Carrier route record

DLSEA→MIA

Delta Air Lines · Seattle to Miami

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 SEA to MIA lands a B, level with the national rate of 77.6%. Roughly one flight in five 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 365 flights. The strongest carrier on the route with a full sample, Alaska Airlines, flew 391 at 78.8% on time.

The US average over the same twelve months is 77.6%, 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.

On time

78.2%

Arrivals under 15 minutes late, of 358 completed flights

Avg delay

60 min

Across delayed arrivals only, not all flights

Cancelled

1.37%

National rate is 1.81%

Whole route

77.3%

Every carrier on SEA to MIA, 1,179 flights

These figures cover 365 flights between July 2025 and June 2026. The median arrival is 6 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

Delta against every carrier on SEA to MIA

Delta flew 365 of the 1,179 flights on SEA to MIA, about 31% of it. Its 78.2% on time is about level with the route as a whole at 77.3%. Ranked by on-time rate it is number two of three carriers on the route, where the range runs from Alaska at 78.8% to American at 75.1%.

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.

On-time performance by carrier on SEA to MIA
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	391	78.8%	38 min	0.77%	B+
Delta Air Lines (this page)	365	78.2%	60 min	1.37%	B
American Airlines	423	75.1%	58 min	0.71%	B+

No carrier stands out. Alaska tops the table at 78.8%, 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 SEA to MIA

DL 50608:0078.2% · 365 flights

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 flights76.7%
11am–2pm—
2pm–5pm—
5pm–8pm—
8pm–5am—

Scale runs 70% to 90%. Rows with fewer than 30 flights show a dash.

Book the early departure. Flights leaving between 5am–8am arrive on time 84.8% of the time, against 76.7% for the morning window. That is 8 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 8am–11am window is the busiest, with 299 flights scheduled in it.

Across the year

Delta on SEA to MIA through the year

Darker means more delays. Figures are on-time percentages.October best, August worst

August is the worst month at 58.1% on time, 35 points below October, the best at 93.5%. 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 84.9% on time and Friday the weakest at 70.6%. 14 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 56% of the 4,695 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.

Carrier
56%
National Airspace System
40%
Extreme weather
3%
Late-arriving aircraft
0%

40% 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 1,877 minutes out of 4,695, against a national NAS share of 21%. The split is drawn from 4,695 reported delay minutes.

The largest share of delay minutes, 56%, 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 2,652 of the 4,695 delay minutes reported here.

Cancellations sit at 1.37% against a national 1.81%, which is ordinary. Diversions add another 0.55%.

Connections and stage length

What a delay here 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 2,724 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 61.6% on time across 365 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 78.2% 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.

Where to go next

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.

Key figures