Route reliability record
SEA→BOS
Seattle to Boston
- Flights
- 2,554
- Period
- September 2025 to August 2026
- Distance
- 2,496 mi
- Carriers
- 3
This route sits in the middle of the pack with a B. Close to three flights in four arrive on time by the DOT's definition, 2 points worse than the national rate of 77.3%. The late ones land about an hour behind schedule. That rests on 2,554 SEA to BOS flights, of which Delta Air Lines flew 1,067 at 79.8% on time.
Best time to fly SEA to BOS: a 5am–8am departure, ideally on a Saturday. Flights in that window arrived on time 92.0% of the time and Saturday flights 79.1%, against 75.1% for the route overall.
The US average over the same twelve months is 77.3%, 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
75.1%
Arrivals under 15 minutes late, of 2,505 completed flights
Avg delay
64 min
Across delayed arrivals only, not all flights
Cancelled
1.49%
National rate is 1.89%
Best carrier
Delta
79.8% on time across 1,067 flights
These figures cover 2,554 flights between September 2025 and August 2026. Beyond the headline rate, 8.1% of arrivals land a full hour or more behind schedule, against 8.3% nationally.
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Carrier by carrier
Which airline is most reliable on SEA to BOS
The two carriers here are closely matched: 2 points separate the best from the worst. At that spread, schedule and price are better tie-breakers than reliability. Sample sizes are shown so you can see which rows are worth weighing.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 1,067 | 79.8% | 71 min | 1.69% | B |
| Alaska Airlines | 1,240 | 77.5% | 51 min | 1.13% | B |
| JetBlue Airwaysthin sample | 247 | 43.2% | 78 min | 2.43% | F |
No carrier stands out. Delta tops the table at 79.8%, 2 points clear of the bottom, which is inside the range this kind of sample moves year to year.
Flight by flight
Flights on SEA to BOS
The flight numbers below flew this route at least 250 times in the period, busiest first. Each has its own record, with its schedule and delay causes.
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Best ways to make this trip
The most reliable ways to fly Seattle to Boston
Every airline on this route, ranked by how often a scheduled flight actually operated and arrived on time, with its best departure time and most punctual flight number. No nearby airport pair beat these.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. SEA → BOSSeattle to Boston | Delta Air Lines | 78.5% | 79.8% | 1.69% | 5am–8am, Fri | DL 459 07:50 |
| 2. SEA → BOSSeattle to Boston | Alaska Airlines | 76.6% | 77.5% | 1.13% | 5am–8am, Sat | AS 460 15:59 |
Success is the share of scheduled flights that operated and arrived less than 15 minutes late, so a cancelled flight counts against it. “(route)” means the best time comes from every carrier on the route rather than this airline alone.
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 40 points, against 2 points between the best and worst carrier.
- 5am–8am359 flights92.0%
- 8am–11am557 flights74.5%
- 11am–2pm309 flights51.8%
- 2pm–5pm591 flights76.2%
- 5pm–8pm—
- 8pm–5am738 flights76.6%
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 51.8% on time against 92.0% 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 738 flights scheduled in it.
Explore
Filter SEA to BOS by airline, time, month and day
Pick a breakdown and a carrier. The dashed mark on each row is the same slice for every US flight.
On SEA to BOS, the best departure window is 5am–8am at 92.0% on time and the worst is 11am–2pm at 51.8%, a wide gap of 40.2 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am92.0%
- 8–11am74.5%
- 11am–2pm51.8%
- 2–5pm76.2%
- 5–8pm—
- 8pm–5am76.6%
best all US flightsScale 45% to 95%. Cells under 30 flights show a dash.
Across the year
When delays peak on this route
This route's year swings by 20 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 66.1% on time, 20 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. August was the busiest month here, with 322 flights.
Across the week, Saturday is the strongest day at 79.1% on time and Thursday the weakest at 72.3%, a spread of 7 points.
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 40,000 reported delay minutes.
- 38%National Airspace System
- 31%Carrier
- 25%Late-arriving aircraft
- 6%Extreme weather
- 0%Security
38% 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 15,000 minutes out of 40,000, against a national NAS share of 21%. The split is drawn from 39,753 reported delay minutes.
Cancellations sit at 1.49% against a national 1.89%, which is ordinary. Diversions add another 0.43%.
Connections and stage length
What a delay here actually costs
8.1% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about an hour, 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 70.1% on time across 2,555 flights, 5 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 5 points of difference.
The sample behind these figures is 2,554 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.
Work out the risk of missing a connection in BOS after this flight
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 66.1%, a swing of 20 points. If the dates move at all, move them out of July before you think about anything else. 2,505 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 arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 24.9% of the 2,554 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on SEA to BOS?
Delta Air Lines leads at 79.8% on time across 1,067 flights. Alaska Airlines is at the other end on 77.5%, a gap of 2 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 92.0% on time. The 11am–2pm window is the weakest at 51.8%, 40 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 66.1% on time, compared with 86.5% in March. Across the year that is 20 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 SEA to BOS79.8%
- Alaska Airlines on SEA to BOS77.5%
- JetBlue Airways on SEA to BOS43.2%
- SEA to LAX, a related route82.0%
- DCA to BOS, a related route70.1%
- SEA to PHX, a related route78.2%
- Delta Air Lines on-time record79.8%
- Alaska Airlines on-time record77.5%
- JetBlue Airways on-time record43.2%
- The best time of day to fly, according to 7.05 million flight records
- Connecting flights: how much buffer the data says you need
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.
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Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.