Carrier route record

DLBOS→SEA

Delta Air Lines · Boston to Seattle

Flights: 1,006
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 BOS to SEA 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 an hour behind schedule. That rests on 1,006 flights, with a median arrival right on schedule.

The national on-time rate over the same period is 77.6%, so Delta on BOS to SEA runs 6 points behind the country as a whole. Across a thousand flights that is about 60 extra late arrivals.

On time

71.6%

Arrivals under 15 minutes late, of 988 completed flights

Avg delay

55 min

Across delayed arrivals only, not all flights

Cancelled

1.59%

National rate is 1.81%

Whole route

71.4%

Every carrier on BOS to SEA, 2,461 flights

These figures cover 1,006 flights between July 2025 and June 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.

Carrier by carrier

Delta against every carrier on BOS to SEA

Delta flew 1,006 of the 2,461 flights on BOS to SEA, about 41% of it. Its 71.6% on time is about level with the route as a whole at 71.4%. Ranked by on-time rate it is number two of three carriers on the route, where the range runs from JetBlue at 75.6% to Alaska at 70.5%. 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 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 (this page)	1,006	71.6%	55 min	1.59%	C
Alaska Airlines	1,208	70.5%	50 min	1.16%	C

Flight by flight

Delta flight numbers on BOS to SEA

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 14 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–8am360 flights75.3%
8am–11am5 flights—
11am–2pm234 flights78.0%
2pm–5pm43 flights67.4%
5pm–8pm363 flights64.5%
8pm–5am1 flights—

Scale runs 55% to 85%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 64.5% 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 5pm–8pm window is the busiest, with 363 flights scheduled in it.

Across the year

Delta on BOS to SEA through the year

Darker means more delays. Figures are on-time percentages.August best, March worst

Delays peak in March, when the on-time rate falls to 33.3%. August is the strongest month at 92.9%. Moving a flexible trip across that boundary buys 60 points. June was the busiest month here, with 113 flights.

Across the week, Saturday is the strongest day at 73.9% on time and Thursday the weakest at 67.8%. 6 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 16,000 reported delay minutes.

Carrier
37%
National Airspace System
32%
Late-arriving aircraft
29%
Extreme weather
2%

The largest share of delay minutes, 37%, 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 5,694 of the 16,000 delay minutes reported here. The split is drawn from 15,520 reported delay minutes.

32% 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 5,025 minutes out of 16,000, against a national NAS share of 21%.

Cancellations sit at 1.59% against a national 1.81%, which is ordinary. Diversions add another 0.20%.

Connections and stage length

What a delay here costs

6.6% 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 81.2% on time across 1,005 flights, 10 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 10 points of difference.

One caution: this is a thin sample. 1,006 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 BOS to SEA

Put together: treat all of this as provisional. 1,006 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 71.6% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 988 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