Carrier route record
UABOS→SFO
United Airlines · Boston to San Francisco
- Flights
- 1,867
- 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 BOS to SFO lands a B, about a point 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 1,867 flights. The strongest carrier on the route with a full sample, Delta Air Lines, flew 898 at 71.8% on time.
The US average over the same twelve months is 77.6%, so there is little to separate United on BOS to SFO from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.
Key figures
On time
76.3%
Arrivals under 15 minutes late, of 1,843 completed flights
Avg delay
66 min
Across delayed arrivals only, not all flights
Cancelled
1.12%
National rate is 1.81%
Whole route
72.6%
Every carrier on BOS to SFO, 4,676 flights
These figures cover 1,867 flights between July 2025 and June 2026. Beyond the headline rate, 8.9% 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 BOS to SFO
United flew 1,867 of the 4,676 flights on BOS to SFO, about 40% of it. Its 76.3% on time is 4 points better than the route as a whole, which runs 72.6%. Ranked by on-time rate it is number two of five carriers on the route, where the range runs from Southwest at 100.0% to Alaska at 61.6%. Two of those carriers flew fewer than 365 flights here, so treat their positions as rough.
The two carriers here are closely matched: about 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 |
|---|---|---|---|---|---|
| Southwest Airlinesthin sample | 1 | 100.0% | — | 0.00% | A+ |
| United Airlines (this page) | 1,867 | 76.3% | 66 min | 1.12% | B |
| Delta Air Lines | 898 | 71.8% | 63 min | 1.45% | C |
| JetBlue Airways | 1,676 | 70.5% | 65 min | 1.31% | C |
| Alaska Airlinesthin sample | 234 | 61.6% | 66 min | 0.43% | D+ |
No carrier stands out. Delta tops the table at 71.8%, about 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 BOS to SFO
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 25 points, against about a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am414 flights82.5%
- 8am–11am429 flights84.9%
- 11am–2pm310 flights80.9%
- 2pm–5pm277 flights74.2%
- 5pm–8pm346 flights60.1%
- 8pm–5am91 flights59.6%
Scale runs 50% to 90%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 59.6% on time, against 84.9% for the 8am–11am slot. By evening the aircraft flying these rotations have absorbed a full day of small delays and there is no slack left to recover in. The 8am–11am window is the busiest, with 429 flights scheduled in it.
Across the year
United on BOS to SFO through the year
June is the worst month at 55.8% on time, 32 points below August, the best at 88.1%. Summer afternoon convection and peak-season schedules land on the same weeks every year, so this is a pattern you can book around rather than a run of bad luck. June was the busiest month here, with 201 flights.
Across the week, Tuesday is the strongest day at 82.8% on time and Thursday the weakest at 70.5%. 12 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 46% of the 29,000 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.
- 46%National Airspace System
- 29%Late-arriving aircraft
- 24%Carrier
- 1%Extreme weather
46% 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 29,000, against a national NAS share of 21%. The split is drawn from 28,832 reported delay minutes.
The practical consequence: the 8am–11am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 84.9% on time here.
Cancellations sit at 1.12% against a national 1.81%, which is ordinary. Diversions add another 0.16%.
Connections and stage length
What a delay here costs
8.9% 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 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 79.1% on time across 1,866 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 1,867 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 BOS to SFO
Put together: the calendar is the biggest lever here, not the carrier or the hour. August runs 88.1% on time and June 55.8%, a swing of 32 points. If the dates move at all, move them out of June before you think about anything else. 1,843 completed flights over 12 months sit behind that.
Related
Where to go next
- BOS to SFO, every carrier
- United on SFO to BOS, the return leg
- Delta Air Lines on BOS to SFO71.8%
- JetBlue Airways on BOS to SFO70.5%
- Alaska Airlines on BOS to SFO61.6%
- United Airlines network-wide record
- Boston Logan International (BOS) delay statistics
- San Francisco International (SFO) delay statistics
- The best time of day to fly, according to 7.04 million flight records
- Worst months to fly in the US, ranked by the data
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.