Regardless of your methodology, Scrum, Waterfall, or whatever, your team has a velocity. Whether you choose to measure it or not is irrelevant. Every day, your team decreases the amount of work they have remaining in the current release, sprint or iteration. The average amount they decrease that work remaining per day is the velocity of your team.
In Scrum velocity is normally measured in story points. Story points are not absolute values and if used properly do not equate to time and are unique in value to the team that created them. They are a measurement of what the team believes the effort involved in finishing the story will be. It is quite normal for one team to take a story and deem it worth 5 story points and to have another team take the same story and deem it 9 points. This could be a result of a couple of reasons: it is possible that the first team is far more familiar with the job at hand and believes it less work, or the teams have a different way of measuring and the 5 points for the first team equate to the 9 points of the second.
This doesn’t make one team’s estimate wrong; as a matter of fact it is just the opposite, it makes that team’s estimate more accurate since they created it, it makes the estimate relative to the team doing the work. This is a very important aspect of Scrum.
A common problem that arises in companies with multiple Scrum teams is that you cannot compare velocities across teams. As previously mentioned the point assignment methodology for 2 teams can differ so their story point completion can’t be compared. Just because 1 team finishes 40 story points in a sprint and another only accomplishes 25 doesn’t mean that the second team is full of slackers. You shouldn’t do this or you will start to get estimate inflation and teams will begin to fail.
Done properly measuring a team’s velocity with story points will make future estimating a much easier task.