Carrier route record

DLBOS→SJU

Delta Air Lines · Boston to San Juan

Flights: 398
Period: July 2025 to June 2026
Share of route: 22%
Carriers: 4

Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Delta Air Lines.

Delta on BOS to SJU lands a B, 2 points worse than the national rate of 77.6%. About one flight in four arrives 15 minutes late or worse, and those flights are late by about an hour on average. That is a workable record, not a spotless one. That rests on 398 flights, with a median arrival 3 minutes early.

The US average over the same twelve months is 77.6%, 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.

On time

76.0%

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 1.81%

Whole route

69.0%

Every carrier on BOS to SJU, 1,819 flights

These figures cover 398 flights between July 2025 and June 2026. The median arrival is 3 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.

Carrier by carrier

Delta against every carrier on BOS to SJU

Delta flew 398 of the 1,819 flights on BOS to SJU, about 22% of it. Its 76.0% on time is 7 points better than the route as a whole, which runs 69.0%. Ranked by on-time rate it is first of four carriers on the route, where the range runs from Delta at 76.0% 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.

On-time performance by carrier on BOS to SJU
Airline	Flights	On time	Avg delay	Cancelled	Grade
Delta Air Lines (this page)	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

Flight by flight

Delta flight numbers on BOS to SJU

DL 178008:1078.1% · 365 flights

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 76.0% is the reliable figure here.

Across the year

Delta on BOS to SJU through the year

Darker means more delays. Figures are on-time percentages.July best, February worst

February is the weak month at 61.3% on time, against 96.8% 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. 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 72.2%. 9 points is a smaller lever than departure hour, but it is free.

Delay causes

Why Delta runs late on this route

One category carries most of the weight here. Carrier accounts for 42% of the 5,346 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.

Carrier
42%
National Airspace System
37%
Late-arriving aircraft
17%
Extreme weather
3%

The largest share of delay minutes, 42%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 2,264 of the 5,346 delay minutes reported here. The split is drawn from 5,346 reported delay minutes.

37% 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 1,993 minutes out of 5,346, against a national NAS share of 21%.

Cancellations sit at 1.51% against a national 1.81%, which is ordinary. Diversions add another 0.00%.

Connections and stage length

What a delay here costs

5.4% of arrivals land an hour or more late, close to the national 8.1%. That is the figure that decides a connection, and it says 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 78.4% on time across 398 flights, within 2 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 76.0% 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.

Where to go next

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.

Key figures