Route reliability record
SEA→SLC
Seattle to Salt Lake City
- Flights
- 4,107
- Period
- July 2025 to June 2026
- Distance
- 689 mi
- Carriers
- 4
This route lands a B+, 6 points better than the national rate of 77.6%. About one flight in six arrive 15 minutes late or worse, and those flights are late by about 50 minutes on average. That is a workable record, not a spotless one. That rests on 4,107 flights, of which Delta Air Lines flew 2,219 at 87.0% on time.
Delta Air Lines is the most reliable of the three carriers covered here at 87.0% on time across 2,219 flights.
Key figures
On time
83.6%
Arrivals under 15 minutes late, of 4,065 completed flights
Avg delay
50 min
Across delayed arrivals only, not all flights
Cancelled
0.93%
National rate is 1.81%
Best carrier
Delta
87.0% on time across 2,219 flights
These figures cover 4,107 flights between July 2025 and June 2026. The median arrival is 7 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.
Carrier by carrier
Which airline is most reliable on SEA to SLC
The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 87.0% of the time; Alaska Airlines manages 79.6%. Sample sizes sit next to every figure, because a two-point gap across 1,372 flights means less than a ten-point one.
| Airline | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| Delta Air Lines | 2,219 | 87.0% | 58 min | 0.54% | A+ |
| SkyWest Airlines | 418 | 81.3% | 49 min | 0.00% | B+ |
| Alaska Airlines | 1,372 | 79.6% | 41 min | 1.68% | B |
| Frontier Airlinesthin sample | 98 | 72.6% | 55 min | 3.06% | C− |
Delta leads Alaska by 7 points. Real, but small enough that a better departure time on the weaker carrier can outweigh it.
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 15 points, against 7 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am732 flights94.1%
- 8am–11am619 flights87.5%
- 11am–2pm1,063 flights79.0%
- 2pm–5pm449 flights81.0%
- 5pm–8pm949 flights80.5%
- 8pm–5am295 flights79.9%
Scale runs 70% 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 79.0% on time against 94.1% 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 11am–2pm window is the busiest, with 1,063 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.
December is the weak month at 76.6% on time, against 92.4% in April. Winter delays cluster: a single storm system can account for most of a month's shortfall, so an average December hides both very good weeks and genuinely unflyable days. August was the busiest month here, with 408 flights.
Across the week, Wednesday is the strongest day at 85.9% on time and Thursday the weakest at 81.1%. 5 points is a smaller lever than departure hour, but it is free.
Delay causes
Why this route gets delayed
One category carries most of the weight here. Carrier accounts for 48% of the 33,000 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.
- 48%Carrier
- 39%Late-arriving aircraft
- 11%National Airspace System
- 2%Extreme weather
- 0%Security
39% 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 33,000 reported delay minutes here, that single category outweighs weather by 23 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 33,289 reported delay minutes.
The largest share of delay minutes, 48%, falls under carrier control: crew scheduling, maintenance, fuelling, baggage. Unlike weather, that is a management outcome, and it is the category where the gap between two carriers on the same schedule is genuinely earned. It accounts for 16,000 of the 33,000 delay minutes reported here.
Cancellations sit at 0.93% against a national 1.81%, which is ordinary. Diversions add another 0.10%.
Connections and stage length
What a delay here actually costs
Badly late arrivals are rare here: only 4.2% 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.
The 689 miles between these two airports put this in the middle of the US network, around two and a half to three hours of block time. That is enough cruise to absorb a ten-minute late pushback and not enough to absorb thirty. It is also long enough that the aircraft usually flies three or four legs a day rather than six, which slows how fast delay compounds.
The return leg is a different operation and performs like one: it runs 80.2% on time across 4,108 flights, 3 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 3 points of difference.
The sample behind these figures is 4,107 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. April runs 92.4% on time and December 76.6%, a swing of 16 points. If the dates move at all, move them out of December before you think about anything else. 4,065 completed flights over 12 months sit behind that.
Common questions
SEA to SLC questions
How often is SEA to SLC delayed?
About one flight in six arrive at least 15 minutes late, which is the Department of Transportation's definition of a delayed arrival. That works out to 16.4% of the 4,107 flights in this analysis, against 22.4% nationally. Delayed flights are late by about 50 minutes on average.
Which airline is most reliable on SEA to SLC?
Delta Air Lines leads at 87.0% on time across 2,219 flights. Alaska Airlines is at the other end on 79.6%, a gap of 7 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 SEA to SLC?
The 5am–8am departure window is the strongest at 94.1% on time. The 11am–2pm window is the weakest at 79.0%, 15 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 December delays on SEA to SLC?
December is the weakest month at 76.6% on time, compared with 92.4% in April. 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
- SLC to SEA — the return legseparate record
- Seattle-Tacoma International (SEA) delay statistics
- Salt Lake City International (SLC) delay statistics
- Delta Air Lines on-time record87.0%
- SkyWest Airlines on-time record81.3%
- Alaska Airlines on-time record79.6%
- 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. SEA to SLC and SLC to SEA 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.