Carrier route record

DLIAD→SEA

Delta Air Lines · Washington to Seattle

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 IAD to SEA 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 an hour and a half 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 688 at 79.7% on time.

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

On time

77.2%

Arrivals under 15 minutes late, of 355 completed flights

Avg delay

93 min

Across delayed arrivals only, not all flights

Cancelled

1.39%

National rate is 1.81%

Whole route

76.6%

Every carrier on IAD to SEA, 1,884 flights

These figures cover 361 flights between July 2025 and June 2026. The median arrival is 6 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 IAD to SEA

Delta flew 361 of the 1,884 flights on IAD to SEA, about 19% of it. Its 77.2% on time is about level with the route as a whole at 76.6%. Ranked by on-time rate it is number two of three carriers on the route, where the range runs from United at 79.7% to Alaska at 73.7%.

The choice of carrier is not cosmetic here. United Airlines arrives on time 79.7% of the time; Alaska Airlines manages 73.7%. 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 IAD to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	688	79.7%	62 min	0.58%	B+
Delta Air Lines (this page)thin sample	361	77.2%	93 min	1.39%	B
Alaska Airlines	835	73.7%	53 min	1.20%	C

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 IAD to SEA

DL 101507:0077.6% · 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.

There are not enough flights in each departure window to split Delta's IAD to SEA record by time of day without the percentages moving on noise. The headline rate of 77.2% is the reliable figure here.

Across the year

Delta on IAD to SEA through the year

Jul–Sep 202592 flights89.1%
Oct–Dec 202589 flights68.2%
Jan–Mar 202689 flights63.5%
Apr–Jun 202691 flights86.7%

Scale runs 55% to 95%. 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 77.2% on time across 361 flights.

Delta on IAD to SEA did not run the same all year. Its best quarter was Jul–Sep 2025 at 89.1% on time and its weakest Jan–Mar 2026 at 63.5%, a swing of 26 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, Monday is the strongest day at 88.2% on time and Thursday the weakest at 68.0%. 20 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 69% of the 7,494 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
69%
National Airspace System
23%
Extreme weather
7%
Late-arriving aircraft
1%

The largest share of delay minutes, 69%, 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 5,195 of the 7,494 delay minutes reported here. The split is drawn from 7,494 reported delay minutes.

Cancellations sit at 1.39% against a national 1.81%, which is ordinary. Diversions add another 0.28%.

Connections and stage length

What a delay here costs

7.3% 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 an hour and a half, which a normal layover absorbs.

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 77.0% 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 IAD to SEA

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 77.2% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 355 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