Route reliability record

PHL→DEN

Philadelphia to Denver

Flights: 3,005
Period: July 2025 to June 2026
Distance: 1,558 mi
Carriers: 4

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 3,005 flights, of which United Airlines flew 953 at 83.9% on time.

United Airlines is the most reliable of the three carriers covered here at 83.9% on time across 953 flights.

On time

74.9%

Arrivals under 15 minutes late, of 2,957 completed flights

Avg delay

74 min

Across delayed arrivals only, not all flights

Cancelled

1.13%

National rate is 1.81%

Best carrier

United

83.9% on time across 953 flights

These figures cover 3,005 flights between July 2025 and June 2026. Beyond the headline rate, 8.6% 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 PHL to DEN

The choice of carrier is not cosmetic here. United Airlines arrives on time 83.9% of the time; Southwest Airlines manages 69.0%. Sample sizes sit next to every figure, because a two-point gap across 879 flights means less than a ten-point one.

On-time performance by carrier on PHL to DEN
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	953	83.9%	92 min	0.84%	B+
American Airlines	877	73.3%	66 min	1.60%	C
Southwest Airlines	879	69.0%	57 min	0.80%	C+
Frontier Airlinesthin sample	296	68.4%	112 min	1.69%	C

The spread between United and Southwest is 15 points, wide enough that it should decide the booking. On a hundred trips that is 15 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 15 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am471 flights84.2%
8am–11am856 flights86.0%
11am–2pm391 flights72.8%
2pm–5pm508 flights73.3%
5pm–8pm512 flights55.8%
8pm–5am267 flights65.1%

Scale runs 45% to 90%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 55.8% on time, against 86.0% for the 8am–11am 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 856 flights scheduled in it.

Across the year

When delays peak on this route

This route's year swings by 30 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.November best, December worst

December is the weak month at 55.6% on time, against 85.7% in November. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. July was the busiest month here, with 317 flights.

Across the week, Wednesday is the strongest day at 78.1% on time and Thursday the weakest at 72.2%. 6 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 55,000 reported delay minutes.

Carrier
37%
Late-arriving aircraft
34%
National Airspace System
23%
Extreme weather
5%
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 55,000 reported delay minutes here, that single category outweighs weather by 6 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 54,649 reported delay minutes.

The largest share of delay minutes, 37%, 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 20,000 of the 55,000 delay minutes reported here.

Cancellations sit at 1.13% against a national 1.81%, which is ordinary. Diversions add another 0.47%.

Connections and stage length

What a delay here actually costs

8.6% 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 are the a little over an hour kind, which a normal layover absorbs.

At 1,558 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 runs 73.1% on time across 3,003 flights, within 2 points 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 3,005 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. November runs 85.7% on time and December 55.6%, a swing of 30 points. If the dates move at all, move them out of December before you think about anything else. 2,957 completed flights over 12 months sit behind that.

Common questions

PHL to DEN questions

How often is PHL to DEN 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 25.1% of the 3,005 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 PHL to DEN?

United Airlines leads at 83.9% on time across 953 flights. Southwest Airlines is at the other end on 69.0%, a gap of 15 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 PHL to DEN?

The 8am–11am departure window is the strongest at 86.0% on time. The 5pm–8pm window is the weakest at 55.8%, 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 December delays on PHL to DEN?

December is the weakest month at 55.6% on time, compared with 85.7% in November. Across the year that is 30 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. PHL to DEN and DEN to PHL 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