Carrier route record
DLSJU→BOS
Delta Air Lines · San Juan to Boston
- Flights
- 398
- Period
- September 2025 to August 2026
- Share of route
- 21%
- Carriers
- 4
Historical statistics for September 2025 to August 2026, not live status. For today’s flights, check with Delta Air Lines.
Delta on SJU to BOS lands a B, level with the national rate of 77.3%. Roughly one flight in four arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That rests on 398 flights, with a median arrival 8 minutes early.
The US average over the same twelve months is 77.3%, so there is little to separate Delta on SJU to BOS from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
77.4%
Arrivals under 15 minutes late, of 390 completed flights
Avg delay
69 min
Across delayed arrivals only, not all flights
Cancelled
1.51%
National rate is 1.89%
Whole route
70.8%
Every carrier on SJU to BOS, 1,916 flights
These figures cover 398 flights between September 2025 and August 2026. The median arrival is 8 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 SJU to BOS
Delta flew 398 of the 1,916 flights on SJU to BOS, about 21% of it. Its 77.4% on time is 7 points better than the route as a whole, which runs 70.8%. Ranked by on-time rate it is first of four carriers on the route, where the range runs from Delta at 77.4% to JetBlue at 68.5%. 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 | 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− |
Flight by flight
Delta flight numbers on SJU to BOS
Timetable by Aviasales, a Travelpayouts partner. Booking through it may earn this site a commission at no cost to you; it changes no figure here.
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.
- 5am–8am—
- 8am–11am—
- 11am–2pm122 flights73.8%
- 2pm–5pm276 flights79.1%
- 5pm–8pm—
- 8pm–5am—
Scale runs 65% to 85%. Rows with fewer than 30 flights show a dash.
The weak window here is the middle of the day, not the evening: 11am–2pm runs 73.8% on time against 79.1% for 2pm–5pm. That shape usually means a departure bank rather than an accumulating delay, so the recovery later in the afternoon is real rather than noise. The 2pm–5pm window is the busiest, with 276 flights scheduled in it.
Across the year
Delta on SJU to BOS through the year
Delays peak in April, when the on-time rate falls to 64.7%. July is the strongest month at 86.7%. Moving a flexible trip across that boundary buys 22 points. December was the busiest month here, with 43 flights.
Across the week, Saturday is the strongest day at 83.8% on time and Sunday the weakest at 68.5%, a spread of 15 points.
Arrival by arrival
How late Delta on SJU to BOS runs when it is late
Delta on SJU to BOS often beats the schedule: 138 of 390 completed flights, 35%, arrived 15 minutes or more early. The 88 late arrivals split 37 under half an hour, 27 between 30 and 60 minutes, and 24 an hour or more.
- 15+ min early138 flights35.4%
- Within 15 min164 flights42.1%
- 15–29 min late37 flights9.5%
- 30–59 min late27 flights6.9%
- 1–2 hours late14 flights3.6%
- 2+ hours late10 flights2.6%
Scale runs 0% to 100%.
Share of 390 completed flights by arrival against schedule.
14 of 80 late departures made up the time and arrived on time, 18% of them. In the other direction, 22 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 BOS.
The worst days
Worst days for Delta on SJU to BOS
The latest arrival of the period was on 5 January 2026, 15 hours 48 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The same cause led on three of the five worst days.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 5 January 2026 | 15 hours 48 minutes | Carrier |
| 30 January 2026 | 7 hours 2 minutes | Carrier |
| 24 February 2026 | 5 hours 56 minutes | Late-arriving aircraft |
| 27 December 2025 | 3 hours 37 minutes | Carrier |
| 7 September 2025 | 3 hours 24 minutes | Late-arriving aircraft |
Delay causes
Why Delta runs late on this route
Late-arriving aircraft accounts for 46% of the 6,083 delay minutes carriers reported here, and the remaining four split what is left.
- 46%Late-arriving aircraft
- 39%Carrier
- 15%National Airspace System
46% of delay minutes trace back to a late-arriving aircraft, meaning the plane was already behind before it got here. Across 6,083 reported delay minutes here, that single category outweighs weather by 46 to one. The split is drawn from 6,083 reported delay minutes.
The largest share of delay minutes, 39%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. It accounts for 2,348 of the 6,083 delay minutes reported here.
Cancellations sit at 1.51% against a national 1.89%, which is ordinary. Diversions add another 0.50%.
Connections and stage length
What a delay here costs
6.2% 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 a little over 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 74.5% 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 SJU to BOS
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 77.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 390 completed flights over 12 months sit behind that.
Related
Where to go next
- SJU to BOS, every carrier
- Delta on BOS to SJU, the return leg
- Spirit Airlines on SJU to BOS73.3%
- JetBlue Airways on SJU to BOS68.5%
- Delta Air Lines network-wide record
- Luis Munoz Marin International (SJU) delay statistics
- Boston Logan International (BOS) delay statistics
- Why the first flight of the morning is statistically safest
- Worst months to fly in the US, ranked by the data
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.