For practical purposes, if the **Reynolds number** is less than 2000, the flow is laminar. The accepted transition Reynolds number for flow in a circular pipe is Re_{d,crit} = 2300. For laminar flow, the **head loss is proportional to velocity** rather than velocity squared. Thus the **friction factor is inversely proportional to velocity**.

**The Darcy friction factor** for laminar (slow) flows is a consequence of **Poiseuille’s law** that and it is given by following equations:

**Laminar flow:**

**Re < 2000**- ‘low’ velocity
- Fluid particles move in
**straight lines** - Layers of water flow over one another at different speeds with
**virtually no mixing**between layers. - The flow velocity profile for laminar flow in circular pipes is parabolic in shape, with a maximum flow at the center of the pipe and a minimum flow at the pipe walls.
- The average flow velocity is approximately one-half of the maximum velocity.
- Simple mathematical analysis is possible.
**Rare in practice in water systems**.