Route reliability record
SFO→BOS
San Francisco to Boston
- Flights
- 4,674
- Period
- July 2025 to June 2026
- Distance
- 2,704 mi
- Carriers
- 5
This route lands a B, level with the national rate of 77.6%. Roughly one flight in four arrive 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 4,674 flights, of which United Airlines flew 1,866 at 79.1% on time.
United Airlines is the most reliable of the three carriers covered here at 79.1% on time across 1,866 flights.
Key figures
On time
76.9%
Arrivals under 15 minutes late, of 4,604 completed flights
Avg delay
74 min
Across delayed arrivals only, not all flights
Cancelled
1.35%
National rate is 1.81%
Best carrier
United
79.1% on time across 1,866 flights
These figures cover 4,674 flights between July 2025 and June 2026. Beyond the headline rate, 9.3% 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
Which airline is most reliable on SFO to BOS
The three carriers here are closely matched: 4 points 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 | 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. United tops the table at 79.1%, 4 points clear of the bottom, which is inside the range this kind of sample moves year to year.
Departure window
The best time of day to leave
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 17 points, against 4 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am804 flights87.2%
- 8am–11am864 flights74.3%
- 11am–2pm1,115 flights69.9%
- 2pm–5pm681 flights78.1%
- 5pm–8pm1 flights—
- 8pm–5am1,209 flights77.6%
Scale runs 60% to 95%. 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 69.9% on time against 87.2% 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 8pm–5am window is the busiest, with 1,209 flights scheduled in it.
Across the year
When delays peak on this route
This route's year swings by 16 points from best month to worst, which is wide. An annual average tells you very little about the week you are actually flying, so read the strip rather than the headline if your dates are fixed.
June is the worst month at 68.9% on time, 16 points below August, the best at 84.8%. 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 502 flights.
Across the week, Saturday is the strongest day at 84.2% on time and Thursday the weakest at 70.6%. 14 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
Carriers report a reason for every delay minute they cause, in five DOT categories. The split below is by minutes, not by number of flights, so a handful of very long weather delays weighs as much as many short ones. It covers 78,000 reported delay minutes.
- 39%National Airspace System
- 37%Late-arriving aircraft
- 22%Carrier
- 3%Extreme weather
37% 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 78,000 reported delay minutes here, that single category outweighs weather by 14 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 78,166 reported delay minutes.
39% 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 30,000 minutes out of 78,000, against a national NAS share of 21%.
Cancellations sit at 1.35% against a national 1.81%, which is ordinary. Diversions add another 0.15%.
Connections and stage length
What a delay here actually costs
For anyone connecting, the number to weigh is not the on-time rate but the tail behind it. 9.3% 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 is a different operation and performs like one: it runs 72.6% on time across 4,676 flights, 4 points below this direction. Departure banks, aircraft rotations and the congestion at each end all differ, so scoring the two directions together would average away a real 4 points of difference.
The sample behind these figures is 4,674 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 this route
Put together: the calendar is the biggest lever here, not the carrier or the hour. August runs 84.8% on time and June 68.9%, a swing of 16 points. If the dates move at all, move them out of June before you think about anything else. 4,604 completed flights over 12 months sit behind that.
Common questions
SFO to BOS questions
How often is SFO to BOS delayed?
Roughly one flight in four arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 23.1% of the 4,674 flights in this analysis, against 22.4% nationally. Delayed flights are late by a little over an hour on average.
Which airline is most reliable on SFO to BOS?
United Airlines leads at 79.1% on time across 1,866 flights. Delta Air Lines is at the other end on 75.1%, a gap of 4 points. Sample size matters as much as the percentage: a rate built from a few hundred flights moves around far more than one built from ten thousand.
What time of day has the fewest delays on SFO to BOS?
The 5am–8am departure window is the strongest at 87.2% on time. The 11am–2pm window is the weakest at 69.9%, 17 points behind. Later departures inherit delays from earlier flights the same aircraft has already flown, which is why the curve slopes the way it does.
How bad are June delays on SFO to BOS?
June is the weakest month at 68.9% on time, compared with 84.8% in August. Across the year that is 16 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.
Related
Where to go next
- BOS to SFO — the return legseparate record
- San Francisco International (SFO) delay statistics
- Boston Logan International (BOS) delay statistics
- United Airlines on-time record79.1%
- Alaska Airlines on-time record77.3%
- JetBlue Airways on-time record75.4%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
Direction matters. SFO to BOS and BOS to SFO are scored separately because they are separate operations, with different departure banks and different aircraft. Compare the return leg.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, July 2025 to June 2026. Updated monthly. How this data is collected.