Viscosity Index Calculator

Find Viscosity Index from KV40 & KV100

Kinematic Viscosity @ 40°C

cSt

Kinematic Viscosity @ 100°C

cSt

Example Oils

Viscosity values must be greater than zero.

How the Viscosity Index Calculator Works

Calculate the Viscosity Index of lubricating oils per ASTM D2270

Viscosity Index (VI) measures how much an oil’s viscosity changes with temperature. A higher VI means the oil’s viscosity is more stable across temperature changes — it thins less when hot and thickens less when cold. This is critical for engine oils, hydraulic fluids, and industrial lubricants that must perform across wide temperature ranges.

When KV40 ≥ H (VI ≤ 100)

VI = ((L − KV40) / (L − H)) × 100

Linear formula — KV40 meets or exceeds reference

When KV40 < H (VI > 100)

VI = ((10^N − 1) / 0.00715) + 100

Logarithmic formula — KV40 is below reference

Worked Example — SAE 5W-30 Synthetic Oil

KV40 = 57 cSt, KV100 = 10.3 cSt

KV100 ≤ 70 → L = 155.4, H = 86.51 (ASTM D2270 table)

KV40 (57) < H (86.51) → VI > 100, use logarithmic formula

N = log₁₀(86.51 / 57) / log₁₀(10.3) = 0.1787 → VI = ((10^0.1787 − 1) / 0.00715) + 100 = 171 (Ultra High)

Understanding Viscosity Index

What VI tells you about lubricant performance

All oils thin out as temperature increases. The Viscosity Index quantifies how much thinning occurs. An oil with VI = 0 (naphthenic base) changes dramatically with temperature, while an oil with VI = 200 (high-end synthetic) maintains nearly constant viscosity.

Oil Type	Typical VI	Characteristics
Naphthenic mineral oil	0–35	Poor temperature stability; used where narrow temp range
Group I paraffinic base oil	80–100	Standard mineral oil; adequate for moderate conditions
Group II/III hydrocracked	100–130	Improved oxidation resistance and VI; modern engine oils
PAO synthetic	130–160	Excellent low and high-temp performance; extended drain intervals
Silicone / Ester	200–400	Near-constant viscosity; aerospace and extreme applications

ASTM D2270 Formula Explained

The two branches of the Viscosity Index calculation

L and H Source: KV100 ≤ 70 cSt (Table Lookup)

L is the viscosity at 40°C of a VI=0 oil with the same KV100 as the sample. H is the viscosity at 40°C of a VI=100 oil with the same KV100 as the sample. Both L and H are read from the ASTM D2270 reference table using interpolation.

The formula branch is then chosen by comparing KV40 to H, not by KV100 alone.

L and H Source: KV100 > 70 cSt (Polynomial Formulas)

For KV100 above 70 cSt, L and H cannot be read from the standard table and are instead computed from polynomial formulas:
L = 0.83531 × KV100² + 14.6731 × KV100 − 216.246
H = 0.16841 × KV100² + 11.8493 × KV100 − 96.947
The same branch logic applies — if KV40 ≥ H use the linear formula, otherwise use the logarithmic formula.

N = (log H − log KV40) / log KV100

VI = ((10^N − 1) / 0.00715) + 100

ASTM D2270 rounds VI to the nearest whole number using ASTM E-29 half-even rounding: values exactly halfway between two numbers round to the nearest even number (e.g. 117.5 → 118, 118.5 → 118). VI below 0 is reported as 0.

Key Considerations & Common Mistakes

Important factors to consider when using Viscosity Index

VI Is Not Viscosity

VI measures the rate of change of viscosity with temperature, not the viscosity itself. Two oils can have the same VI but very different viscosities. Always specify both the viscosity grade and VI when selecting lubricants.

Shear Stability Matters

High VI achieved through VI improver additives can shear down in service, reducing the effective VI over time. Synthetic base oils maintain their VI even under high shear conditions.

KV100 Must Be Lower Than KV40

Oil always thins as temperature increases. If your KV100 value equals or exceeds KV40, the measurement is likely incorrect or the temperatures were swapped.

Temperature Precision

Small errors in temperature measurement (even ±0.1°C) can significantly affect the calculated VI. Per ASTM D445, viscometer bath temperature must be controlled within ±0.02°C for accurate kinematic viscosity measurements used in VI calculations.

Frequently Asked Questions

Common questions and detailed answers

Viscosity Index (VI) is a dimensionless number that measures how much an oil's viscosity changes with temperature. A higher VI means the oil's viscosity is more stable — it thins less at high temperatures and thickens less at low temperatures. VI is calculated from the oil's kinematic viscosity at 40°C and 100°C per ASTM D2270. Mineral oils typically have VI of 80–110, while synthetic oils can reach 140–200+.

Viscosity Index is calculated per ASTM D2270 using kinematic viscosity at 40°C (KV40) and 100°C (KV100). First, determine L and H reference values: if KV100 ≤ 70 cSt, interpolate L and H from the ASTM D2270 table; if KV100 > 70 cSt, compute them from polynomial formulas. Then, if KV40 ≥ H (giving VI ≤ 100), use VI = ((L − KV40) / (L − H)) × 100. If KV40 < H (giving VI > 100), use N = log₁₀(H / KV40) / log₁₀(KV100), then VI = ((10^N − 1) / 0.00715) + 100. The result is rounded to the nearest whole number per ASTM E-29.

ASTM D2270 uses L (KV40 of a VI=0 oil) and H (KV40 of a VI=100 oil), both having the same KV100 as the sample. When KV40 ≥ H (VI ≤ 100): VI = ((L − KV40) / (L − H)) × 100. When KV40 < H (VI > 100): N = log₁₀(H / KV40) / log₁₀(KV100), then VI = ((10^N − 1) / 0.00715) + 100. For KV100 ≤ 70 cSt, L and H come from the standard table. For KV100 > 70 cSt, they are computed from polynomial formulas: L = 0.83531·KV100² + 14.6731·KV100 − 216.246 and H = 0.16841·KV100² + 11.8493·KV100 − 96.947.

For modern engine oils, a VI of 120–160 is considered good. Conventional mineral engine oils typically have VI around 95–110. Synthetic and semi-synthetic oils achieve 130–160+. High-performance and long-life synthetic oils can exceed 170. SAE multi-grade oils (like 5W-30) use VI improver additives and base oil selection to achieve high VI, allowing the oil to flow adequately when cold while maintaining protective film strength when hot.

Viscosity is the oil's resistance to flow at a specific temperature — it tells you how thick or thin the oil is. Viscosity Index tells you how much that viscosity changes as temperature changes. Two oils can have the same viscosity at 100°C but very different viscosities at 40°C, giving them different VI values. Think of viscosity as a speed measurement and VI as how stable that speed remains across different conditions.

1) Measure KV40 and KV100 per ASTM D445. 2) Determine L and H: if KV100 ≤ 70 cSt, interpolate from the ASTM D2270 table; if KV100 > 70 cSt, use the polynomial formulas. 3) Check the branch: if KV40 ≥ H, use the linear formula VI = ((L − KV40) / (L − H)) × 100. If KV40 < H, use the logarithmic formula: N = log₁₀(H / KV40) / log₁₀(KV100), then VI = ((10^N − 1) / 0.00715) + 100. 4) Round to the nearest integer per ASTM E-29.

ASTM D2270 is the standard practice for calculating Viscosity Index from kinematic viscosity at 40°C and 100°C, published by ASTM International. First introduced in 1964 and last updated in 2024 (D2270-24 Active), it provides the reference tables and equations that convert two viscosity measurements into a single VI number. It is the globally recognized standard used by oil manufacturers, laboratories, and specification writers. ISO 2909 is the equivalent international standard.

Yes, if you know VI and one viscosity value, you can calculate the other. Given VI and KV100: first determine L and H (table for KV100 ≤ 70, formulas for KV100 > 70). If the target VI ≤ 100, use KV40 = L − (VI/100)(L − H). If VI > 100, use N = log₁₀((VI − 100) × 0.00715 + 1), then KV40 = H / KV100^N. Given VI and KV40: an iterative (binary search) approach finds the KV100 that produces the target VI, since both the L/H source and formula branch depend on the unknown KV100 value.

Three main factors affect VI: 1) Base oil type — paraffinic oils have higher natural VI than naphthenic oils. Synthetic base stocks (PAO, esters) have inherently higher VI than mineral oils. 2) VI improver additives — long-chain polymer molecules that expand at high temperatures to counteract thinning, boosting VI by 20–50 points. 3) Molecular structure — more linear, saturated hydrocarbon chains give higher VI. Aromatic and branched molecules reduce VI.

VI matters most when equipment operates across wide temperature ranges — like car engines that must start at −20°C and run at 100°C+. A high VI oil maintains adequate film thickness when hot (preventing wear) while remaining pumpable when cold (preventing startup damage). Low VI oils may be acceptable for indoor industrial equipment with stable operating temperatures, where cost savings outweigh the benefits of temperature stability.

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Last updated May 9, 2026