Route reliability record

DEN→BOS

Denver to Boston

Flights: 3,903
Period: July 2025 to June 2026
Distance: 1,754 mi
Carriers: 4

This route sits in the middle of the pack with a C. About seven flights in ten arrive on time by the DOT's definition, 6 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 3,903 flights, of which United Airlines flew 1,911 at 77.1% on time.

United Airlines is the most reliable of the four carriers covered here at 77.1% on time across 1,911 flights.

On time

71.4%

Arrivals under 15 minutes late, of 3,852 completed flights

Avg delay

70 min

Across delayed arrivals only, not all flights

Cancelled

1.18%

National rate is 1.81%

Best carrier

United

77.1% on time across 1,911 flights

These figures cover 3,903 flights between July 2025 and June 2026. Beyond the headline rate, 11.7% 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 DEN to BOS

The choice of carrier is not cosmetic here. United Airlines arrives on time 77.1% of the time; Southwest Airlines manages 59.7%. Sample sizes sit next to every figure, because a two-point gap across 910 flights means less than a ten-point one.

On-time performance by carrier on DEN to BOS
Airline	Flights	On time	Avg delay	Cancelled	Grade
United Airlines	1,911	77.1%	70 min	0.84%	B+
Delta Air Lines	400	75.8%	56 min	1.50%	B
JetBlue Airways	682	68.6%	82 min	2.05%	C
Southwest Airlines	910	59.7%	66 min	1.10%	D

The spread between United and Southwest is 17 points, wide enough that it should decide the booking. On a hundred trips that is 17 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 35 points, against 17 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am129 flights96.1%
8am–11am1,014 flights76.3%
11am–2pm698 flights70.4%
2pm–5pm822 flights65.0%
5pm–8pm696 flights61.1%
8pm–5am544 flights80.6%

Scale runs 55% to 100%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 61.1% on time, against 96.1% 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 1,014 flights scheduled in it.

Across the year

When delays peak on this route

This route's year swings by 34 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.December best, September worst

Delays peak in September, when the on-time rate falls to 51.4%. December is the strongest month at 85.7%. Moving a flexible trip across that boundary buys 34 points. July was the busiest month here, with 427 flights.

Across the week, Saturday is the strongest day at 83.3% on time and Monday the weakest at 68.0%. 15 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. National Airspace System accounts for 43% of the 77,000 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.

National Airspace System
43%
Late-arriving aircraft
37%
Carrier
19%
Extreme weather
1%
Security
0%

37% 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 77,000 reported delay minutes here, that single category outweighs weather by 37 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 76,533 reported delay minutes.

43% 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 33,000 minutes out of 77,000, against a national NAS share of 21%.

Cancellations sit at 1.18% against a national 1.81%, which is ordinary. Diversions add another 0.13%.

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. 11.7% 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.

At 1,754 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 76.5% on time across 3,899 flights, 5 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 5 points of difference.

The sample behind these figures is 3,903 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. December runs 85.7% on time and September 51.4%, a swing of 34 points. If the dates move at all, move them out of September before you think about anything else. 3,852 completed flights over 12 months sit behind that.

Common questions

DEN to BOS questions

How often is DEN to BOS delayed?

Roughly one flight in three arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 28.6% of the 3,903 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 DEN to BOS?

United Airlines leads at 77.1% on time across 1,911 flights. Southwest Airlines is at the other end on 59.7%, a gap of 17 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 BOS?

The 5am–8am departure window is the strongest at 96.1% on time. The 5pm–8pm window is the weakest at 61.1%, 35 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 September delays on DEN to BOS?

September is the weakest month at 51.4% on time, compared with 85.7% in December. Across the year that is 34 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 BOS and BOS 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