Route reliability record
BOS→SEA
Boston to Seattle
- Flights
- 2,555
- Period
- September 2025 to August 2026
- Distance
- 2,496 mi
- Carriers
- 3
This route sits in the middle of the pack with a C. About seven flights in ten arrive on time by the DOT's definition, 7 points worse than the national rate of 77.3%. The late ones land about an hour behind schedule. That rests on 2,555 BOS to SEA flights, of which Delta Air Lines flew 1,068 at 71.1% on time.
Best time to fly BOS to SEA: a 8am–11am departure, ideally on a Wednesday. Flights in that window arrived on time 76.2% of the time and Wednesday flights 75.3%, against 70.1% for the route overall.
The national on-time rate over the same period is 77.3%, so this route runs 7 points behind the country as a whole. Across a thousand flights that is about 72 extra late arrivals.
Key figures
On time
70.1%
Arrivals under 15 minutes late, of 2,503 completed flights
Avg delay
55 min
Across delayed arrivals only, not all flights
Cancelled
1.49%
National rate is 1.89%
Best carrier
Delta
71.1% on time across 1,068 flights
These figures cover 2,555 flights between September 2025 and August 2026. Beyond the headline rate, 8.0% 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 BOS to SEA
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 | 1,068 | 71.1% | 57 min | 1.59% | C |
| JetBlue Airwaysthin sample | 247 | 70.8% | 64 min | 1.62% | C |
| Alaska Airlines | 1,240 | 69.1% | 52 min | 1.37% | C |
No carrier stands out. Delta tops the table at 71.1%, 2 points clear of the bottom, which is inside the range this kind of sample moves year to year.
Flight by flight
Flights on BOS to SEA
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 Boston to Seattle
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. BOS → SEABoston to Seattle | Delta Air Lines | 70.0% | 71.1% | 1.59% | 8am–11am, Sat | DL 384 07:15 |
| 2. BOS → SEABoston to Seattle | Alaska Airlines | 68.2% | 69.1% | 1.37% | 2pm–5pm, Wed | AS 459 07:00 |
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 17 points, against 2 points between the best and worst carrier.
- 5am–8am826 flights75.2%
- 8am–11am391 flights76.2%
- 11am–2pm247 flights75.2%
- 2pm–5pm228 flights75.7%
- 5pm–8pm848 flights59.5%
- 8pm–5am15 flights—
Scale runs 50% to 80%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 59.5% on time, against 76.2% for the 8am–11am slot. By evening the aircraft flying these rotations have absorbed a full day of small delays and there is no slack left to recover in. The 5pm–8pm window is the busiest, with 848 flights scheduled in it.
Explore
Filter BOS to SEA 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 BOS to SEA, the best departure window is 8am–11am at 76.2% on time and the worst is 5pm–8pm at 59.5%, a wide gap of 16.7 points. All US flights leaving 8am–11am ran 83.9%.
- 5–8am75.2%
- 8–11am76.2%
- 11am–2pm75.2%
- 2–5pm75.7%
- 5–8pm59.5%
- 8pm–5am—
best all US flightsScale 50% to 95%. Cells under 30 flights show a dash.
Across the year
When delays peak on this route
This route's year swings by 45 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.
December is the weak month at 39.2% on time, against 84.2% in September. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. August was the busiest month here, with 322 flights.
Across the week, Wednesday is the strongest day at 75.3% on time and Thursday the weakest at 64.5%, a spread of 11 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 41,000 reported delay minutes.
- 37%Late-arriving aircraft
- 36%National Airspace System
- 24%Carrier
- 3%Extreme weather
- 0%Security
37% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 41,000 reported delay minutes here, that single category outweighs weather by 15 to one. The split is drawn from 41,378 reported delay minutes.
36% 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 41,000, against a national NAS share of 21%.
Cancellations sit at 1.49% against a national 1.89%, which is ordinary. Diversions add another 0.55%.
Connections and stage length
What a delay here actually costs
8.0% 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 75.1% on time across 2,554 flights, 5 points above 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 2,555 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 SEA 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. September runs 84.2% on time and December 39.2%, a swing of 45 points. If the dates move at all, move them out of December before you think about anything else. 2,503 completed flights over 12 months sit behind that.
Common questions
BOS to SEA questions
How often is BOS to SEA delayed?
Roughly one flight in three arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 29.9% of the 2,555 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on BOS to SEA?
Delta Air Lines leads at 71.1% on time across 1,068 flights. Alaska Airlines is at the other end on 69.1%, 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 BOS to SEA?
The 8am–11am departure window is the strongest at 76.2% on time. The 5pm–8pm window is the weakest at 59.5%, 17 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 December delays on BOS to SEA?
December is the weakest month at 39.2% on time, compared with 84.2% in September. Across the year that is 45 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
- SEA to BOS — the return legseparate record
- Boston Logan International (BOS) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- Delta Air Lines on BOS to SEA71.1%
- JetBlue Airways on BOS to SEA70.8%
- Alaska Airlines on BOS to SEA69.1%
- BOS to DCA, a related route72.6%
- LAX to SEA, a related route85.0%
- BOS to ORD, a related route71.9%
- Delta Air Lines on-time record71.1%
- JetBlue Airways on-time record70.8%
- Alaska Airlines on-time record69.1%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.05 million flight records
Direction matters. BOS to SEA and SEA to BOS 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.