What is the difference between ISO VG and SAE?

ISO VG (Viscosity Grade) classifies industrial lubricants by kinematic viscosity at 40°C. SAE grades are used for automotive oils: SAE crankcase grades (e.g. 10W-30) are based on viscosity at 100°C, while SAE gear grades (e.g. 75W-90) use a different scale. The systems measure at different temperatures and serve different markets.

Why are there different viscosity scales?

Different industries developed their own standards independently. ISO VG (ISO 3448) is used for industrial lubricants, SAE J300 for automotive engine oils, SAE J306 for automotive gear oils, and AGMA 9005 for industrial gears. Each standard was designed for its specific application and has different test conditions.

AGMA (American Gear Manufacturers Association) defines lubricant grades for industrial gearing per AGMA 9005. AGMA grades 1-8A correspond roughly to ISO VG 46 through VG 1000. The AGMA system is commonly used in North America for specifying gear lubricants.

What is the difference between cSt and cP?

cSt (centistokes, mm²/s) is kinematic viscosity — it measures flow under gravity. cP (centipoise, mPa·s) is dynamic viscosity — it measures resistance to flow under applied force. They are related by: cP = cSt × density (g/cm³). For most lubricating oils with density ~0.87 g/cm³, cP ≈ 0.87 × cSt.

How to read multigrade SAE oil?

Multigrade SAE oils like 10W-30 have two numbers: the 'W' (Winter) number indicates cold-start viscosity, and the second number indicates viscosity at operating temperature (100°C). A 10W-30 oil flows like a 10W at cold temperatures but provides the protection of a 30-grade at operating temperature. Multigrade oils use viscosity index improvers to achieve this behavior.

Free Engineering Tool

Viscosity Converter

Cross-reference between ISO VG, SAE crankcase, SAE gear, and AGMA viscosity grades. Also convert between cSt, cP, and SUS units.

ISO VG SAE J300 / J306 AGMA 9005 cSt ↔ cP ↔ SUS

Results

ISO VG Grade

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SAE Crankcase Grade

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SAE Gear Grade

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AGMA Grade

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Viscosity Range

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Dynamic Viscosity

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SUS (Saybolt Universal)

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ISO VG / SAE / AGMA Cross-Reference

Approximate equivalence between viscosity grading systems. These are not exact conversions — each system uses different measurement conditions.

ISO VG	cSt Range (40 °C)	SAE Crankcase	SAE Gear	AGMA

Unit Conversion Formulas

Dynamic viscosity (cP) = Kinematic viscosity (cSt) × Density (g/cm³)

Where: 1 cP = 1 mPa·s, and 1 cSt = 1 mm²/s. Oil density is typically 0.85–0.90 g/cm³ (850–900 kg/m³) at 15 °C.

SUS ≈ cSt × 4.635 (approximate, for cSt > 50)

The Saybolt Universal Seconds (SUS) approximation is valid for viscosities above ~50 cSt. For lower viscosities, a more complex formula is needed, but the above gives reasonable results for most industrial oils.

About the Grading Systems

ISO VG (ISO 3448) — Industrial lubricants; 18 grades VG 2 to VG 1500; kinematic viscosity at 40 °C ± 10%.
SAE Crankcase (SAE J300) — Automotive engine oils; grades like 10W, 20, 30, 40, 50; measured at 100 °C.
SAE Gear (SAE J306) — Automotive gear oils; grades like 75W, 80W-90, 140, 250; measured at 100 °C.
AGMA (AGMA 9005) — Industrial gear lubricants; grades 1 through 8A; aligned with ISO VG.

⚠️ Important: Cross-references are approximate. ISO VG measures at 40 °C while SAE measures at 100 °C. Always verify with the specific oil product data sheet for exact viscosity values at your operating temperature.

Viscosity Converter | ISO VG ↔ SAE ↔ AGMA

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

Results

ISO VG / SAE / AGMA Cross-Reference

Unit Conversion Formulas

About the Grading Systems