Route reliability record
BOS→SJU
Boston to San Juan
- Flights
- 1,819
- Period
- July 2025 to June 2026
- Distance
- 1,674 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, 9 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 1,819 BOS to SJU flights, of which Delta Air Lines flew 398 at 76.0% on time.
Best time to fly BOS to SJU: a 5am–8am departure, ideally on a Wednesday. Flights in that window arrived on time 78.4% of the time and Wednesday flights 73.1%, against 69.0% for the route overall.
The national on-time rate over the same period is 77.6%, so this route runs 9 points behind the country as a whole. Across a thousand flights that is about 86 extra late arrivals.
Key figures
On time
69.0%
Arrivals under 15 minutes late, of 1,767 completed flights
Avg delay
74 min
Across delayed arrivals only, not all flights
Cancelled
2.69%
National rate is 1.81%
Best carrier
Delta
76.0% on time across 398 flights
These figures cover 1,819 flights between July 2025 and June 2026. Beyond the headline rate, 11.8% 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 BOS to SJU
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 76.0% of the time; JetBlue Airways manages 69.2%. Sample sizes sit next to every figure, because a two-point gap across 1,232 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 398 | 76.0% | 57 min | 1.51% | B |
| JetBlue Airways | 1,232 | 69.2% | 71 min | 2.44% | C |
| Spirit Airlinesthin sample | 155 | 54.5% | 105 min | 7.74% | F− |
| Frontier Airlinesthin sample | 34 | 42.4% | 113 min | 2.94% | F |
Delta leads JetBlue by 7 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.
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 | 74.9% | 76.0% | 1.51% | 8am–11am, Fri | DL 1780 08:10 |
| 2. BOS → SJUBoston to San Juan | JetBlue Airways | 67.5% | 69.2% | 2.44% | 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 25 points, against 7 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am194 flights78.4%
- 8am–11am715 flights76.6%
- 11am–2pm312 flights69.6%
- 2pm–5pm171 flights56.2%
- 5pm–8pm228 flights59.1%
- 8pm–5am199 flights53.9%
Scale runs 45% to 85%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 53.9% on time, against 78.4% 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 715 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 24 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 80.0% in July. 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, Wednesday is the strongest day at 73.1% on time and Sunday the weakest at 65.0%. 8 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 40,000 reported delay minutes.
- 35%National Airspace System
- 33%Late-arriving aircraft
- 29%Carrier
- 3%Extreme weather
- 0%Security
35% 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 40,000, against a national NAS share of 21%. The split is drawn from 40,471 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 78.4% on time here.
Cancellations run at 2.69%, against 1.81% nationally. That is about one flight in 40 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
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 11.8% 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 1,674 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 runs 71.8% on time across 2,001 flights, within 3 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 1,819 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 calendar is the biggest lever here, not the carrier or the hour. July runs 80.0% on time and February 55.8%, a swing of 24 points. If the dates move at all, move them out of February before you think about anything else. 1,767 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 31.0% of the 1,819 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 BOS to SJU?
Delta Air Lines leads at 76.0% on time across 398 flights. JetBlue Airways is at the other end on 69.2%, a gap of 7 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 78.4% on time. The 8pm–5am window is the weakest at 53.9%, 25 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 80.0% in July. Across the year that is 24 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 SJU76.0%
- JetBlue Airways on BOS to SJU69.2%
- Spirit Airlines on BOS to SJU54.5%
- BOS to DCA, a related route74.0%
- MCO to SJU, a related route73.0%
- BOS to ORD, a related route72.7%
- Delta Air Lines on-time record76.0%
- JetBlue Airways on-time record69.2%
- Spirit Airlines on-time record54.5%
- 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, July 2025 to June 2026. Last updated 15 September 2026; recomputed monthly. How this data is collected.