Carrier route record
DLBOS→SJU
Delta Air Lines · Boston to San Juan
- Flights
- 398
- Period
- September 2025 to August 2026
- Share of route
- 23%
- Carriers
- 4
Historical statistics for September 2025 to August 2026, not live status. For today’s flights, check with Delta Air Lines.
Delta on BOS to SJU sits in the middle of the pack with a C. Close to three flights in four arrive on time by the DOT's definition, 3 points worse than the national rate of 77.3%. The late ones land about an hour behind schedule. That rests on 398 flights, with a median arrival 4 minutes early.
The US average over the same twelve months is 77.3%, so there is little to separate Delta on BOS to SJU from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
74.5%
Arrivals under 15 minutes late, of 392 completed flights
Avg delay
57 min
Across delayed arrivals only, not all flights
Cancelled
1.51%
National rate is 1.89%
Whole route
66.8%
Every carrier on BOS to SJU, 1,717 flights
These figures cover 398 flights between September 2025 and August 2026. The median arrival is 4 minutes early, which is the part an average hides: most flights here are fine, and the on-time rate is decided by a tail of badly late ones. On-time means under 15 minutes late, the DOT's own threshold.
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Carrier by carrier
Delta against every carrier on BOS to SJU
Delta flew 398 of the 1,717 flights on BOS to SJU, about 23% of it. Its 74.5% on time is 8 points better than the route as a whole, which runs 66.8%. Ranked by on-time rate it is first of four carriers on the route, where the range runs from Delta at 74.5% to Frontier at 42.4%. Two of those carriers flew fewer than 365 flights here, so treat their positions as rough.
Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines (this page) | 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 |
Flight by flight
Delta flight numbers on BOS to SJU
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Departure window
When Delta runs best on this route
Departure time is the single lever a traveller controls, and it moves the odds more than most people expect. Each bar below is the on-time rate for flights scheduled to leave inside that window, across 398 flights.
There are not enough flights in each departure window to split Delta's BOS to SJU record by time of day without the percentages moving on noise. The headline rate of 74.5% is the reliable figure here.
Across the year
Delta on BOS to SJU through the year
February is the weak month at 61.3% on time, against 87.1% 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. December was the busiest month here, with 43 flights.
Across the week, Friday is the strongest day at 81.5% on time and Thursday the weakest at 66.7%, a spread of 15 points.
Arrival by arrival
How late Delta on BOS to SJU runs when it is late
The 100 late arrivals split 44 under half an hour, 32 between 30 and 60 minutes, and 24 an hour or more.
- 15+ min early87 flights22.2%
- Within 15 min205 flights52.3%
- 15–29 min late44 flights11.2%
- 30–59 min late32 flights8.2%
- 1–2 hours late14 flights3.6%
- 2+ hours late10 flights2.6%
Scale runs 0% to 100%.
Share of 392 completed flights by arrival against schedule.
16 of 69 late departures made up the time and arrived on time, 23% of them. In the other direction, 47 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at SJU.
The worst days
Worst days for Delta on BOS to SJU
The latest arrival of the period was on 24 February 2026, 6 hours 31 minutes behind schedule; most of its delay minutes were filed as a late-arriving aircraft. The five worst days are spread across February, December, January, September and April, with no single month standing out.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 24 February 2026 | 6 hours 31 minutes | Late-arriving aircraft |
| 26 December 2025 | 5 hours 6 minutes | National Airspace System |
| 5 January 2026 | 4 hours 49 minutes | Carrier |
| 7 September 2025 | 3 hours 49 minutes | Carrier |
| 4 April 2026 | 3 hours 45 minutes | Late-arriving aircraft |
Delay causes
Why Delta runs late on this route
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 5,688 reported delay minutes.
- 41%Carrier
- 36%National Airspace System
- 20%Late-arriving aircraft
- 3%Extreme weather
The largest share of delay minutes, 41%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 2,308 of the 5,688 delay minutes reported here. The split is drawn from 5,688 reported delay minutes.
36% 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 2,054 minutes out of 5,688, against a national NAS share of 21%.
Cancellations sit at 1.51% against a national 1.89%, which is ordinary. Diversions add another 0.00%.
Connections and stage length
What a delay here costs
6.1% of arrivals land an hour or more late, close to the national 8.3%, so a 60 to 75 minute layover is reasonable here while anything under 45 minutes is a bet. Most delays on this route last about an hour, which a normal layover absorbs.
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 77.4% on time across 398 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.
One caution: this is a thin sample. 398 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking Delta on BOS to SJU
Put together: treat all of this as provisional. 398 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 74.5% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 392 completed flights over 12 months sit behind that.
Related
Where to go next
- BOS to SJU, every carrier
- Delta on SJU to BOS, the return leg
- JetBlue Airways on BOS to SJU66.5%
- Spirit Airlines on BOS to SJU52.7%
- Delta Air Lines network-wide record
- Boston Logan International (BOS) delay statistics
- Luis Munoz Marin International (SJU) delay statistics
- Worst months to fly in the US, ranked by the data
- The best day of the week to fly, and why it is the second lever
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.