If you want to know your team's capacity, you have to limit the number of tasks they work on at the same time. Once you limit WIP, several interesting things will happen:
* A backlog of tasks will emerge.
* You will be able to measure how much time is spent on each task.
* Tasks will get finished faster.
The first two results are not very surprising because by introducing WIP limits, you have effectively eliminated multitasking, but how on earth, do tasks get finished faster?
Unlike computers with multiple processor cores, our brains have one or at best two cores. Without WIP limits, when there are too many tasks to work on, we spend more time on switching tasks than the tasks themselves.
Bottlenecks become visible. Since everyone is working on a limited number of tasks, some finish theirs on time, some get overloaded, and some cannot finish their work because they need input from those who are overloaded. Team members with free capacity can help those who are overloaded. Better yet, they can even come up with ideas on how to fix the newly discovered bottlenecks.
Disclaimer: I am the author of http://flow.io , a lean project management application based on kanban.