Guide
Hose Size Guide: Inner Diameter, Flow Rate and Pressure Drop
How to read hose sizes, convert inches to millimetres and choose the right ID for your flow rate.
Hose sizes are commonly expressed in **inner diameter (ID)**, in inches or millimetres.
## Common hose sizes
| Inch (nominal) | Approx. ID (mm) |
|---|---|
| 1/4" | 6.3 mm |
| 3/8" | 9.5 mm |
| 1/2" | 12.5 mm |
| 3/4" | 19 mm |
| 1" | 25 mm |
| 1-1/4" | 31.5 mm |
| 1-1/2" | 38 mm |
| 2" | 50 mm |
## Why size matters
- Too small: high pressure drop, excessive velocity, premature wear
- Too large: wasted cost, heavier assembly, harder routing
## Flow velocity rule of thumb
- Hydraulic pressure lines: 6-8 m/s
- Hydraulic return lines: 2-4 m/s
- Air lines: 15-25 m/s
- Water lines: 1-3 m/s
Choose the smallest ID that keeps velocity within these ranges for your flow rate.