The fair place to meet is Portland, OR — the city closest to the midpoint of Boise and Seattle. From the farther side that’s about 7 hr 14 min of driving.
Recommended midpoint
Portland, OR
From Boise
7 hr 14 min
345 mi to Portland
From Seattle
3 hr 38 min
145 mi to Portland
Seattle has the shorter trip; the split is off by about 3 hr 36 min. The alternatives below can even it out. Boise and Seattle are about 405 miles apart.
Boise and Seattle are about 405 miles apart by road. Split the difference and you arrive near Portland, the city closest to the halfway point between them. That puts roughly 7 hr 14 min of driving on the Boise side and 3 hr 38 min on the Seattle side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Portland suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Portland doesn't have what you're after, Sacramento and Salt Lake City are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Boise and Seattle.
Portland is the most populous city in the U.S. state of Oregon. Located in the Pacific Northwest at the confluence of the Willamette and Columbia rivers, it is the 28th-most populous city in the United States, sixth-most populous on the West Coast, and… Read more →
Boise is the capital and most populous city in the U.S. state of Idaho. It is the county seat of Ada County. The population of the city was 235,685 at the 2020 census. Read more →
Seattle is the most populous city in the U.S. state of Washington and the Pacific Northwest region of North America. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.