Carrier route record
DLMIA→SEA
Delta Air Lines · Miami to Seattle
- Flights
- 366
- Period
- September 2025 to August 2026
- Share of route
- 30%
- Carriers
- 3
Historical statistics for September 2025 to August 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 366 flights. The strongest carrier on the route with a full sample, American Airlines, flew 475 at 72.5% on time.
The national on-time rate over the same period is 77.3%, so Delta on MIA to SEA runs 14 points behind the country as a whole. Across a thousand flights that is about 139 extra late arrivals.
Key figures
On time
63.4%
Arrivals under 15 minutes late, of 358 completed flights
Avg delay
90 min
Across delayed arrivals only, not all flights
Cancelled
1.64%
National rate is 1.89%
Whole route
68.7%
Every carrier on MIA to SEA, 1,233 flights
These figures cover 366 flights between September 2025 and August 2026. Beyond the headline rate, 11.5% of arrivals land a full hour or more behind schedule, against 8.3% nationally.
Fare search by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.
Carrier by carrier
Delta against every carrier on MIA to SEA
Delta flew 366 of the 1,233 flights on MIA to SEA, about 30% of it. Its 63.4% on time is 5 points worse than the route as a whole, which runs 68.7%. Ranked by on-time rate it is last of three carriers on the route, where the range runs from American at 72.5% to Delta at 63.4%.
The two carriers here are closely matched: 3 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 | 475 | 72.5% | 52 min | 0.42% | C+ |
| Alaska Airlines | 392 | 69.1% | 48 min | 1.28% | C |
| Delta Air Lines (this page) | 366 | 63.4% | 90 min | 1.64% | D |
No carrier stands out. American tops the table at 72.5%, 3 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
Timetable by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.
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 366 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 63.4% is the reliable figure here.
Across the year
Delta on MIA to SEA through the year
October is the best month on the calendar at 83.9% on time. Shoulder season thins the schedule and takes the summer weather with it, and the operation gets its margin back. July was the busiest month here, with 32 flights.
Across the week, Tuesday is the strongest day at 76.9% on time and Sunday the weakest at 52.0%, a spread of 25 points.
Arrival by arrival
How late Delta on MIA to SEA runs when it is late
The 131 late arrivals split 38 under half an hour, 52 between 30 and 60 minutes, and 41 an hour or more.
- 15+ min early77 flights21.5%
- Within 15 min150 flights41.9%
- 15–29 min late38 flights10.6%
- 30–59 min late52 flights14.5%
- 1–2 hours late24 flights6.7%
- 2+ hours late17 flights4.7%
Scale runs 0% to 100%.
Share of 358 completed flights by arrival against schedule.
19 of 114 late departures made up the time and arrived on time, 17% of them. In the other direction, 36 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 MIA to SEA
The latest arrival of the period was on 29 November 2025, 17 hours 51 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on five of the five worst days.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 29 November 2025 | 17 hours 51 minutes | Carrier |
| 21 December 2025 | 17 hours 33 minutes | Carrier |
| 4 December 2025 | 16 hours 48 minutes | Carrier |
| 7 September 2025 | 13 hours 43 minutes | Carrier |
| 9 April 2026 | 7 hours 51 minutes | Carrier |
Delay causes
Why Delta runs late on this route
Carrier accounts for 56% of the 12,000 delay minutes carriers reported here, and the remaining four split what is left.
- 56%Carrier
- 24%Late-arriving aircraft
- 16%National Airspace System
- 4%Extreme weather
The largest share of delay minutes, 56%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 6,664 of the 12,000 delay minutes reported here. The split is drawn from 11,818 reported delay minutes.
Cancellations sit at 1.64% against a national 1.89%, which is ordinary. Diversions add another 0.55%.
Connections and stage length
What a delay here costs
11.5% of arrivals land a full hour or more behind schedule, against 8.3% nationally. Ninety minutes is the honest minimum for a connection 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 80.7% 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. 366 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. 366 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 63.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 358 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 SEA72.5%
- Alaska Airlines on MIA to SEA69.1%
- 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, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.