Carrier route record

DLSEA→BOS

Delta Air Lines · Seattle to Boston

Flights: 1,005
Period: July 2025 to June 2026
Share of route: 41%
Carriers: 3

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

Delta on SEA to BOS lands a B, 4 points better than the national rate of 77.6%. About one flight in five arrives 15 minutes late or worse, and those flights are late by about an hour on average. That is a workable record, not a spotless one. That rests on 1,005 flights, with a median arrival 8 minutes early.

The US average over the same twelve months is 77.6%, so there is little to separate Delta 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

81.2%

Arrivals under 15 minutes late, of 983 completed flights

Avg delay

68 min

Across delayed arrivals only, not all flights

Cancelled

1.49%

National rate is 1.81%

Whole route

74.5%

Every carrier on SEA to BOS, 2,459 flights

These figures cover 1,005 flights between July 2025 and June 2026. The median arrival is 8 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

Delta against every carrier on SEA to BOS

Delta flew 1,005 of the 2,459 flights on SEA to BOS, about 41% of it. Its 81.2% on time is 7 points better than the route as a whole, which runs 74.5%. Ranked by on-time rate it is first 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 (this page)	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+

Flight by flight

Delta flight numbers on SEA to BOS

Departure window

When Delta runs best on this route

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 22 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–8am300 flights92.1%
8am–11am65 flights73.8%
11am–2pm52 flights82.7%
2pm–5pm225 flights69.9%
5pm–8pm—
8pm–5am363 flights80.3%

Scale runs 60% to 100%. Rows with fewer than 30 flights show a dash.

Book the early departure. Flights leaving between 5am–8am arrive on time 92.1% of the time, against 69.9% for the afternoon window. That is 22 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 8pm–5am window is the busiest, with 363 flights scheduled in it.

Across the year

Delta on SEA to BOS through the year

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

May is the best month on the calendar at 93.7% on time. Shoulder season thins the schedule and takes the summer weather with it, and the operation gets its margin back. June was the busiest month here, with 113 flights.

Across the week, Tuesday is the strongest day at 85.5% on time and Sunday the weakest at 77.3%. 8 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Delta runs late on this route

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 13,000 reported delay minutes.

Carrier
36%
National Airspace System
33%
Late-arriving aircraft
22%
Extreme weather
9%

33% 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 4,093 minutes out of 13,000, against a national NAS share of 21%. The split is drawn from 12,525 reported delay minutes.

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 4,530 of the 13,000 delay minutes reported here.

Cancellations sit at 1.49% against a national 1.81%, which is ordinary. Diversions add another 0.70%.

Connections and stage length

What a delay here costs

5.5% 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 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 71.6% on time across 1,006 flights, 10 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 10 points of difference.

One caution: this is a thin sample. 1,005 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.

What to do with this

Booking Delta on SEA to BOS

Put together: treat all of this as provisional. 1,005 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 81.2% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 983 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