 to VG 1500.

How accurate is the Walther equation?

Very accurate for mineral oils (-20 to 150°C). Less accurate for synthetics, very low temps (near pour point), or very high temps (near flash point).

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 and pour point estimation.

Walther / ASTM D341Viscosity IndexISO VGPour Point

Viscosity Results

Viscosity at Target Temperature

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Viscosity Index (VI)

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Operating Range Check

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Walther Constant A

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Walther Constant B

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Reference: ν40 / ν100

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Estimated Pour Point

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Dynamic Viscosity (η)

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Walther Chart — Viscosity vs Temperature

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

Example — VG 68 at 80°C

Given: ν₄₀=68 cSt, ν₁₀₀=8.7 cSt

Walther: A=8.8445, B=3.5912

At 80°C (353.15 K): log log(ν+0.7)=8.8445−3.5912×log(353.15)=8.8445−9.1470=−0.3025

ν = 10^(10^(−0.3025))−0.7 = 17.6 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.

Oil Viscosity–Temperature Calculator | Walther Equation | VI | Free Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026