Route reliability record

BOS→SEA

Boston to Seattle

Flights: 2,461
Period: July 2025 to June 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, 6 points worse than the national rate of 77.6%. The late ones land about 55 minutes behind schedule. That rests on 2,461 flights, of which Delta Air Lines flew 1,006 at 71.6% on time.

The national on-time rate over the same period is 77.6%, so this route runs 6 points behind the country as a whole. Across a thousand flights that is about 62 extra late arrivals.

On time

71.4%

Arrivals under 15 minutes late, of 2,420 completed flights

Avg delay

54 min

Across delayed arrivals only, not all flights

Cancelled

1.26%

National rate is 1.81%

Best carrier

Delta

71.6% on time across 1,006 flights

These figures cover 2,461 flights between July 2025 and June 2026. The median arrival is 2 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 BOS to SEA

The two carriers here are closely matched: about a point 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.

On-time performance by carrier on BOS to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
JetBlue Airwaysthin sample	247	75.6%	72 min	0.40%	B+
Delta Air Lines	1,006	71.6%	55 min	1.59%	C
Alaska Airlines	1,208	70.5%	50 min	1.16%	C

No carrier stands out. Delta tops the table at 71.6%, about a point clear of the bottom, which is inside the range this kind of sample moves year to year.

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 about a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am866 flights76.1%
8am–11am351 flights76.4%
11am–2pm234 flights78.0%
2pm–5pm202 flights77.6%
5pm–8pm806 flights60.9%
8pm–5am2 flights—

Scale runs 50% to 85%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 60.9% on time, against 78.0% for the 11am–2pm 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 5am–8am window is the busiest, with 866 flights scheduled in it.

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.

Darker means more delays. Figures are on-time percentages.September best, December worst

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. June was the busiest month here, with 278 flights.

Across the week, Monday is the strongest day at 75.6% on time and Thursday the weakest at 64.7%. 11 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 37,000 reported delay minutes.

Late-arriving aircraft
38%
National Airspace System
34%
Carrier
25%
Extreme weather
2%
Security
0%

38% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 37,000 reported delay minutes here, that single category outweighs weather by 16 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit. The split is drawn from 37,321 reported delay minutes.

34% 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 13,000 minutes out of 37,000, against a national NAS share of 21%.

Cancellations sit at 1.26% against a national 1.81%, which is ordinary. Diversions add another 0.41%.

Connections and stage length

What a delay here actually costs

7.2% 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 about 55 minutes 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 74.5% on time across 2,459 flights, 3 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 3 points of difference.

The sample behind these figures is 2,461 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. 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. 2,420 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 arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 28.6% of the 2,461 flights in this analysis, against 22.4% nationally. Delayed flights are late by about 55 minutes on average.

Which airline is most reliable on BOS to SEA?

Delta Air Lines leads at 71.6% on time across 1,006 flights. Alaska Airlines is at the other end on 70.5%, a gap of about a point. 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 11am–2pm departure window is the strongest at 78.0% on time. The 5pm–8pm window is the weakest at 60.9%, 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.

Where to go next

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.

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.

Key figures