Carrier route record

DLSJU→BOS

Delta Air Lines · San Juan to Boston

Flights: 398
Period: July 2025 to June 2026
Share of route: 20%
Carriers: 4

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

Delta on SJU to BOS lands a B, about a point better than the national rate of 77.6%. About one flight in five arrives 15 minutes late or worse, and those flights are late by a little over an hour on average. That is a workable record, not a spotless one. That rests on 398 flights, with a median arrival 9 minutes early.

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

On time

78.4%

Arrivals under 15 minutes late, of 389 completed flights

Avg delay

70 min

Across delayed arrivals only, not all flights

Cancelled

1.51%

National rate is 1.81%

Whole route

71.8%

Every carrier on SJU to BOS, 2,001 flights

These figures cover 398 flights between July 2025 and June 2026. The median arrival is 9 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 SJU to BOS

Delta flew 398 of the 2,001 flights on SJU to BOS, about 20% of it. Its 78.4% on time is 7 points better than the route as a whole, which runs 71.8%. Ranked by on-time rate it is first of four carriers on the route, where the range runs from Delta at 78.4% to Frontier at 69.7%. 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 SJU to BOS
Airline	Flights	On time	Avg delay	Cancelled	Grade
Delta Air Lines (this page)	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

Flight by flight

Delta flight numbers on SJU to BOS

DL 178014:2580.5% · 324 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.

5am–8am—
8am–11am—
11am–2pm184 flights79.7%
2pm–5pm214 flights77.3%
5pm–8pm—
8pm–5am—

Scale runs 70% to 85%. Rows with fewer than 30 flights show a dash.

The 11am–2pm window is the strongest at 79.7% on time. The full set of windows spans 2 points.

Across the year

Delta on SJU to BOS through the year

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

Delays peak in April, when the on-time rate falls to 64.7%. July is the strongest month at 100.0%. Moving a flexible trip across that boundary buys 35 points. December was the busiest month here, with 43 flights.

Across the week, Friday is the strongest day at 83.0% on time and Wednesday the weakest at 72.2%. 11 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. Late-arriving aircraft accounts for 44% of the 5,841 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.

Late-arriving aircraft
44%
Carrier
39%
National Airspace System
16%

44% 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 5,841 reported delay minutes here, that single category outweighs weather by 44 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 5,841 reported delay minutes.

The largest share of delay minutes, 39%, 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 2,301 of the 5,841 delay minutes reported here.

Cancellations sit at 1.51% against a national 1.81%, which is ordinary. Diversions add another 0.75%.

Connections and stage length

What a delay here costs

5.7% 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 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 76.0% 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 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 78.4% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 389 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