Flight number record
UA 1665
United Airlines · San Francisco to Boston
- Route
- SFO → BOS
- Departs
- 23:30
- Flights
- 343
- Period
- July 2025 to June 2026
Historical statistics for July 2025 to June 2026, not live status. For today’s UA 1665, check with United Airlines.
UA 1665 earns a B. More than four flights in five arrive inside the 15-minute window the DOT uses to define an on-time arrival, which puts it 7 points better than the national rate of 77.6%. When something does go wrong the average delay runs roughly 45 minutes. That rests on 343 flights, with a median arrival 12 minutes early.
Set against the rest of the country, UA 1665's record is 7 points better than the US average of 77.6%. That gap is worth roughly 71 fewer late arrivals for every thousand flights.
Key figures
On time
84.7%
Arrivals under 15 minutes late, of 339 completed flights
Avg delay
47 min
Across delayed arrivals only, not all flights
Cancelled
1.17%
National rate is 1.81%
1 hour+ late
4.1%
National rate is 8.1%
These figures cover 343 flights between July 2025 and June 2026. The median arrival is 12 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.
Schedule
When UA 1665 flies
UA 1665 flies SFO to BOS without a single fixed slot. Its most common scheduled departure was 23:30, used on 114 of 343 flights, with 23:25 (77 flights) and 23:20 (69 flights) next. Carriers retime flights through the season, so the rates here blend slightly different positions in the aircraft's day.
- Departs 23:30114 flights33.2%
- Departs 23:2577 flights22.4%
- Departs 23:2069 flights20.1%
Scale runs 0% to 100%. Rows with fewer than 30 flights show a dash.
Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.
A 23:30 departure sits in the 8pm–5am window. Across every US flight scheduled in that window the on-time rate is 70.5%, and UA 1665 runs 14 points better at 84.7%. By that time of day most aircraft have flown several legs already, and whatever went wrong earlier is still in the schedule.
Against the route
UA 1665 next to SFO to BOS
Every carrier on SFO to BOS combined arrived on time 76.9% of the time across 4,674 flights, so UA 1665 is 8 points ahead of it. United flew 1,866 flights on the same city pair under all its flight numbers, including this one, and those ran 79.1% on time. Across its whole network United Airlines managed 79.0%, which this flight beats by 6 points.
| Compared with | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| UA 1665thin sample | 343 | 84.7% | 47 min | 1.17% | B |
| All United Airlines on SFO–BOS | 1,866 | 79.1% | 73 min | 1.02% | B |
| All carriers on SFO–BOS | 4,674 | 76.9% | 74 min | 1.35% | B |
| United Airlines, whole network | 821,041 | 79.0% | 72 min | 1.01% | B |
Across the year
UA 1665 quarter by quarter
UA 1665 did not run the same all year. Its best quarter was Jul–Sep 2025 at 92.4% on time and its weakest Jan–Mar 2026 at 78.9%, a swing of 14 points on roughly 86 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.
- Jul–Sep 202592 flights92.4%
- Oct–Dec 202589 flights82.0%
- Jan–Mar 202675 flights78.9%
- Apr–Jun 202687 flights83.9%
Scale runs 70% to 100%. Rows with fewer than 30 flights show a dash.
By weekday, UA 1665 did best on Tuesday at 91.5% on time and worst on Sunday at 78.0%. Each day rests on about 49 flights, so a gap of 14 points is worth noticing but could still narrow in another year.
Delay causes
Why UA 1665 runs late
One category carries most of the weight here. Late-arriving aircraft accounts for 46% of the 2,457 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.
- 46%Late-arriving aircraft
- 40%Carrier
- 14%National Airspace System
46% 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 2,457 reported delay minutes here, that single category outweighs weather by 46 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 2,457 delay minutes reported on UA 1665.
Cancellations sit at 1.17% against a national 1.81%, which is ordinary. Diversions add another 0.00%.
Connections and routes
What a delay on this flight costs
Badly late arrivals are rare here: only 4.1% land an hour or more behind schedule, against 8.1% nationally. Connections built on this flight are about as safe as US domestic connections get, though a cancelled flight still ignores every layover you give it.
At 2,704 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 same number turns up on five other city pairs in the data, including MSY to SFO (117), ONT to SFO (71) and SEA to SFO (24), but none flew often enough under UA 1665 to report a rate for. Carriers move flight numbers between routes, so the figures on this page apply to SFO to BOS only.
One caution: this is a thin sample. 343 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 UA 1665
For a booking, read UA 1665 as 84.7% on time with 1.17% cancelled over 343 flights. Those are rates from the past twelve months, not a forecast for any particular day.
Related
Where to go next
- SFO to BOS, every carrier and departure
- BOS to SFO, the return direction
- Delta Air Lines 400 on the same route89.1%
- JetBlue Airways 1734 on the same route70.6%
- United Airlines 2647 on the same route81.1%
- JetBlue Airways 734 on the same route78.3%
- United Airlines 2356 on the same route61.6%
- JetBlue Airways 434 on the same route85.1%
- San Francisco International (SFO) delay statistics
- Boston Logan International (BOS) delay statistics
- United Airlines on-time record
- Every United Airlines flight number with a record
- The most and least reliable flight numbers in the US
- Why the first flight of the morning is statistically safest
A flight number here means United Airlines’s own number as it reports to the DOT. How the figures are computed is on the methodology page.
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.