Route reliability record
SEA→IAD
Seattle to Washington
- Flights
- 1,886
- Period
- July 2025 to June 2026
- Distance
- 2,305 mi
- Carriers
- 3
This route lands a B+, 4 points better than the national rate of 77.6%. About one flight in six 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 1,886 SEA to IAD flights, of which United Airlines flew 690 at 86.5% on time.
Best time to fly SEA to IAD: a 11am–2pm departure, ideally on a Wednesday. Flights in that window arrived on time 87.1% of the time and Wednesday flights 87.8%, against 81.9% for the route overall.
Set against the rest of the country, this route's record is 4 points better than the US average of 77.6%. That gap is worth roughly 43 fewer late arrivals for every thousand flights.
Key figures
On time
81.9%
Arrivals under 15 minutes late, of 1,866 completed flights
Avg delay
60 min
Across delayed arrivals only, not all flights
Cancelled
0.64%
National rate is 1.81%
Best carrier
United
86.5% on time across 690 flights
These figures cover 1,886 flights between July 2025 and June 2026. The median arrival is 8 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 SEA to IAD
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.
| 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 Linesthin 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
Flights on SEA to IAD
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 Seattle to Washington
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. SEA → DCASeattle to Washington | Delta Air Lines | 89.3% | 92.1% | 3.01% | 5am–8am, Sat | DL 786 07:20 |
| 2. SEA → IADSeattle to Washington | United Airlines | 85.9% | 86.5% | 0.72% | 8am–11am, Tue | — |
| 3. SEA → BWISeattle to Baltimore | Alaska Airlines | 82.4% | 82.8% | 0.50% | 8pm–5am, Sat | AS 378 10:11 |
| 4. SEA → IADSeattle to Washington | Alaska Airlines | 80.1% | 80.4% | 0.36% | 8pm–5am, Wed | AS 367 23:30 |
| 5. SEA → DCASeattle to Washington | Alaska Airlines | 75.6% | 76.4% | 1.10% | 8am–11am, Sat | AS 4 08:00 |
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 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 1,886 flights.
- 5am–8am—
- 8am–11am726 flights86.2%
- 11am–2pm134 flights87.1%
- 2pm–5pm433 flights77.5%
- 5pm–8pm17 flights—
- 8pm–5am576 flights78.8%
Scale runs 70% 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 87.1% on time, ahead of 2pm–5pm at 77.5%. 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 8am–11am window is the busiest, with 726 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 20 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.
July is the worst month at 69.4% on time, 20 points below April, the best at 89.0%. Summer afternoon convection and peak-season schedules land on the same weeks every year, so this is a pattern you can book around rather than a run of bad luck. July was the busiest month here, with 245 flights.
Across the week, Wednesday is the strongest day at 87.8% on time and Saturday the weakest at 78.9%. 9 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
One category carries most of the weight here. Carrier accounts for 45% of the 20,000 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.
- 45%Carrier
- 37%Late-arriving aircraft
- 15%National Airspace System
- 3%Extreme weather
- 0%Security
37% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. That is a cascade, and cascades start small and grow through the day. Across 20,000 reported delay minutes here, that single category outweighs weather by 11 to one. It is the strongest argument for booking the first departure of the morning, when the aircraft has been sitting at the gate overnight with nothing to inherit. The split is drawn from 20,210 reported delay minutes.
The largest share of delay minutes, 45%, 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 9,121 of the 20,000 delay minutes reported here.
Cancellations are rare at 0.64%, below the national 1.81% and low enough to earn the plus on the grade. Roughly one flight in 100 is called off; the risk here is arriving late, not not arriving.
Connections and stage length
What a delay here actually costs
5.6% 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 76.6% on time across 1,884 flights, 5 points below 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 1,886 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 IAD 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. April runs 89.0% on time and July 69.4%, a swing of 20 points. If the dates move at all, move them out of July before you think about anything else. 1,866 completed flights over 12 months sit behind that.
Common questions
SEA to IAD questions
How often is SEA to IAD delayed?
About one flight in six arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 18.1% of the 1,886 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on SEA to IAD?
United Airlines leads at 86.5% on time across 690 flights. Alaska Airlines is at the other end on 80.4%, 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 SEA to IAD?
The 11am–2pm departure window is the strongest at 87.1% on time. The 2pm–5pm window is the weakest at 77.5%, 10 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 July delays on SEA to IAD?
July is the weakest month at 69.4% on time, compared with 89.0% in April. Across the year that is 20 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
- IAD to SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- Washington Dulles International (IAD) delay statistics
- United Airlines on SEA to IAD86.5%
- Alaska Airlines on SEA to IAD80.4%
- Delta Air Lines on SEA to IAD77.0%
- SEA to LAX, a related route83.4%
- ATL to IAD, a related route76.5%
- SEA to PHX, a related route79.2%
- United Airlines on-time record86.5%
- Alaska Airlines on-time record80.4%
- Delta Air Lines on-time record77.0%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. SEA to IAD and IAD to SEA 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.