Route reliability record
STL→BOS
St. Louis to Boston
- Flights
- 1,610
- Period
- July 2025 to June 2026
- Distance
- 1,047 mi
- Carriers
- 3
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, 7 points worse than the national rate of 77.6%. The late ones land about an hour behind schedule. That rests on 1,610 STL to BOS flights, of which Envoy Air flew 418 at 75.9% on time.
Best time to fly STL to BOS: a 8am–11am departure, ideally on a Friday. Flights in that window arrived on time 87.3% of the time and Friday flights 75.8%, against 70.3% for the route overall.
The national on-time rate over the same period is 77.6%, so this route runs 7 points behind the country as a whole. Across a thousand flights that is about 73 extra late arrivals.
Key figures
On time
70.3%
Arrivals under 15 minutes late, of 1,586 completed flights
Avg delay
67 min
Across delayed arrivals only, not all flights
Cancelled
1.49%
National rate is 1.81%
Best carrier
Envoy
75.9% on time across 418 flights
These figures cover 1,610 flights between July 2025 and June 2026. Beyond the headline rate, 10.3% 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 STL to BOS
The choice of carrier is not cosmetic here. Envoy Air arrives on time 75.9% of the time; Southwest Airlines manages 66.3%. Sample sizes sit next to every figure, because a two-point gap across 934 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Republic Airwaysthin sample | 258 | 76.0% | 88 min | 3.10% | B− |
| Envoy Air | 418 | 75.9% | 79 min | 1.67% | B |
| Southwest Airlines | 934 | 66.3% | 59 min | 0.96% | C+ |
Envoy leads Southwest by 10 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Best ways to make this trip
The most reliable ways to fly St. Louis to Boston
Every airline on this route, ranked by how often a scheduled flight actually operated and arrived on time, with its best departure time and most punctual flight number. No nearby airport pair beat these.
| Option | Airline | Success | On time | Cancelled | Best departure | Most punctual flight |
|---|---|---|---|---|---|---|
| 1. STL → BOSSt. Louis to Boston | Envoy Air | 74.6% | 75.9% | 1.67% | 8am–11am, Tue | — |
| 2. STL → BOSSt. Louis to Boston | Southwest Airlines | 65.7% | 66.3% | 0.96% | 8am–11am, Tue | — |
Success is the share of scheduled flights that operated and arrived less than 15 minutes late, so a cancelled flight counts against it. “(route)” means the best time comes from every carrier on the route rather than this airline alone.
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 33 points, against 10 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am38 flights86.8%
- 8am–11am446 flights87.3%
- 11am–2pm242 flights71.7%
- 2pm–5pm301 flights58.0%
- 5pm–8pm442 flights64.4%
- 8pm–5am141 flights54.0%
Scale runs 45% to 95%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 54.0% on time, against 87.3% 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 446 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.
Delays peak in October, when the on-time rate falls to 53.7%. August is the strongest month at 84.1%. Moving a flexible trip across that boundary buys 30 points. October was the busiest month here, with 150 flights.
Across the week, Friday is the strongest day at 75.8% on time and Thursday the weakest at 65.4%. 10 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 47% of the 31,000 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.
- 47%Late-arriving aircraft
- 28%National Airspace System
- 20%Carrier
- 3%Extreme weather
- 2%Security
47% 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 31,000 reported delay minutes here, that single category outweighs weather by 17 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 31,322 reported delay minutes.
The practical consequence: the 8am–11am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 87.3% on time here.
Cancellations sit at 1.49% against a national 1.81%, which is ordinary. Diversions add another 0.00%.
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. 10.3% 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 1,047 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 76.5% on time across 1,611 flights, 6 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 6 points of difference.
The sample behind these figures is 1,610 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.
Work out the risk of missing a connection in BOS after this flight
What to do with this
Booking this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. August runs 84.1% on time and October 53.7%, a swing of 30 points. If the dates move at all, move them out of October before you think about anything else. 1,586 completed flights over 12 months sit behind that.
Common questions
STL to BOS questions
How often is STL to BOS delayed?
Roughly one flight in three arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 29.7% of the 1,610 flights in this analysis, against 22.4% nationally. Delayed flights are late by about an hour on average.
Which airline is most reliable on STL to BOS?
Envoy Air leads at 75.9% on time across 418 flights. Southwest Airlines is at the other end on 66.3%, a gap of 10 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 STL to BOS?
The 8am–11am departure window is the strongest at 87.3% on time. The 8pm–5am window is the weakest at 54.0%, 33 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 October delays on STL to BOS?
October is the weakest month at 53.7% on time, compared with 84.1% in August. 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.
Related
Where to go next
- BOS to STL — the return legseparate record
- St. Louis Lambert International (STL) delay statistics
- Boston Logan International (BOS) delay statistics
- Republic Airways on STL to BOS76.0%
- Envoy Air on STL to BOS75.9%
- Southwest Airlines on STL to BOS66.3%
- STL to ATL, a related route85.3%
- DCA to BOS, a related route71.4%
- STL to DEN, a related route74.9%
- Republic Airways on-time record76.0%
- Envoy Air on-time record75.9%
- Southwest Airlines on-time record66.3%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. STL to BOS and BOS to STL are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
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Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.