Carrier route record

DLSEA→IAD

Delta Air Lines · Seattle to Washington

Flights: 361
Period: July 2025 to June 2026
Share of route: 19%
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 IAD lands a B, level with the national rate of 77.6%. Roughly one flight in four arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That is a workable record, not a spotless one. That rests on 361 flights. The strongest carrier on the route with a full sample, United Airlines, flew 690 at 86.5% on time.

The US average over the same twelve months is 77.6%, so there is little to separate Delta on SEA to IAD from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.

On time

77.0%

Arrivals under 15 minutes late, of 356 completed flights

Avg delay

75 min

Across delayed arrivals only, not all flights

Cancelled

1.11%

National rate is 1.81%

Whole route

81.9%

Every carrier on SEA to IAD, 1,886 flights

These figures cover 361 flights between July 2025 and June 2026. Beyond the headline rate, 10.1% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.

Carrier by carrier

Delta against every carrier on SEA to IAD

Delta flew 361 of the 1,886 flights on SEA to IAD, about 19% of it. Its 77.0% on time is 5 points worse than the route as a whole, which runs 81.9%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from United at 86.5% to Delta at 77.0%.

The choice of carrier is not cosmetic here. United Airlines arrives on time 86.5% of the time; Alaska Airlines manages 80.4%. Sample sizes sit next to every figure, because a two-point gap across 835 flights means less than a ten-point one.

On-time performance by carrier on SEA to IAD
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	690	86.5%	79 min	0.72%	A+
Alaska Airlines	835	80.4%	41 min	0.36%	B+
Delta Air Lines (this page)thin sample	361	77.0%	75 min	1.11%	B

United leads Alaska by 6 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.

Flight by flight

Delta flight numbers on SEA to IAD

DL 54121:5576.7% · 349 flights

Departure window

When Delta runs best on this route

Departure time is the single lever a traveller controls, and it moves the odds more than most people expect. Each bar below is the on-time rate for flights scheduled to leave inside that window, across 361 flights.

5am–8am—
8am–11am—
11am–2pm—
2pm–5pm247 flights78.1%
5pm–8pm17 flights—
8pm–5am97 flights74.2%

Scale runs 65% to 85%. Rows with fewer than 30 flights show a dash.

The 2pm–5pm window is the strongest at 78.1% on time. The full set of windows spans 4 points.

Across the year

Delta on SEA to IAD through the year

Jul–Sep 202592 flights75.0%
Oct–Dec 202589 flights77.3%
Jan–Mar 202689 flights72.1%
Apr–Jun 202691 flights83.3%

Scale runs 65% to 90%. Rows with fewer than 30 flights show a dash.

Monthly volumes are too low here to break the year into twelve reliable readings. What the twelve-month total says is 77.0% on time across 361 flights.

Delta on SEA to IAD did not run the same all year. Its best quarter was Apr–Jun 2026 at 83.3% on time and its weakest Jan–Mar 2026 at 72.1%, a swing of 11 points on roughly 90 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.

Across the week, Wednesday is the strongest day at 86.0% on time and Monday the weakest at 70.6%. 15 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Delta runs late on this route

One category carries most of the weight here. Carrier accounts for 64% of the 6,167 delay minutes carriers reported, and the remaining four split what is left. The split is by minutes rather than by flights, so a few very long delays count for as much as many short ones.

Carrier
64%
Late-arriving aircraft
20%
National Airspace System
10%
Extreme weather
6%

The largest share of delay minutes, 64%, 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 3,917 of the 6,167 delay minutes reported here. The split is drawn from 6,167 reported delay minutes.

Cancellations sit at 1.11% against a national 1.81%, which is ordinary. Diversions add another 0.28%.

Connections and stage length

What a delay here costs

For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 10.1% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.

At 2,305 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 runs 77.2% on time across 361 flights, within half a point of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.

One caution: this is a thin sample. 361 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 IAD

Put together: treat all of this as provisional. 361 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 77.0% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 356 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