Route reliability record

DEN→SAN

Denver to San Diego

Flights: 5,949
Period: July 2025 to June 2026
Distance: 853 mi
Carriers: 5

This route sits in the middle of the pack with a B+. Close to three flights in four arrive on time by the DOT's definition, 2 points worse than the national rate of 77.6%. The late ones land about an hour behind schedule. That rests on 5,949 flights, of which Alaska Airlines flew 433 at 82.9% on time.

Alaska Airlines is the most reliable of the four carriers covered here at 82.9% on time across 433 flights.

On time

75.6%

Arrivals under 15 minutes late, of 5,894 completed flights

Avg delay

56 min

Across delayed arrivals only, not all flights

Cancelled

0.66%

National rate is 1.81%

Best carrier

Alaska

82.9% on time across 433 flights

These figures cover 5,949 flights between July 2025 and June 2026. The median arrival is 4 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

Which airline is most reliable on DEN to SAN

The choice of carrier is not cosmetic here. Alaska Airlines arrives on time 82.9% of the time; Southwest Airlines manages 69.5%. Sample sizes sit next to every figure, because a two-point gap across 2,389 flights means less than a ten-point one.

On-time performance by carrier on DEN to SAN
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlines	433	82.9%	56 min	1.15%	B
United Airlines	2,081	79.8%	59 min	0.48%	B+
Frontier Airlines	703	79.6%	63 min	0.85%	B+
SkyWest Airlinesthin sample	343	75.0%	40 min	0.87%	B+
Southwest Airlines	2,389	69.5%	55 min	0.63%	C+

The spread between Alaska and Southwest is 13 points, wide enough that it should decide the booking. On a hundred trips that is 13 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 30 points, against 13 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am978 flights91.7%
8am–11am715 flights83.2%
11am–2pm1,293 flights80.6%
2pm–5pm810 flights75.3%
5pm–8pm992 flights62.2%
8pm–5am1,161 flights63.3%

Scale runs 55% to 95%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 62.2% on time, against 91.7% 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 11am–2pm window is the busiest, with 1,293 flights scheduled in it.

Across the year

When delays peak on this route

This route's year swings by 19 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.February best, June worst

June is the worst month at 65.0% on time, 19 points below February, the best at 84.1%. 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. December was the busiest month here, with 538 flights.

Across the week, Tuesday is the strongest day at 81.0% on time and Thursday the weakest at 69.4%. 12 points is a smaller lever than departure hour, but it is free.

Delay causes

Why this route gets delayed

One category carries most of the weight here. Late-arriving aircraft accounts for 44% of the 81,000 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.

Late-arriving aircraft
44%
Carrier
30%
National Airspace System
25%
Extreme weather
1%
Security
0%

44% 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 81,000 reported delay minutes here, that single category outweighs weather by 35 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 80,731 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 91.7% on time here.

Cancellations are rare at 0.66%, below the national 1.81% and low enough to earn the plus on the grade. Roughly one flight in 100 is called off; the risk here is arriving late, not not arriving.

Connections and stage length

What a delay here actually costs

7.3% 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 are the about an hour kind, which a normal layover absorbs.

The 853 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 79.2% on time across 5,947 flights, 4 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 4 points of difference.

The sample behind these figures is 5,949 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. February runs 84.1% on time and June 65.0%, a swing of 19 points. If the dates move at all, move them out of June before you think about anything else. 5,894 completed flights over 12 months sit behind that.

Common questions

DEN to SAN questions

How often is DEN to SAN 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 24.4% of the 5,949 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.

Which airline is most reliable on DEN to SAN?

Alaska Airlines leads at 82.9% on time across 433 flights. Southwest Airlines is at the other end on 69.5%, a gap of 13 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 DEN to SAN?

The 5am–8am departure window is the strongest at 91.7% on time. The 5pm–8pm window is the weakest at 62.2%, 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 June delays on DEN to SAN?

June is the weakest month at 65.0% on time, compared with 84.1% in February. Across the year that is 19 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. DEN to SAN and SAN to DEN 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