Carrier route record
DLMIA→SEA
Delta Air Lines · Miami to Seattle
- Flights
- 365
- Period
- July 2025 to June 2026
- Share of route
- 31%
- Carriers
- 3
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Delta Air Lines.
Delta on MIA to SEA draws a D. Roughly one flight in three arrives at least 15 minutes late, and the typical late arrival is an hour and a half behind schedule. Build slack into anything you connect to. That rests on 365 flights. The strongest carrier on the route with a full sample, American Airlines, flew 423 at 69.5% on time.
The national on-time rate over the same period is 77.6%, so Delta on MIA to SEA runs 16 points behind the country as a whole. Across a thousand flights that is about 160 extra late arrivals.
Key figures
On time
61.6%
Arrivals under 15 minutes late, of 357 completed flights
Avg delay
93 min
Across delayed arrivals only, not all flights
Cancelled
1.64%
National rate is 1.81%
Whole route
65.7%
Every carrier on MIA to SEA, 1,180 flights
These figures cover 365 flights between July 2025 and June 2026. Beyond the headline rate, 12.3% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.
Carrier by carrier
Delta against every carrier on MIA to SEA
Delta flew 365 of the 1,180 flights on MIA to SEA, about 31% of it. Its 61.6% on time is 4 points worse than the route as a whole, which runs 65.7%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from American at 69.5% to Delta at 61.6%.
The two carriers here are closely matched: 4 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 |
|---|---|---|---|---|---|
| American Airlines | 423 | 69.5% | 56 min | 0.47% | C+ |
| Alaska Airlines | 392 | 65.4% | 50 min | 1.53% | C |
| Delta Air Lines (this page) | 365 | 61.6% | 93 min | 1.64% | D |
No carrier stands out. American tops the table at 69.5%, 4 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 MIA to SEA
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 365 flights.
There are not enough flights in each departure window to split Delta's MIA to SEA record by time of day without the percentages moving on noise. The headline rate of 61.6% is the reliable figure here.
Across the year
Delta on MIA to SEA through the year
August is the worst month at 48.4% on time, 36 points below October, the best at 83.9%. 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. January was the busiest month here, with 31 flights.
Across the week, Tuesday is the strongest day at 75.5% on time and Friday the weakest at 52.0%. 24 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 59% of the 13,000 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.
- 59%Carrier
- 24%Late-arriving aircraft
- 14%National Airspace System
- 3%Extreme weather
- 0%Security
The largest share of delay minutes, 59%, 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 7,508 of the 13,000 delay minutes reported here. The split is drawn from 12,762 reported delay minutes.
Cancellations sit at 1.64% against a national 1.81%, which is ordinary. Diversions add another 0.55%.
Connections and stage length
What a delay here costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 12.3% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.
At 2,724 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 78.2% on time across 365 flights, 17 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 17 points of difference.
One caution: this is a thin sample. 365 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 MIA to SEA
Put together: treat all of this as provisional. 365 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 61.6% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 357 completed flights over 12 months sit behind that.
Related
Where to go next
- MIA to SEA, every carrier
- Delta on SEA to MIA, the return leg
- American Airlines on MIA to SEA69.5%
- Alaska Airlines on MIA to SEA65.4%
- Delta Air Lines network-wide record
- Miami International (MIA) delay statistics
- Seattle-Tacoma International (SEA) delay statistics
- How often a connection fails at the major US hubs
- Connecting flights: how much buffer the data says you need
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.