Route reliability record
IAD→SEA
Washington to Seattle
- Flights
- 1,884
- Period
- July 2025 to June 2026
- Distance
- 2,305 mi
- Carriers
- 3
This route lands a B, about a point worse than the national rate of 77.6%. About one flight in four 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 1,884 IAD to SEA flights, of which United Airlines flew 688 at 79.7% on time.
Best time to fly IAD to SEA: a 11am–2pm departure, ideally on a Saturday. Flights in that window arrived on time 91.5% of the time and Saturday flights 78.9%, against 76.6% for the route overall.
The US average over the same twelve months is 77.6%, so there is little to separate this route from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
76.6%
Arrivals under 15 minutes late, of 1,861 completed flights
Avg delay
63 min
Across delayed arrivals only, not all flights
Cancelled
1.01%
National rate is 1.81%
Best carrier
United
79.7% on time across 688 flights
These figures cover 1,884 flights between July 2025 and June 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.
Carrier by carrier
Which airline is most reliable on IAD to SEA
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.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 688 | 79.7% | 62 min | 0.58% | B+ |
| Delta Air Linesthin 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
Flights on IAD to SEA
The flight numbers below flew this route at least 250 times in the period, busiest first. Each has its own record, with its schedule and delay causes.
Best ways to make this trip
The most reliable ways to fly Washington to Seattle
Every airline on this route and on the routes between nearby airports, ranked by how often a scheduled flight actually operated and arrived on time. 3 of the top 5 use a different airport at one end.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. IAD → SEAWashington to Seattle | United Airlines | 79.2% | 79.7% | 0.58% | 11am–2pm, Sat | UA 326 17:55 |
| 2. DCA → SEAWashington to Seattle | Alaska Airlines | 75.9% | 76.7% | 1.10% | 8am–11am, Sat | AS 1 08:00 |
| 3. IAD → SEAWashington to Seattle | Alaska Airlines | 72.8% | 73.7% | 1.20% | 8am–11am, Thu | AS 351 07:00 |
| 4. BWI → SEABaltimore to Seattle | Alaska Airlines | 68.2% | 68.8% | 0.83% | 8am–11am, Sun | AS 379 19:10 |
| 5. DCA → SEAWashington to Seattle | Delta Air Lines | 64.1% | 66.1% | 3.01% | 2pm–5pm, Sat | — |
Success is the share of scheduled flights that operated and arrived less than 15 minutes late, so a cancelled flight counts against it. “(route)” means the best time comes from every carrier on the route rather than this airline alone.
Departure window
The best time of day to leave
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 24 points, against 6 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am688 flights78.4%
- 8am–11am304 flights85.4%
- 11am–2pm165 flights91.5%
- 2pm–5pm—
- 5pm–8pm680 flights67.6%
- 8pm–5am47 flights69.6%
Scale runs 60% to 95%. Rows with fewer than 30 flights show a dash.
The best window here is not the first bank but the 11am–2pm slot at 91.5% on time, ahead of 5pm–8pm at 67.6%. Where the curve peaks mid-day rather than at dawn, the constraint is usually congestion at one end rather than the aircraft's own rotation. The 5am–8am window is the busiest, with 688 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 44 points from best month to worst, which is wide. An annual average tells you very little about the week you are actually flying, so read the strip rather than the headline if your dates are fixed.
December is the weak month at 48.4% on time, against 92.2% in September. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. July was the busiest month here, with 245 flights.
Across the week, Saturday is the strongest day at 78.9% on time and Sunday the weakest at 74.1%. 5 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
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 27,000 reported delay minutes.
- 42%Carrier
- 31%National Airspace System
- 17%Late-arriving aircraft
- 10%Extreme weather
- 1%Security
The largest share of delay minutes, 42%, 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 11,000 of the 27,000 delay minutes reported here. The split is drawn from 27,433 reported delay minutes.
Cancellations sit at 1.01% against a national 1.81%, which is ordinary. Diversions add another 0.21%.
Connections and stage length
What a delay here actually costs
7.0% 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 2,305 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 is a different operation and performs like one: it runs 81.9% on time across 1,886 flights, 5 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 5 points of difference.
The sample behind these figures is 1,884 flights, enough that the headline rate is stable to within roughly a point. The hour-block and month splits divide that further, so read the thinner bars with more caution than the headline.
Work out the risk of missing a connection in SEA after this flight
What to do with this
Booking this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. September runs 92.2% on time and December 48.4%, a swing of 44 points. If the dates move at all, move them out of December before you think about anything else. 1,861 completed flights over 12 months sit behind that.
Common questions
IAD to SEA questions
How often is IAD to SEA delayed?
About one flight in four arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 23.4% of the 1,884 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on IAD to SEA?
United Airlines leads at 79.7% on time across 688 flights. Alaska Airlines is at the other end on 73.7%, a gap of 6 points. Sample size matters as much as the percentage: a rate built from a few hundred flights moves around far more than one built from ten thousand.
What time of day has the fewest delays on IAD to SEA?
The 11am–2pm departure window is the strongest at 91.5% on time. The 5pm–8pm window is the weakest at 67.6%, 24 points behind. Later departures inherit delays from earlier flights the same aircraft has already flown, which is why the curve slopes the way it does.
How bad are December delays on IAD to SEA?
December is the weakest month at 48.4% on time, compared with 92.2% in September. Across the year that is 44 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.
Related
Where to go next
- SEA to IAD — the return legseparate record
- Washington Dulles International (IAD) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- United Airlines on IAD to SEA79.7%
- Delta Air Lines on IAD to SEA77.2%
- Alaska Airlines on IAD to SEA73.7%
- IAD to ATL, a related route79.2%
- LAX to SEA, a related route85.4%
- IAD to DEN, a related route81.7%
- United Airlines on-time record79.7%
- Delta Air Lines on-time record77.2%
- Alaska Airlines on-time record73.7%
- The best time of day to fly, according to 7.04 million flight records
- Worst months to fly in the US, ranked by the data
Direction matters. IAD to SEA and SEA to IAD are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
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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.