Finding total amperage in circuit panel?

Is there a way where I can tally up the total amp rating of the breakers in my panel and "compute" my service amperage? I'm hoping to be able to install "slimline" breakers to make room for one or two more 20 amp circuits in my garage. Currently, my panel is full. The house is fairly new and was built within the past ten years. I'm fairly confident in my abilities working with electricity and inside breaker panels, so I don't think professional service is needed in this case. Thanks for the info.