Carrier route record
DLSEA→MIA
Delta Air Lines · Seattle to Miami
- Flights
- 365
- 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 SEA to MIA lands a B, 3 points better than the national rate of 77.3%. About one flight in five arrives 15 minutes late or worse, and those flights are late by about an hour on average. That rests on 365 flights. The strongest carrier on the route with a full sample, Alaska Airlines, flew 391 at 80.5% on time.
The US average over the same twelve months is 77.3%, so there is little to separate Delta on SEA to MIA from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
80.7%
Arrivals under 15 minutes late, of 357 completed flights
Avg delay
65 min
Across delayed arrivals only, not all flights
Cancelled
1.37%
National rate is 1.89%
Whole route
79.7%
Every carrier on SEA to MIA, 1,231 flights
These figures cover 365 flights between September 2025 and August 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.
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Carrier by carrier
Delta against every carrier on SEA to MIA
Delta flew 365 of the 1,231 flights on SEA to MIA, about 30% of it. Its 80.7% on time is about level with the route as a whole at 79.7%. Ranked by on-time rate it is first of three carriers on the route, where the range runs from Delta at 80.7% to American at 78.4%.
The two carriers here are closely matched: 2 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 |
|---|---|---|---|---|---|
| Delta Air Lines (this page) | 365 | 80.7% | 65 min | 1.37% | B |
| Alaska Airlines | 391 | 80.5% | 40 min | 0.26% | B+ |
| American Airlines | 475 | 78.4% | 68 min | 0.84% | B+ |
No carrier stands out. Alaska tops the table at 80.5%, 2 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 SEA to MIA
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 365 flights.
- 5am–8am66 flights84.8%
- 8am–11am299 flights79.7%
- 11am–2pm—
- 2pm–5pm—
- 5pm–8pm—
- 8pm–5am—
Scale runs 70% to 90%. Rows with fewer than 30 flights show a dash.
The early departures do better, though not dramatically: 84.8% on time for the 5am–8am window against 79.7% for 8am–11am. Worth taking if the schedule allows, not worth an extra connection. The 8am–11am window is the busiest, with 299 flights scheduled in it.
Across the year
Delta on SEA to MIA through the year
- Jul–Sep 202530 flights—
- Oct–Dec 202592 flights85.4%
- Jan–Mar 202690 flights73.9%
- Apr–Jun 202691 flights84.6%
- Jul–Sep 202662 flights83.3%
Scale runs 65% to 90%. Rows with fewer than 30 flights show a dash.
Monthly volumes are too low here to break the year into twelve reliable readings. What the twelve-month total says is 80.7% on time across 365 flights.
Delta on SEA to MIA did not run the same all year. Its best quarter was Oct–Dec 2025 at 85.4% on time and its weakest Jan–Mar 2026 at 73.9%, a swing of 12 points on roughly 91 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.
Across the week, Wednesday is the strongest day at 84.3% on time and Monday the weakest at 74.5%, a spread of 10 points.
Arrival by arrival
How late Delta on SEA to MIA runs when it is late
Delta on SEA to MIA often beats the schedule: 130 of 357 completed flights, 36%, arrived 15 minutes or more early. The 69 late arrivals split 23 under half an hour, 26 between 30 and 60 minutes, and 20 an hour or more.
- 15+ min early130 flights36.4%
- Within 15 min158 flights44.3%
- 15–29 min late23 flights6.4%
- 30–59 min late26 flights7.3%
- 1–2 hours late9 flights2.5%
- 2+ hours late11 flights3.1%
Scale runs 0% to 100%.
Share of 357 completed flights by arrival against schedule.
6 of 36 late departures made up the time and arrived on time, 17% of them. In the other direction, 39 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 MIA.
The worst days
Worst days for Delta on SEA to MIA
The latest arrival of the period was on 5 June 2026, 5 hours 56 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 |
|---|---|---|
| 5 June 2026 | 5 hours 56 minutes | Carrier |
| 1 June 2026 | 4 hours 17 minutes | Carrier |
| 12 February 2026 | 4 hours 16 minutes | Carrier |
| 30 August 2026 | 3 hours 59 minutes | Carrier |
| 15 May 2026 | 3 hours 22 minutes | Carrier |
Delay causes
Why Delta runs late on this route
Carrier accounts for 58% of the 4,511 delay minutes carriers reported here, and the remaining four split what is left.
- 58%Carrier
- 38%National Airspace System
- 3%Extreme weather
- 0%Late-arriving aircraft
38% 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 1,719 minutes out of 4,511, against a national NAS share of 21%. The split is drawn from 4,511 reported delay minutes.
The largest share of delay minutes, 58%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 2,633 of the 4,511 delay minutes reported here.
Cancellations sit at 1.37% against a national 1.89%, which is ordinary. Diversions add another 0.82%.
Connections and stage length
What a delay here costs
5.6% 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 2,724 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 63.4% on time across 366 flights, 17 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 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 SEA to MIA
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 80.7% 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
- SEA to MIA, every carrier
- Delta on MIA to SEA, the return leg
- Alaska Airlines on SEA to MIA80.5%
- American Airlines on SEA to MIA78.4%
- Delta Air Lines network-wide record
- Seattle-Tacoma International (SEA) delay statistics
- Miami International (MIA) delay statistics
- The best day of the week to fly, and why it is the second lever
- How to read this site: grades, sample sizes, and honest caveats
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.