Carrier route record
UASFO→BOS
United Airlines · San Francisco to Boston
- Flights
- 1,866
- Period
- July 2025 to June 2026
- Share of route
- 40%
- Carriers
- 5
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with United Airlines.
United on SFO to BOS lands a B, 2 points 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 1,866 flights. The strongest carrier on the route with a full sample, JetBlue Airways, flew 1,674 at 75.4% on time.
The US average over the same twelve months is 77.6%, so there is little to separate United on SFO 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
79.1%
Arrivals under 15 minutes late, of 1,844 completed flights
Avg delay
73 min
Across delayed arrivals only, not all flights
Cancelled
1.02%
National rate is 1.81%
Whole route
76.9%
Every carrier on SFO to BOS, 4,674 flights
These figures cover 1,866 flights between July 2025 and June 2026. Beyond the headline rate, 9.0% of arrivals land a full hour or more behind schedule. That is the number that wrecks a connection, and it is not visible in an on-time percentage alone.
Carrier by carrier
United against every carrier on SFO to BOS
United flew 1,866 of the 4,674 flights on SFO to BOS, about 40% of it. Its 79.1% on time is 2 points better than the route as a whole, which runs 76.9%. Ranked by on-time rate it is first of five carriers on the route, where the range runs from United at 79.1% to Southwest at 0.0%. Two of those carriers flew fewer than 365 flights here, so treat their positions as rough.
The two carriers here are closely matched: half a point separate the best from the worst. At that spread, schedule and price are better tie-breakers than reliability. Sample sizes are shown so you can see which rows are worth weighing.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| United Airlines (this page) | 1,866 | 79.1% | 73 min | 1.02% | B |
| Alaska Airlinesthin sample | 234 | 77.3% | 57 min | 0.43% | B+ |
| JetBlue Airways | 1,674 | 75.4% | 76 min | 1.79% | B |
| Delta Air Lines | 899 | 75.1% | 73 min | 1.45% | B |
| Southwest Airlinesthin sample | 1 | 0.0% | 49 min | 0.00% | F+ |
No carrier stands out. JetBlue tops the table at 75.4%, half a point clear of the bottom, which is inside the range this kind of sample moves year to year.
Flight by flight
United flight numbers on SFO to BOS
Departure window
When United runs best on this route
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 22 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am77 flights93.4%
- 8am–11am561 flights73.8%
- 11am–2pm373 flights71.3%
- 2pm–5pm297 flights83.7%
- 5pm–8pm—
- 8pm–5am558 flights85.0%
Scale runs 65% to 100%. 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 71.3% on time against 93.4% for 5am–8am. 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 8am–11am window is the busiest, with 561 flights scheduled in it.
Across the year
United on SFO to BOS through the year
Delays peak in September, when the on-time rate falls to 69.5%. February is the strongest month at 85.6%. Moving a flexible trip across that boundary buys 16 points. June was the busiest month here, with 201 flights.
Across the week, Saturday is the strongest day at 84.0% on time and Thursday the weakest at 75.1%. 9 points is a smaller lever than departure hour, but it is free.
Delay causes
Why United runs late on this route
One category carries most of the weight here. National Airspace System accounts for 44% of the 28,000 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.
- 44%National Airspace System
- 36%Late-arriving aircraft
- 19%Carrier
- 2%Extreme weather
36% 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 28,000 reported delay minutes here, that single category outweighs weather by 22 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 28,322 reported delay minutes.
44% 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 13,000 minutes out of 28,000, against a national NAS share of 21%.
Cancellations sit at 1.02% against a national 1.81%, which is ordinary. Diversions add another 0.16%.
Connections and stage length
What a delay here costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 9.0% of arrivals land a full hour or more behind schedule, against 8.1% nationally. A 45-minute layover does not survive that, and no amount of walking quickly changes it. Ninety minutes is the honest minimum here, and two hours if the onward flight is on a separate ticket.
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 return leg runs 76.3% on time across 1,867 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.
The sample behind these figures is 1,866 flights, enough that the headline rate is stable to within roughly a point. The hour-block and month splits divide that further, so read the thinner bars with more caution than the headline.
What to do with this
Booking United on SFO to BOS
Put together: the calendar is the biggest lever here, not the carrier or the hour. February runs 85.6% on time and September 69.5%, a swing of 16 points. If the dates move at all, move them out of September before you think about anything else. 1,844 completed flights over 12 months sit behind that.
Related
Where to go next
- SFO to BOS, every carrier
- United on BOS to SFO, the return leg
- Alaska Airlines on SFO to BOS77.3%
- JetBlue Airways on SFO to BOS75.4%
- Delta Air Lines on SFO to BOS75.1%
- United Airlines network-wide record
- San Francisco International (SFO) delay statistics
- Boston Logan International (BOS) delay statistics
- Why the first flight of the morning is statistically safest
- How often a connection fails at the major US hubs
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.