Route reliability record
BOS→SJU
Boston to San Juan
- Flights
- 1,717
- Period
- September 2025 to August 2026
- Distance
- 1,674 mi
- Carriers
- 4
This route draws a C. About one flight in three arrives at least 15 minutes late, and the typical late arrival is a little over an hour behind schedule. Build slack into anything you connect to. That rests on 1,717 BOS to SJU flights, of which Delta Air Lines flew 398 at 74.5% on time.
Best time to fly BOS to SJU: a 5am–8am departure, ideally on a Tuesday. Flights in that window arrived on time 80.8% of the time and Tuesday flights 71.2%, against 66.8% for the route overall.
The national on-time rate over the same period is 77.3%, so this route runs 11 points behind the country as a whole. Across a thousand flights that is about 105 extra late arrivals.
Key figures
On time
66.8%
Arrivals under 15 minutes late, of 1,664 completed flights
Avg delay
76 min
Across delayed arrivals only, not all flights
Cancelled
2.97%
National rate is 1.89%
Best carrier
Delta
74.5% on time across 398 flights
These figures cover 1,717 flights between September 2025 and August 2026. Beyond the headline rate, 12.9% of arrivals land a full hour or more behind schedule, against 8.3% nationally.
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Carrier by carrier
Which airline is most reliable on BOS to SJU
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 74.5% of the time; JetBlue Airways manages 66.5%. Sample sizes sit next to every figure, because a two-point gap across 1,142 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 398 | 74.5% | 57 min | 1.51% | C |
| JetBlue Airways | 1,142 | 66.5% | 73 min | 2.80% | C |
| Spirit Airlinesthin sample | 143 | 52.7% | 108 min | 8.39% | F− |
| Frontier Airlinesthin sample | 34 | 42.4% | 113 min | 2.94% | F |
Delta leads JetBlue by 8 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Flight by flight
Flights on BOS to SJU
The flight numbers below flew this route at least 250 times in the period, busiest first. Each has its own record, with its schedule and delay causes.
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Best ways to make this trip
The most reliable ways to fly Boston to San Juan
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. BOS → SJUBoston to San Juan | Delta Air Lines | 73.4% | 74.5% | 1.51% | 8am–11am, Fri | DL 1780 08:05 |
| 2. BOS → SJUBoston to San Juan | JetBlue Airways | 64.6% | 66.5% | 2.80% | 5am–8am, Tue | B6 861 08:00 |
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 36 points, against 8 points between the best and worst carrier.
- 5am–8am247 flights80.8%
- 8am–11am670 flights72.6%
- 11am–2pm213 flights68.6%
- 2pm–5pm161 flights55.6%
- 5pm–8pm238 flights59.0%
- 8pm–5am188 flights45.0%
Scale runs 35% to 85%.
The 8pm–5am window is where this operation falls apart: 45.0% on time, against 80.8% 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 670 flights scheduled in it.
Explore
Filter BOS to SJU by airline, time, month and day
Pick a breakdown and a carrier. The dashed mark on each row is the same slice for every US flight.
On BOS to SJU, the best departure window is 5am–8am at 80.8% on time and the worst is 8pm–5am at 45.0%, a wide gap of 35.8 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am80.8%
- 8–11am72.6%
- 11am–2pm68.6%
- 2–5pm55.6%
- 5–8pm59.0%
- 8pm–5am45.0%
best all US flightsScale 35% to 95%.
Across the year
When delays peak on this route
This route's year swings by 23 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.
February is the weak month at 55.8% on time, against 78.6% in May. 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. March was the busiest month here, with 204 flights.
Across the week, Tuesday is the strongest day at 71.2% on time and Thursday the weakest at 62.7%, a spread of 9 points.
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 42,000 reported delay minutes.
- 34%Late-arriving aircraft
- 33%National Airspace System
- 30%Carrier
- 2%Extreme weather
- 0%Security
34% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 42,000 reported delay minutes here, that single category outweighs weather by 15 to one. The split is drawn from 41,665 reported delay minutes.
33% 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 14,000 minutes out of 42,000, against a national NAS share of 21%.
Cancellations run at 2.97%, against 1.89% nationally. That is roughly one flight in 30 cancelled outright. A cancelled flight is invisible in every on-time figure on this page, so the punctuality numbers above describe only the flights that actually operated.
Connections and stage length
What a delay here actually costs
12.9% of arrivals land a full hour or more behind schedule, against 8.3% nationally. Ninety minutes is the honest minimum for a connection here, and two hours if the onward flight is on a separate ticket.
At 1,674 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 70.8% on time across 1,916 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 1,717 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 SJU after this flight
What to do with this
Booking this route
Put together: the delay figures matter less here than the 2.97% cancellation rate. A late arrival costs you a little over an hour; a cancelled flight costs you the day. Book the 5am–8am departure so there is still a schedule behind you if it goes wrong, and treat a same-day connection as optimistic. 1,664 completed flights over 12 months sit behind that.
Common questions
BOS to SJU questions
How often is BOS to SJU delayed?
About 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 33.2% of the 1,717 flights in this analysis, against 22.7% nationally. Delayed flights are late by a little over an hour on average.
Which airline is most reliable on BOS to SJU?
Delta Air Lines leads at 74.5% on time across 398 flights. JetBlue Airways is at the other end on 66.5%, a gap of 8 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 BOS to SJU?
The 5am–8am departure window is the strongest at 80.8% on time. The 8pm–5am window is the weakest at 45.0%, 36 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 February delays on BOS to SJU?
February is the weakest month at 55.8% on time, compared with 78.6% in May. Across the year that is 23 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
- SJU to BOS — the return legseparate record
- Boston Logan International (BOS) delay statistics
- Luis Munoz Marin International (SJU) delay statistics
- Delta Air Lines on BOS to SJU74.5%
- JetBlue Airways on BOS to SJU66.5%
- Spirit Airlines on BOS to SJU52.7%
- BOS to DCA, a related route72.6%
- MCO to SJU, a related route73.8%
- BOS to ORD, a related route71.9%
- Delta Air Lines on-time record74.5%
- JetBlue Airways on-time record66.5%
- Spirit Airlines on-time record52.7%
- The US routes with the most cancellations
- Why the first flight of the morning is statistically safest
Direction matters. BOS to SJU and SJU to BOS 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, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.