Viscosity Index & Temperature Calculator

Compute VI and the viscosity–temperature curve from ν₄₀ and ν₁₀₀

Enter the kinematic viscosity at 40 °C and 100 °C and the tool returns the ASTM D2270 viscosity index along with the ASTM D341 (Walther) viscosity at any temperature.

1Enter kinematic viscosities

Viscosity at 40 °C

ν₄₀ [mm²/s (cSt)]

Viscosity at 100 °C

ν₁₀₀ [mm²/s (cSt)]

2Results

Viscosity Index (ASTM D2270)154VI > 100 (higher-VI regime)

Reference L0-VI reference at Y141.96mm²/s

Reference H100-VI reference at Y89.61mm²/s

SAE J300 high-temp gradeSAE 30

Low-temperature (W) grades require CCS / MRV data and are not computed here.

Viscosity–Temperature Curve (ASTM D341 / Walther)

Probe temperature80 °C

Predicted viscosity17.41mm²/s

The curve fits the Walther equation through the two input points (40 °C / 100 °C). White markers show the inputs; the cyan marker and vertical line track the slider temperature.

A7.82297

B3.03118

Y = ν₁₀₀10.900 mm²/s

U = ν₄₀64.000 mm²/s

One page with the inputs, results, equations, standards, and source URL.

Equations and standards

ASTM D2270ISO 2909ASTM D341SAE J300

\mathrm{VI} = \frac{L - U}{L - H} \times 100 \qquad (U \geq H,\ \mathrm{VI} \leq 100)

L and H are the 40 °C viscosities of the reference oils for the same nu100, interpolated from the ASTM D2270 Annex A1 table.

N = \frac{\log_{10} H - \log_{10} U}{\log_{10} Y}, \qquad \mathrm{VI} = \frac{10^{N} - 1}{0.00715} + 100

Used when U < H, i.e. for oils with VI above 100.

L = 0.8353\,Y^{2} + 14.67\,Y - 216, \qquad H = 0.1684\,Y^{2} + 11.85\,Y - 97

Above nu100 = 70 mm²/s these quadratics replace the table (ASTM D2270).

\log_{10}\!\left[\log_{10}(\nu + 0.7)\right] = A - B\,\log_{10} T

ASTM D341 (Walther). A and B are fixed by the 40 °C and 100 °C points, then give viscosity at any temperature. T is absolute.

Symbols and units

$U = \nu_{40}$	Kinematic viscosity of the test oil at 40 °C	mm²/s
$Y = \nu_{100}$	Kinematic viscosity of the test oil at 100 °C	mm²/s
$L$	40 °C viscosity of the VI = 0 reference oil with the same nu100	mm²/s
$H$	40 °C viscosity of the VI = 100 reference oil with the same nu100	mm²/s
$A,\ B$	Oil-specific constants of the Walther relation	—
$T$	Absolute temperature	K

Valid range and limitations

·ASTM D2270 is defined for nu100 >= 2.0 mm²/s. It does not apply to lighter fluids.
·nu40 must exceed nu100. Inverted inputs mean a measurement error or two temperatures swapped.
·VI is a practical comparison index roughly over 60–350. Treat values far outside that range as extrapolation.
·The Walther relation holds only in the liquid range — not below the cloud or pour point, and not in the high-shear regime of shear-thinning multigrades.

Reproduce in Excel

Viscosity index, VI <= 100 case

=(L-U)/(L-H)*100

U = nu40; L and H are read from the ASTM D2270 Annex A1 table for the sample's nu100.

Viscosity index, VI > 100 case

=(10^((LOG10(H)-LOG10(U))/LOG10(Y))-1)/0.00715+100

U = nu40, Y = nu100, H from the table. Use this branch when U < H.

Walther constants B and A

B: =(LOG10(LOG10($A$1+0.7))-LOG10(LOG10($B$1+0.7)))/(LOG10(373.15)-LOG10(313.15))
A: =LOG10(LOG10($A$1+0.7))+$B$3*LOG10(313.15)

A1 = nu40, B1 = nu100. Put B in cell B3, then compute A.

Viscosity at any temperature (ASTM D341)

=10^(10^($A$3-$B$3*LOG10(C1+273.15)))-0.7

A3 = A, B3 = B, C1 = temperature in °C. Result in mm²/s.

Assumptions & Limitations

VI uses the ASTM D2270 L/H reference table with linear interpolation for Y ≤ 70 mm²/s, and the standard's polynomials (L = 0.8353·Y² + 14.67·Y − 216, H = 0.1684·Y² + 11.85·Y − 97) for Y > 70.
The viscosity–temperature curve extrapolates a two-point Walther fit, log₁₀(log₁₀(ν + 0.7)) = A − B · log₁₀(T). Real oils may deviate noticeably below 0 °C or above 150 °C.
We assume Newtonian behaviour with no shear-rate dependence. Multigrade oils may show temporary or permanent shear loss (TSL/PSL) in service due to polymer additives.
SAE J300 grading shown here is for high-temperature classification only. W-grade classification requires CCS / MRV data and is out of scope.

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2Results

Viscosity Index (ASTM D2270)154VI > 100 (higher-VI regime)

Reference L0-VI reference at Y141.96mm²/s

Reference H100-VI reference at Y89.61mm²/s

SAE J300 high-temp gradeSAE 30

Low-temperature (W) grades require CCS / MRV data and are not computed here.

Viscosity–Temperature Curve (ASTM D341 / Walther)

Probe temperature80 °C

Predicted viscosity17.41mm²/s

The curve fits the Walther equation through the two input points (40 °C / 100 °C). White markers show the inputs; the cyan marker and vertical line track the slider temperature.

A7.82297

B3.03118

Y = ν₁₀₀10.900 mm²/s

U = ν₄₀64.000 mm²/s

One page with the inputs, results, equations, standards, and source URL.

Assumptions & Limitations

VI uses the ASTM D2270 L/H reference table with linear interpolation for Y ≤ 70 mm²/s, and the standard's polynomials (L = 0.8353·Y² + 14.67·Y − 216, H = 0.1684·Y² + 11.85·Y − 97) for Y > 70.

The viscosity–temperature curve extrapolates a two-point Walther fit, log₁₀(log₁₀(ν + 0.7)) = A − B · log₁₀(T). Real oils may deviate noticeably below 0 °C or above 150 °C.

We assume Newtonian behaviour with no shear-rate dependence. Multigrade oils may show temporary or permanent shear loss (TSL/PSL) in service due to polymer additives.

SAE J300 grading shown here is for high-temperature classification only. W-grade classification requires CCS / MRV data and is out of scope.