Route reliability record
SEA→BOS
Seattle to Boston
- Flights
- 2,459
- Period
- July 2025 to June 2026
- Distance
- 2,496 mi
- Carriers
- 3
This route sits in the middle of the pack with a C. Close to three flights in four arrive on time by the DOT's definition, 3 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 2,459 flights, of which Delta Air Lines flew 1,005 at 81.2% on time.
The US average over the same twelve months is 77.6%, so there is little to separate this route from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
74.5%
Arrivals under 15 minutes late, of 2,419 completed flights
Avg delay
68 min
Across delayed arrivals only, not all flights
Cancelled
1.14%
National rate is 1.81%
Best carrier
Delta
81.2% on time across 1,005 flights
These figures cover 2,459 flights between July 2025 and June 2026. The median arrival is 5 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
Which airline is most reliable on SEA to BOS
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 81.2% of the time; Alaska Airlines manages 73.7%. Sample sizes sit next to every figure, because a two-point gap across 1,207 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 1,005 | 81.2% | 68 min | 1.49% | B |
| Alaska Airlines | 1,207 | 73.7% | 48 min | 0.99% | C+ |
| JetBlue Airwaysthin sample | 247 | 51.2% | 121 min | 0.40% | F+ |
Delta leads Alaska by 8 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Departure window
The best time of day to leave
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 37 points, against 8 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am314 flights91.8%
- 8am–11am501 flights74.3%
- 11am–2pm312 flights54.7%
- 2pm–5pm588 flights75.1%
- 5pm–8pm—
- 8pm–5am744 flights75.2%
Scale runs 45% to 95%. 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 54.7% on time against 91.8% for 5am–8am. 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 8pm–5am window is the busiest, with 744 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 27 points from best month to worst, which is wide. An annual average tells you very little about the week you are actually flying, so read the strip rather than the headline if your dates are fixed.
July is the worst month at 59.5% on time, 27 points below March, the best at 86.5%. 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. June was the busiest month here, with 278 flights.
Across the week, Tuesday is the strongest day at 77.5% on time and Sunday the weakest at 72.2%. 5 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 42,000 reported delay minutes.
- 36%Carrier
- 32%National Airspace System
- 26%Late-arriving aircraft
- 6%Extreme weather
- 0%Security
The largest share of delay minutes, 36%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 15,000 of the 42,000 delay minutes reported here. The split is drawn from 41,955 reported delay minutes.
Cancellations sit at 1.14% against a national 1.81%, which is ordinary. Diversions add another 0.49%.
Connections and stage length
What a delay here actually costs
7.9% 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 are the a little over an hour kind, 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 71.4% on time across 2,461 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 2,459 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 this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. March runs 86.5% on time and July 59.5%, a swing of 27 points. If the dates move at all, move them out of July before you think about anything else. 2,419 completed flights over 12 months sit behind that.
Common questions
SEA to BOS questions
How often is SEA to BOS delayed?
About one flight in four arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 25.5% of the 2,459 flights in this analysis, against 22.4% nationally. Delayed flights are late by a little over an hour on average.
Which airline is most reliable on SEA to BOS?
Delta Air Lines leads at 81.2% on time across 1,005 flights. Alaska Airlines is at the other end on 73.7%, a gap of 8 points. Sample size matters as much as the percentage: a rate built from a few hundred flights moves around far more than one built from ten thousand.
What time of day has the fewest delays on SEA to BOS?
The 5am–8am departure window is the strongest at 91.8% on time. The 11am–2pm window is the weakest at 54.7%, 37 points behind. Later departures inherit delays from earlier flights the same aircraft has already flown, which is why the curve slopes the way it does.
How bad are July delays on SEA to BOS?
July is the weakest month at 59.5% on time, compared with 86.5% in March. Across the year that is 27 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.
Related
Where to go next
- BOS to SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- Boston Logan International (BOS) delay statistics
- Delta Air Lines on-time record81.2%
- Alaska Airlines on-time record73.7%
- JetBlue Airways on-time record51.2%
- The best time of day to fly, according to 7.04 million flight records
- Worst months to fly in the US, ranked by the data
Direction matters. SEA to BOS and BOS to SEA are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.