Route reliability record
SJU→BOS
San Juan to Boston
- Flights
- 2,001
- 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, 6 points worse than the national rate of 77.6%. The late ones land a little over an hour behind schedule. That rests on 2,001 SJU to BOS flights, of which Delta Air Lines flew 398 at 78.4% on time.
Best time to fly SJU to BOS: a 5am–8am departure, ideally on a Tuesday. Flights in that window arrived on time 80.5% of the time and Tuesday flights 78.0%, against 71.8% for the route overall.
The national on-time rate over the same period is 77.6%, so this route runs 6 points behind the country as a whole. Across a thousand flights that is about 58 extra late arrivals.
Key figures
On time
71.8%
Arrivals under 15 minutes late, of 1,939 completed flights
Avg delay
72 min
Across delayed arrivals only, not all flights
Cancelled
2.75%
National rate is 1.81%
Best carrier
Delta
78.4% on time across 398 flights
These figures cover 2,001 flights between July 2025 and June 2026. Beyond the headline rate, 10.8% 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 SJU to BOS
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 78.4% of the time; JetBlue Airways manages 69.9%. Sample sizes sit next to every figure, because a two-point gap across 1,415 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 398 | 78.4% | 70 min | 1.51% | B |
| Spirit Airlinesthin sample | 154 | 72.8% | 56 min | 3.90% | C− |
| JetBlue Airways | 1,415 | 69.9% | 75 min | 2.97% | C |
| Frontier Airlinesthin sample | 34 | 69.7% | 45 min | 2.94% | C |
Delta leads JetBlue by 9 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
Flight by flight
Flights on SJU to BOS
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 San Juan 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. SJU → BOSSan Juan to Boston | Delta Air Lines | 77.2% | 78.4% | 1.51% | 11am–2pm, Fri | DL 1780 14:25 |
| 2. SJU → BOSSan Juan to Boston | JetBlue Airways | 67.8% | 69.9% | 2.97% | 8am–11am, Tue | B6 862 13:00 |
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 20 points, against 9 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am315 flights80.5%
- 8am–11am159 flights79.6%
- 11am–2pm511 flights74.1%
- 2pm–5pm590 flights69.4%
- 5pm–8pm252 flights60.6%
- 8pm–5am174 flights66.5%
Scale runs 50% to 85%. Rows with fewer than 30 flights show a dash.
The 5pm–8pm window is where this operation falls apart: 60.6% on time, against 80.5% 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 2pm–5pm window is the busiest, with 590 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 18 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.
January is the weak month at 63.6% on time, against 82.1% 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 235 flights.
Across the week, Tuesday is the strongest day at 78.0% on time and Sunday the weakest at 68.2%. 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 39,000 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
- 32%Carrier
- 19%National Airspace System
- 1%Extreme weather
- 0%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 39,000 reported delay minutes here, that single category outweighs weather by 33 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 39,314 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 80.5% on time here.
Cancellations run at 2.75%, against 1.81% nationally. That is roughly 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. 10.8% 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 69.0% on time across 1,819 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 2,001 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. May runs 82.1% on time and January 63.6%, a swing of 18 points. If the dates move at all, move them out of January before you think about anything else. 1,939 completed flights over 12 months sit behind that.
Common questions
SJU to BOS questions
How often is SJU to BOS delayed?
Roughly one flight in four arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 28.2% of the 2,001 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 SJU to BOS?
Delta Air Lines leads at 78.4% on time across 398 flights. JetBlue Airways is at the other end on 69.9%, a gap of 9 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 SJU to BOS?
The 5am–8am departure window is the strongest at 80.5% on time. The 5pm–8pm window is the weakest at 60.6%, 20 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 January delays on SJU to BOS?
January is the weakest month at 63.6% on time, compared with 82.1% in May. Across the year that is 18 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 SJU — the return legseparate record
- Luis Munoz Marin International (SJU) delay statistics
- Boston Logan International (BOS) delay statistics
- Delta Air Lines on SJU to BOS78.4%
- Spirit Airlines on SJU to BOS72.8%
- JetBlue Airways on SJU to BOS69.9%
- SJU to MCO, a related route78.9%
- DCA to BOS, a related route71.4%
- SJU to JFK, a related route79.3%
- Delta Air Lines on-time record78.4%
- Spirit Airlines on-time record72.8%
- JetBlue Airways on-time record69.9%
- The US routes with the most cancellations
- Why the first flight of the morning is statistically safest
Direction matters. SJU to BOS and BOS to SJU 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.