Route reliability record
IAD→SEA
Washington to Seattle
- Flights
- 1,821
- Period
- September 2025 to August 2026
- Distance
- 2,305 mi
- Carriers
- 3
This route lands a B+, level with the national rate of 77.3%. Roughly one flight in four arrives 15 minutes late or worse, and those flights are late by about an hour on average. That rests on 1,821 IAD to SEA flights, of which United Airlines flew 625 at 81.3% on time.
Best time to fly IAD to SEA: a 8am–11am departure, ideally on a Saturday. Flights in that window arrived on time 90.8% of the time and Saturday flights 80.7%, against 77.7% for the route overall.
The US average over the same twelve months is 77.3%, 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
77.7%
Arrivals under 15 minutes late, of 1,799 completed flights
Avg delay
64 min
Across delayed arrivals only, not all flights
Cancelled
0.99%
National rate is 1.89%
Best carrier
United
81.3% on time across 625 flights
These figures cover 1,821 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
Which airline is most reliable on IAD to SEA
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.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines | 625 | 81.3% | 66 min | 0.48% | B+ |
| Delta Air Linesthin 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
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.
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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. 2 of the top 4 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 | 80.9% | 81.3% | 0.48% | 8am–11am, Sat | UA 326 17:55 |
| 2. DCA → SEAWashington to Seattle | Alaska Airlines | 75.9% | 76.4% | 0.69% | 8am–11am, Sat | AS 1 08:00 |
| 3. IAD → SEAWashington to Seattle | Alaska Airlines | 74.5% | 75.4% | 1.20% | 8am–11am, Thu | — |
| 4. BWI → SEABaltimore to Seattle | Alaska Airlines | 67.6% | 68.2% | 0.83% | 8am–11am, Mon | AS 379 18:07 |
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 22 points, against 6 points between the best and worst carrier.
- 5am–8am688 flights77.8%
- 8am–11am297 flights90.8%
- 11am–2pm165 flights89.1%
- 2pm–5pm—
- 5pm–8pm671 flights68.8%
- 8pm–5am—
Scale runs 60% to 95%. Rows with fewer than 30 flights show a dash.
Book the morning departure. Flights leaving between 8am–11am arrive on time 90.8% of the time, against 68.8% for the evening window. That is 22 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 5am–8am window is the busiest, with 688 flights scheduled in it.
Explore
Filter IAD to SEA by airline, time, month and day
Pick a breakdown and a carrier. The dashed mark on each row is the same slice for every US flight.
On IAD to SEA, the best departure window is 8am–11am at 90.8% on time and the worst is 5pm–8pm at 68.8%, a wide gap of 22.0 points. All US flights leaving 8am–11am ran 83.9%.
- 5–8am77.8%
- 8–11am90.8%
- 11am–2pm89.1%
- 2–5pm—
- 5–8pm68.8%
- 8pm–5am—
best all US flightsScale 60% to 95%. Cells under 30 flights show a dash.
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 212 flights.
Across the week, Saturday is the strongest day at 80.7% on time and Tuesday the weakest at 74.9%, a spread of 6 points.
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 26,000 reported delay minutes.
- 41%Carrier
- 32%National Airspace System
- 16%Late-arriving aircraft
- 9%Extreme weather
- 1%Security
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 8,290 minutes out of 26,000, against a national NAS share of 21%. The split is drawn from 25,675 reported delay minutes.
The largest share of delay minutes, 41%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 11,000 of the 26,000 delay minutes reported here.
Cancellations sit at 0.99% against a national 1.89%, which is ordinary. Diversions add another 0.22%.
Connections and stage length
What a delay here actually costs
6.4% 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 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 84.8% on time across 1,822 flights, 7 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 7 points of difference.
The sample behind these figures is 1,821 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. 1,799 completed flights over 12 months sit behind that.
Common questions
IAD to SEA questions
How often is IAD to SEA delayed?
Roughly 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 22.3% of the 1,821 flights in this analysis, against 22.7% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on IAD to SEA?
United Airlines leads at 81.3% on time across 625 flights. Alaska Airlines is at the other end on 75.4%, 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 8am–11am departure window is the strongest at 90.8% on time. The 5pm–8pm window is the weakest at 68.8%, 22 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 SEA81.3%
- Delta Air Lines on IAD to SEA76.6%
- Alaska Airlines on IAD to SEA75.4%
- IAD to ATL, a related route78.0%
- LAX to SEA, a related route85.0%
- IAD to DEN, a related route83.1%
- United Airlines on-time record81.3%
- Delta Air Lines on-time record76.6%
- Alaska Airlines on-time record75.4%
- The best time of day to fly, according to 7.05 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, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.