Route reliability record
SJU→BOS
San Juan to Boston
- Flights
- 1,916
- Period
- September 2025 to August 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, 7 points worse than the national rate of 77.3%. The late ones land a little over an hour behind schedule. That rests on 1,916 SJU to BOS flights, of which Delta Air Lines flew 398 at 77.4% on time.
Best time to fly SJU to BOS: a 5am–8am departure, ideally on a Tuesday. Flights in that window arrived on time 80.1% of the time and Tuesday flights 73.9%, against 70.8% for the route overall.
The national on-time rate over the same period is 77.3%, so this route runs 7 points behind the country as a whole. Across a thousand flights that is about 65 extra late arrivals.
Key figures
On time
70.8%
Arrivals under 15 minutes late, of 1,850 completed flights
Avg delay
74 min
Across delayed arrivals only, not all flights
Cancelled
3.18%
National rate is 1.89%
Best carrier
Delta
77.4% on time across 398 flights
These figures cover 1,916 flights between September 2025 and August 2026. Beyond the headline rate, 11.2% 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 SJU to BOS
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 77.4% of the time; JetBlue Airways manages 68.5%. Sample sizes sit next to every figure, because a two-point gap across 1,342 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 398 | 77.4% | 69 min | 1.51% | B |
| Spirit Airlinesthin sample | 142 | 73.3% | 59 min | 4.23% | C− |
| Frontier Airlinesthin sample | 34 | 69.7% | 45 min | 2.94% | C |
| JetBlue Airways | 1,342 | 68.5% | 77 min | 3.58% | C− |
JetBlue Airways cancels 3.6% of what it schedules here, the highest in the table. A cancelled flight never appears in an on-time rate, so read that column alongside the punctuality one rather than after 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.
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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 | 76.2% | 77.4% | 1.51% | 2pm–5pm, Sat | DL 1780 14:25 |
| 2. SJU → BOSSan Juan to Boston | JetBlue Airways | 66.0% | 68.5% | 3.58% | 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 19 points, against 9 points between the best and worst carrier.
- 5am–8am316 flights80.1%
- 8am–11am112 flights77.5%
- 11am–2pm387 flights72.2%
- 2pm–5pm643 flights70.6%
- 5pm–8pm229 flights61.9%
- 8pm–5am229 flights61.2%
Scale runs 55% to 85%.
The 8pm–5am window is where this operation falls apart: 61.2% on time, against 80.1% 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 2pm–5pm window is the busiest, with 643 flights scheduled in it.
Explore
Filter SJU to BOS 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 SJU to BOS, the best departure window is 5am–8am at 80.1% on time and the worst is 8pm–5am at 61.2%, a wide gap of 18.9 points. All US flights leaving 5am–8am ran 89.3%.
- 5–8am80.1%
- 8–11am77.5%
- 11am–2pm72.2%
- 2–5pm70.6%
- 5–8pm61.9%
- 8pm–5am61.2%
best all US flightsScale 55% to 95%.
Across the year
When delays peak on this route
This route's year swings by 21 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.
August is the worst month at 61.2% on time, 21 points below May, the best at 82.1%. Summer afternoon convection and peak-season schedules land on the same weeks every year, so this is a pattern you can book around rather than a run of bad luck. March was the busiest month here, with 235 flights.
Across the week, Tuesday is the strongest day at 73.9% on time and Sunday the weakest at 67.1%, a spread of 7 points.
Delay causes
Why this route gets delayed
Late-arriving aircraft accounts for 54% of the 40,000 delay minutes carriers reported here, and the remaining four split what is left.
- 54%Late-arriving aircraft
- 28%Carrier
- 17%National Airspace System
- 1%Extreme weather
- 0%Security
54% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 40,000 reported delay minutes here, that single category outweighs weather by 54 to one. The split is drawn from 39,988 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 80.1% on time here.
Cancellations run at 3.18%, against 1.89% nationally. That is about 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
11.2% 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 66.8% on time across 1,717 flights, 4 points below 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 1,916 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 delay figures matter less here than the 3.18% 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. 1,850 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 three arrives at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 29.2% of the 1,916 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 SJU to BOS?
Delta Air Lines leads at 77.4% on time across 398 flights. JetBlue Airways is at the other end on 68.5%, 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 80.1% on time. The 8pm–5am window is the weakest at 61.2%, 19 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 August delays on SJU to BOS?
August is the weakest month at 61.2% on time, compared with 82.1% in May. Across the year that is 21 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 BOS77.4%
- Spirit Airlines on SJU to BOS73.3%
- JetBlue Airways on SJU to BOS68.5%
- SJU to MCO, a related route78.0%
- DCA to BOS, a related route70.1%
- SJU to JFK, a related route77.8%
- Delta Air Lines on-time record77.4%
- Spirit Airlines on-time record73.3%
- Frontier Airlines on-time record69.7%
- 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, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.