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ISO Viscosity Grade Chart

ISO 3448 viscosity grades with their midpoint and allowed range in centistokes at 40 °C. The grade number is the midpoint, and each grade allows plus or minus 10%.

Ref HY-0051ISO 3448

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

ISO VG (viscosity grade) is a classification for industrial oils, set by ISO 3448. Each grade number is the oil's midpoint kinematic viscosity in centistokes (cSt, mm²/s) at 40 °C. An oil meets the grade if its viscosity falls within plus or minus 10% of the midpoint. Hydraulic oils are most often ISO VG 32, 46 or 68.

How to read it

Find the ISO VG on the product label or data sheet, such as "AW 46" or "ISO VG 46".
The number tells you viscosity at 40 °C only. How much the oil thins as it heats is shown by its viscosity index (VI). Check the data sheet.
Equipment makers state the required grade, or a viscosity range at operating temperature. Use their data to select a grade.

Viscosity grade chart

ISO VG	Midpoint at 40 °C (cSt)	Minimum (cSt)	Maximum (cSt)	Typical hydraulic use
10	10	9.0	11.0	Very cold service, light-duty systems
15	15	13.5	16.5	Cold service
22	22	19.8	24.2	Cold climates, low-temperature start-up
32	32	28.8	35.2	Mobile and industrial, cooler climates
46	46	41.4	50.6	Most common industrial and mobile grade
68	68	61.2	74.8	Warm climates, high operating temperature
100	100	90	110	Hot service, some large systems
150	150	135	165	Rare in hydraulics; gear and circulating use

ISO 3448 also defines grades below 10 and above 150, up to VG 3200.

Key points

Too thin an oil causes internal leakage, poor efficiency and wear. Too thick an oil causes slow response, cavitation at cold start and high power loss.
High-VI and multigrade hydraulic oils hold viscosity better across a temperature range.
Mixing grades gives a viscosity in between. Mixing additive types (for example zinc AW and zinc-free) may cause problems. Check compatibility with the supplier.
ISO VG numbers do not convert directly to SAE engine or gear grades. Use an equivalents chart only as a guide.

Common mistakes

Reading "46" as a weight like an engine oil.
Picking a grade by habit instead of from the pump maker's viscosity range.
Ignoring operating temperature. The same grade can be right in winter and too thin in summer.

Governing standards: ISO 3448 (industrial liquid lubricants, viscosity classification); ASTM D445 (kinematic viscosity test); pump and equipment manufacturer requirements.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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