Route reliability record

MIA→SJU

Miami to San Juan

Flights: 2,636
Period: July 2025 to June 2026
Distance: 1,045 mi
Carriers: 3

This route sits in the middle of the pack with a C+. Close to three flights in four arrive on time by the DOT's definition, 3 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 2,636 flights, of which American Airlines flew 2,078 at 76.8% on time.

The US average over the same twelve months is 77.6%, so there is little to separate this route from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.

On time

74.7%

Arrivals under 15 minutes late, of 2,609 completed flights

Avg delay

76 min

Across delayed arrivals only, not all flights

Cancelled

0.95%

National rate is 1.81%

Best carrier

American

76.8% on time across 2,078 flights

These figures cover 2,636 flights between July 2025 and June 2026. Beyond the headline rate, 10.2% 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

Which airline is most reliable on MIA to SJU

The two carriers here are closely matched: 5 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 MIA to SJU
Airline	Flights	On time	Avg delay	Cancelled	Grade
American Airlines	2,078	76.8%	71 min	0.53%	B+
Frontier Airlines	379	71.5%	85 min	1.85%	C
Spirit Airlinesthin sample	179	55.8%	96 min	3.91%	D−

No carrier stands out. American tops the table at 76.8%, 5 points clear of the bottom, which is inside the range this kind of sample moves year to year.

Departure window

The best time of day to leave

Departure time matters more here than the choice of carrier. The gap between the best and worst window is 30 points, against 5 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am335 flights88.9%
8am–11am726 flights84.5%
11am–2pm471 flights68.5%
2pm–5pm523 flights70.3%
5pm–8pm263 flights67.4%
8pm–5am318 flights59.4%

Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.

The 8pm–5am window is where this operation falls apart: 59.4% on time, against 88.9% for the 5am–8am slot. By evening the aircraft flying these rotations have absorbed a full day of small delays and there is no slack left to recover in. 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 23 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.

Darker means more delays. Figures are on-time percentages.May best, July worst

July is the worst month at 62.6% on time, 23 points below May, the best at 85.5%. 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. March was the busiest month here, with 278 flights.

Across the week, Wednesday is the strongest day at 81.1% on time and Sunday the weakest at 65.7%. 15 points is a smaller lever than departure hour, but it is free.

Delay causes

Why this route gets delayed

Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 50,000 reported delay minutes.

Late-arriving aircraft
41%
Carrier
39%
National Airspace System
17%
Extreme weather
3%
Security
0%

41% 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 50,000 reported delay minutes here, that single category outweighs weather by 13 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 50,322 reported delay minutes.

The largest share of delay minutes, 39%, 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 19,000 of the 50,000 delay minutes reported here.

Cancellations sit at 0.95% against a national 1.81%, which is ordinary. Diversions add another 0.08%.

Connections and stage length

What a delay here actually costs

For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 10.2% 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.

The 1,045 miles between these two airports put this in the middle of the US network, around two and a half to three hours of block time. That is enough cruise to absorb a ten-minute late pushback and not enough to absorb thirty. It is also long enough that the aircraft usually flies three or four legs a day rather than six, which slows how fast delay compounds.

The return leg is a different operation and performs like one: it runs 77.7% on time across 2,642 flights, 3 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 3 points of difference.

The sample behind these figures is 2,636 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.

What to do with this

Booking this route

Put together: the calendar is the biggest lever here, not the carrier or the hour. May runs 85.5% on time and July 62.6%, a swing of 23 points. If the dates move at all, move them out of July before you think about anything else. 2,609 completed flights over 12 months sit behind that.

Common questions

MIA to SJU questions

How often is MIA to SJU delayed?

About one flight in four arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 25.3% of the 2,636 flights in this analysis, against 22.4% nationally. Delayed flights are late by a little over an hour on average.

Which airline is most reliable on MIA to SJU?

American Airlines leads at 76.8% on time across 2,078 flights. Frontier Airlines is at the other end on 71.5%, a gap of 5 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 MIA to SJU?

The 5am–8am departure window is the strongest at 88.9% on time. The 8pm–5am window is the weakest at 59.4%, 30 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 MIA to SJU?

July is the weakest month at 62.6% on time, compared with 85.5% in May. Across the year that is 23 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.

Where to go next

Direction matters. MIA to SJU and SJU to MIA are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.

Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.

Key figures