Although Tacoma to SeaTac is roughly 25 miles ( depending on which area of Tacoma ), raveling to the airport requires on the average 40 minutes. There are bottlenecks in the Tacoma Dome area, notorious slow-downs as one approaches the exit to Route 18 and finally the merging of traffic from Federal Way and DesMoines onto I-5. So, get ready and relax while you are at it.