How many credits am I consuming during partial minutes?

For each task, CircleCI rounds up to the nearest second, then to the nearest credit. In practice, the type of resource class you are running controls your credit::time ratio.

For example, a Docker executor with a medium resource class costs 10 credits/minute or 1 credit/every 6 seconds. Since CircleCI will round up to the nearest second, then nearest credit, it would be safe to say that 1 credit is consumed every 6 second block. 

More specifically, let's consider running a task for 10 seconds using a Docker executor with a medium resource class. Since this is not a fraction of a second, we don't need to worry about rounding up to the nearest second. However, it would be rounded up to the nearest credit. Therefore, with this example, a 10 second task consumes 2 credits.

You can view a list of executors with corresponding credits per minute here: https://circleci.com/pricing/#compute-options-table