Carrier route record

DLIAD→SEA

Delta Air Lines · Washington to Seattle

Flights: 361
Period: September 2025 to August 2026
Share of route: 20%
Carriers: 3

Historical statistics for September 2025 to August 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 77.3%. About one flight in four arrives 15 minutes late or worse, and those flights are late by an hour and a half on average. That rests on 361 flights. The strongest carrier on the route with a full sample, United Airlines, flew 625 at 81.3% on time.

The US average over the same twelve months is 77.3%, 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

76.6%

Arrivals under 15 minutes late, of 355 completed flights

Avg delay

88 min

Across delayed arrivals only, not all flights

Cancelled

1.39%

National rate is 1.89%

Whole route

77.7%

Every carrier on IAD to SEA, 1,821 flights

These figures cover 361 flights between September 2025 and August 2026. The median arrival is 5 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.

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Carrier by carrier

Delta against every carrier on IAD to SEA

Delta flew 361 of the 1,821 flights on IAD to SEA, about 20% of it. Its 76.6% on time is about level with the route as a whole at 77.7%. Ranked by on-time rate it is number two of three carriers on the route, where the range runs from United at 81.3% to Alaska at 75.4%.

The two carriers here are closely matched: 6 points 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 IAD to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	625	81.3%	66 min	0.48%	B+
Delta Air Lines (this page)thin sample	361	76.6%	88 min	1.39%	B
Alaska Airlines	835	75.4%	53 min	1.20%	B

No carrier stands out. United tops the table at 81.3%, 6 points clear of the bottom, which is inside the range this kind of sample moves year to year.

Flight by flight

Delta flight numbers on IAD to SEA

DL 101507:0077.0% · 349 flights

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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 76.6% is the reliable figure here.

Across the year

Delta on IAD to SEA through the year

Jul–Sep 202530 flights90.0%
Oct–Dec 202589 flights68.2%
Jan–Mar 202689 flights63.5%
Apr–Jun 202691 flights86.7%
Jul–Sep 202662 flights85.5%

Scale runs 55% to 95%.

Monthly volumes are too low here to break the year into twelve reliable readings. What the twelve-month total says is 76.6% 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 90.0% on time and its weakest Jan–Mar 2026 at 63.5%, a swing of 27 points on roughly 72 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 84.6% on time and Tuesday the weakest at 69.2%, a spread of 15 points.

Arrival by arrival

How late Delta on IAD to SEA runs when it is late

Delta on IAD to SEA often beats the schedule: 112 of 355 completed flights, 32%, arrived 15 minutes or more early. The 83 late arrivals split 32 under half an hour, 24 between 30 and 60 minutes, and 27 an hour or more.

15+ min early112 flights31.5%
Within 15 min160 flights45.1%
15–29 min late32 flights9.0%
30–59 min late24 flights6.8%
1–2 hours late10 flights2.8%
2+ hours late17 flights4.8%

Scale runs 0% to 100%.

Share of 355 completed flights by arrival against schedule.

9 of 52 late departures made up the time and arrived on time, 17% of them. In the other direction, 40 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at SEA.

The worst days

Worst days for Delta on IAD to SEA

The latest arrival of the period was on 13 November 2025, 11 hours 42 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on three of the five worst days.

The latest arrivals of Delta on IAD to SEA in the period
Date	Arrived late by	Largest filed cause
13 November 2025	11 hours 42 minutes	Carrier
26 January 2026	8 hours 30 minutes	Extreme weather
19 March 2026	8 hours 25 minutes	Carrier
4 July 2026	7 hours 10 minutes	Carrier
13 August 2026	6 hours 17 minutes	Late-arriving aircraft

Delay causes

Why Delta runs late on this route

Carrier accounts for 63% of the 7,303 delay minutes carriers reported here, and the remaining four split what is left.

Carrier
63%
National Airspace System
24%
Extreme weather
7%
Late-arriving aircraft
6%

The largest share of delay minutes, 63%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 4,614 of the 7,303 delay minutes reported here. The split is drawn from 7,303 reported delay minutes.

Cancellations sit at 1.39% against a national 1.89%, which is ordinary. Diversions add another 0.28%.

Connections and stage length

What a delay here costs

7.6% of arrivals land an hour or more late, close to the national 8.3%, so 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 76.7% 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 76.6% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 355 completed flights over 12 months sit behind that.

Where to go next

Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated 22 October 2026; recomputed monthly. How this data is collected.

Key figures