Carrier route record

ASSEA→BOS

Alaska Airlines · Seattle to Boston

Flights: 1,207
Period: July 2025 to June 2026
Share of route: 49%
Carriers: 3

Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Alaska Airlines.

Alaska on SEA to BOS sits in the middle of the pack with a C+. Close to three flights in four arrive on time by the DOT's definition, 4 points worse than the national rate of 77.6%. The late ones land roughly 50 minutes behind schedule. That rests on 1,207 flights, with a median arrival 3 minutes early.

The US average over the same twelve months is 77.6%, so there is little to separate Alaska on SEA to BOS from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.

On time

73.7%

Arrivals under 15 minutes late, of 1,192 completed flights

Avg delay

48 min

Across delayed arrivals only, not all flights

Cancelled

0.99%

National rate is 1.81%

Whole route

74.5%

Every carrier on SEA to BOS, 2,459 flights

These figures cover 1,207 flights between July 2025 and June 2026. The median arrival is 3 minutes early, which is the part an average hides: most flights here are fine, and the on-time rate is decided by a tail of badly late ones. On-time means under 15 minutes late, the DOT's own threshold.

Carrier by carrier

Alaska against every carrier on SEA to BOS

Alaska flew 1,207 of the 2,459 flights on SEA to BOS, about 49% of it. Its 73.7% on time is about level with the route as a whole at 74.5%. Ranked by on-time rate it is number two of three carriers on the route, where the range runs from Delta at 81.2% to JetBlue at 51.2%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.

Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.

On-time performance by carrier on SEA to BOS
Airline	Flights	On time	Avg delay	Cancelled	Grade
Delta Air Lines	1,005	81.2%	68 min	1.49%	B
Alaska Airlines (this page)	1,207	73.7%	48 min	0.99%	C+
JetBlue Airwaysthin sample	247	51.2%	121 min	0.40%	F+

Flight by flight

Alaska flight numbers on SEA to BOS

Departure window

When Alaska runs best on this route

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 35 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am14 flights—
8am–11am436 flights74.4%
11am–2pm95 flights43.2%
2pm–5pm363 flights78.4%
5pm–8pm—
8pm–5am299 flights76.4%

Scale runs 35% to 85%. Rows with fewer than 30 flights show a dash.

The weak window here is the middle of the day, not the evening: 11am–2pm runs 43.2% on time against 78.4% for 2pm–5pm. That shape usually means a departure bank rather than an accumulating delay, so the recovery later in the afternoon is real rather than noise. The 8am–11am window is the busiest, with 436 flights scheduled in it.

Across the year

Alaska on SEA to BOS through the year

Darker means more delays. Figures are on-time percentages.January best, August worst

August is the worst month at 54.5% on time, 35 points below January, the best at 89.1%. Summer afternoon convection and peak-season schedules land on the same weeks every year, so this is a pattern you can book around rather than a run of bad luck. May was the busiest month here, with 124 flights.

Across the week, Saturday is the strongest day at 77.8% on time and Thursday the weakest at 69.4%. 8 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Alaska runs late on this route

One category carries most of the weight here. National Airspace System accounts for 47% of the 15,000 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

National Airspace System
47%
Late-arriving aircraft
25%
Carrier
19%
Extreme weather
8%

47% of delay minutes are charged to the National Airspace System: air traffic volume, runway configuration, ground stops. No carrier can price its way out of that, which is why airport choice moves the number here more than carrier choice does. That is 7,097 minutes out of 15,000, against a national NAS share of 21%. The split is drawn from 14,986 reported delay minutes.

Cancellations sit at 0.99% against a national 1.81%, which is ordinary. Diversions add another 0.25%.

Connections and stage length

What a delay here costs

6.5% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last roughly 50 minutes, which a normal layover absorbs.

At 2,496 miles this is transcontinental flying, and long routes get a quiet structural advantage. Scheduled block time on a flight this length carries more padding in absolute minutes, so a twenty-minute late departure often still becomes an on-time arrival. Read the on-time rate here as slightly flattering next to a short-haul route with the same operational quality.

The return leg is a different operation and performs like one: it runs 70.5% on time across 1,208 flights, 3 points below this direction. Departure banks, aircraft rotations and the congestion at each end all differ, so scoring the two directions together would average away a real 3 points of difference.

The sample behind these figures is 1,207 flights, enough that the headline rate is stable to within roughly a point. The hour-block and month splits divide that further, so read the thinner bars with more caution than the headline.

What to do with this

Booking Alaska on SEA to BOS

Put together: the calendar is the biggest lever here, not the carrier or the hour. January runs 89.1% on time and August 54.5%, a swing of 35 points. If the dates move at all, move them out of August before you think about anything else. 1,192 completed flights over 12 months sit behind that.

Where to go next

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.

Key figures