Network bandwidth is not normally a significant factor in your calculations and preparations for cluster sizing, but network bandwidth is vital for accessing information from your cluster by clients, and for exchanging information between nodes.

In general you can calculate your network bandwidth requirements using the formula:

Bandwidth = (operations per second * item size) +
    overhead for rebalancing

Where the operations per second can be calculated using:

Operations per second = Application reads +
    (Application writes * Replica copies)