Lubrication science
SAE and ISO viscosity grades: what the numbers mean
Quick answer
How to read 5W-30 and ISO VG 220, and why you cannot swap one scale for the other.
Two different scales
Engine and some transmission oils use SAE grades. Industrial oils, such as hydraulic, gear, bearing and compressor oils, use ISO VG grades. They measure different things and are not interchangeable.
Reading an SAE multigrade
In 5W-30 the number before the W (winter) sets the cold-start and cold-pumping behaviour. The lower it is, the better the oil flows when cold. The number after the dash sets the viscosity at 100 °C. The higher it is, the thicker the oil is when hot.
SAE J300 winter grades
| Grade | Max cranking viscosity, mPa.s (at °C) | Max pumping viscosity 60,000 mPa.s (at °C) |
|---|---|---|
| 0W | 6,200 (-35) | -40 |
| 5W | 6,600 (-30) | -35 |
| 10W | 7,000 (-25) | -30 |
| 15W | 7,000 (-20) | -25 |
| 20W | 9,500 (-15) | -20 |
| 25W | 13,000 (-10) | -15 |
SAE J300 hot grades
| Grade | Kinematic viscosity at 100 °C, mm²/s | Minimum high-temperature high-shear viscosity, mPa.s |
|---|---|---|
| 20 | 5.6 to under 9.3 | 2.6 |
| 30 | 9.3 to under 12.5 | 2.9 |
| 40 | 12.5 to under 16.3 | 2.9 (0W-40, 5W-40, 10W-40) or 3.5 (15W-40, 20W-40, 25W-40) |
| 50 | 16.3 to under 21.9 | 3.7 |
| 60 | 21.9 to under 26.1 | 3.7 |
ISO viscosity grades (ISO 3448)
| ISO VG | Mid-point viscosity at 40 °C, mm²/s | Allowed range |
|---|---|---|
| 32 | 32 | 28.8 to 35.2 |
| 46 | 46 | 41.4 to 50.6 |
| 68 | 68 | 61.2 to 74.8 |
| 100 | 100 | 90 to 110 |
| 150 | 150 | 135 to 165 |
| 220 | 220 | 198 to 242 |
| 320 | 320 | 288 to 352 |
| 460 | 460 | 414 to 506 |
| 680 | 680 | 612 to 748 |
| 1000 | 1000 | 900 to 1100 |
Rules of thumb
- The ISO grade number is the viscosity at 40 °C. A VG 220 oil is about 220 mm²/s at 40 °C.
- Each ISO step is about 1.5 times thicker than the one before.
- Do not match an SAE number to an ISO grade by name. A conversion chart is only approximate and ignores viscosity index.
- Follow the equipment maker for the grade, then check the operating temperature.
Takeaway. SAE grades describe engine oils by cold and hot behaviour. ISO VG grades give industrial oils a single viscosity at 40 °C. Choose by the scale the equipment maker uses.
Put it to work in the tool