Route reliability record
SFO→BOS
San Francisco to Boston
- Flights
- 4,610
- Period
- June 2025 to May 2026
- Distance
- 2,704 mi
- Carriers
- 5
This route lands a B, about a point better than the national rate of 77.4%. Roughly one flight in five 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 4,610 flights, of which United Airlines flew 1,815 at 78.7% on time.
United Airlines is the most reliable of the three carriers covered here at 78.7% on time across 1,815 flights.
Key figures
On time
78.2%
Arrivals under 15 minutes late, of 4,544 completed flights
Avg delay
73 min
Across delayed arrivals only, not all flights
Cancelled
1.28%
National rate is 1.80%
Best carrier
United
78.7% on time across 1,815 flights
These figures cover 4,610 flights between June 2025 and May 2026. Beyond the headline rate, 8.8% 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: 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 |
|---|---|---|---|---|---|
| United Airlines | 1,815 | 78.7% | 74 min | 0.99% | B+ |
| JetBlue Airways | 1,617 | 78.1% | 73 min | 1.73% | B |
| Delta Air Lines | 883 | 77.9% | 80 min | 1.36% | B |
| Alaska Airlinesthin sample | 294 | 76.8% | 52 min | 0.34% | B+ |
| Southwest Airlinesthin sample | 1 | 0.0% | 49 min | 0.00% | F+ |
No carrier stands out. United tops the table at 78.7%, about a point 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 16 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–8am804 flights87.2%
- 8am–11am851 flights75.6%
- 11am–2pm1,098 flights71.2%
- 2pm–5pm636 flights80.5%
- 5pm–8pm1 flights—
- 8pm–5am1,220 flights79.1%
Scale runs 65% 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 71.2% 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.
Across the year
When delays peak on this route
This route's year swings by 14 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.
Delays peak in October, when the on-time rate falls to 71.1%. August is the strongest month at 84.8%. Moving a flexible trip across that boundary buys 14 points.
Across the week, Saturday is the strongest day at 84.2% on time and Thursday the weakest at 72.9%. 11 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 73,000 reported delay minutes.
- 39%National Airspace System
- 36%Late-arriving aircraft
- 22%Carrier
- 3%Extreme weather
- 0%Security
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 73,000 reported delay minutes here, that single category outweighs weather by 13 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.
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 29,000 minutes out of 73,000, against a national NAS share of 21%.
Cancellations sit at 1.28% against a national 1.80%, which is ordinary. Diversions add another 0.15%.
Connections and stage length
What a delay here actually costs
8.8% of arrivals land an hour or more late, close to the national 8.2%. 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 are the a little over an hour kind, 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 75.3% on time across 4,612 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 4,610 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 October 71.1%, a swing of 14 points. If the dates move at all, move them out of October before you think about anything else.
Common questions
SFO to BOS questions
How often is SFO to BOS delayed?
Roughly one flight in five arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 21.8% of the 4,610 flights in this analysis, against 22.6% 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 78.7% on time across 1,815 flights. Delta Air Lines is at the other end on 77.9%, a gap of about a point. 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 71.2%, 16 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 October delays on SFO to BOS?
October is the weakest month at 71.1% on time, compared with 84.8% in August. Across the year that is 14 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 record78.7%
- JetBlue Airways on-time record78.1%
- Delta Air Lines on-time record77.9%
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.05 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, June 2025 to May 2026. Updated monthly. How this data is collected.