Two separate things get called bandwidth. Keep them apart.
Port speed is per server
How fast a server can move traffic at any instant.
- General Compute (
gc.*): 2.5 Gbps - Network Optimized (
no.*): 10 Gbps
Port speed is a property of the plan and cannot be changed without resizing to a plan that has it.
Transfer is per account
How much total traffic you move in a period. Every account includes a 2 TB pool plus 500 GB for each server, still shared across the account. It is not 2 TB per server, and a plan's own bandwidth figure on the pricing page is not a separate cap the server is held to. Each server adds 500 GB to the same pool; resizing one server to a larger plan does not.
Buying more
Prepaid blocks are $3.00 per TB per calendar month, added to the pool from Billing. Bought mid-month, you pay only for the days left (including today) and still get the full extra TB for this month's pool. A block auto-renews on the 1st at the full monthly rate until you turn renewal off, and you get an email a few days before it does.
Overage on traffic past the pool is the same $3.00 per TB, calculated and posted to account credit in batches. From Billing → Add bandwidth you can disable automatic overage billing. If transfer then exceeds the pool, servers are suspended for the rest of the UTC calendar month instead of charging. They resume when the pool resets, or when you add enough prepaid bandwidth to cover the overage. This does not suspend the account for non-payment and does not start a termination review.
Blocks and overage cost the same per TB. Add a block when you want the extra capacity in the pool up front, including mid-month, when the charge is prorated, or when automatic overage billing is off.
Resize for CPU, buy transfer for traffic
Moving to a bigger plan does not buy you a bigger pool. Each server adds 500 GB. If the constraint is traffic beyond that, buy blocks.
Where to look
Billing shows pool usage for the current period, active blocks and their renewal dates. Each server's Usage Graphs tab shows that server's share of the pool this month (inbound, outbound, and total).
The same figures are on the API: GET /api/v1/account for the pool plus every
server's share this month, and GET /api/v1/servers/<id>/bandwidth for one
server including previous months. See API recipes.