Route reliability record

MIA→IAH

Miami to Houston

Flights: 3,737
Period: July 2025 to June 2026
Distance: 964 mi
Carriers: 6

This route lands a B, level with the national rate of 77.6%. Roughly one flight in four arrive 15 minutes late or worse, and those flights are late by a little over an hour on average. That is a workable record, not a spotless one. That rests on 3,737 flights, of which United Airlines flew 1,479 at 85.9% on time.

United Airlines is the most reliable of the three carriers covered here at 85.9% on time across 1,479 flights.

On time

77.2%

Arrivals under 15 minutes late, of 3,689 completed flights

Avg delay

80 min

Across delayed arrivals only, not all flights

Cancelled

1.12%

National rate is 1.81%

Best carrier

United

85.9% on time across 1,479 flights

These figures cover 3,737 flights between July 2025 and June 2026. Beyond the headline rate, 9.3% 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

Which airline is most reliable on MIA to IAH

The choice of carrier is not cosmetic here. United Airlines arrives on time 85.9% of the time; Spirit Airlines manages 69.7%. Sample sizes sit next to every figure, because a two-point gap across 440 flights means less than a ten-point one.

On-time performance by carrier on MIA to IAH
Airline	Flights	On time	Avg delay	Cancelled	Grade
Envoy Airthin sample	122	86.9%	39 min	0.00%	A+
United Airlines	1,479	85.9%	91 min	0.61%	A+
Frontier Airlinesthin sample	155	78.4%	97 min	1.29%	B
American Airlines	1,527	70.1%	72 min	1.24%	C
Spirit Airlines	440	69.7%	87 min	2.73%	C
Republic Airwaysthin sample	14	64.3%	133 min	0.00%	D+

The spread between United and Spirit is 16 points, wide enough that it should decide the booking. On a hundred trips that is 16 more arrivals inside the on-time window.

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 24 points, against 16 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am535 flights90.8%
8am–11am674 flights88.6%
11am–2pm611 flights83.7%
2pm–5pm673 flights67.1%
5pm–8pm743 flights66.8%
8pm–5am501 flights67.9%

Scale runs 60% to 95%. Rows with fewer than 30 flights show a dash.

Book the early departure. Flights leaving between 5am–8am arrive on time 90.8% of the time, against 66.8% for the evening window. That is 24 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 5pm–8pm window is the busiest, with 743 flights scheduled in it.

Across the year

When delays peak on this route

This route's year swings by 18 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.January best, August worst

August is the worst month at 67.0% on time, 18 points below January, the best at 84.7%. 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 342 flights.

Across the week, Tuesday is the strongest day at 81.0% on time and Sunday the weakest at 69.0%. 12 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 67,000 reported delay minutes.

Late-arriving aircraft
34%
Carrier
32%
National Airspace System
27%
Extreme weather
7%
Security
0%

34% 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 67,000 reported delay minutes here, that single category outweighs weather by 5 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 67,232 reported delay minutes.

The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 90.8% on time here.

Cancellations sit at 1.12% against a national 1.81%, which is ordinary. Diversions add another 0.16%.

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. 9.3% 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.

The 964 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 runs 77.8% on time across 3,736 flights, within half a point of this direction. The two legs are scored separately because they usually differ; on this city pair they happen not to.

The sample behind these figures is 3,737 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. January runs 84.7% on time and August 67.0%, a swing of 18 points. If the dates move at all, move them out of August before you think about anything else. 3,689 completed flights over 12 months sit behind that.

Common questions

MIA to IAH questions

How often is MIA to IAH delayed?

Roughly 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 22.8% of the 3,737 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 IAH?

United Airlines leads at 85.9% on time across 1,479 flights. Spirit Airlines is at the other end on 69.7%, a gap of 16 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 IAH?

The 5am–8am departure window is the strongest at 90.8% on time. The 5pm–8pm window is the weakest at 66.8%, 24 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 August delays on MIA to IAH?

August is the weakest month at 67.0% on time, compared with 84.7% in January. Across the year that is 18 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 IAH and IAH 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