Route reliability record

SLC→SEA

Salt Lake City to Seattle

Flights: 4,108
Period: July 2025 to June 2026
Distance: 689 mi
Carriers: 4

This route lands a B+, 3 points better than the national rate of 77.6%. About one flight in five arrive 15 minutes late or worse, and those flights are late by roughly 55 minutes on average. That is a workable record, not a spotless one. That rests on 4,108 flights, of which Delta Air Lines flew 2,222 at 83.3% on time.

Delta Air Lines is the most reliable of the three carriers covered here at 83.3% on time across 2,222 flights.

On time

80.2%

Arrivals under 15 minutes late, of 4,064 completed flights

Avg delay

53 min

Across delayed arrivals only, not all flights

Cancelled

0.97%

National rate is 1.81%

Best carrier

Delta

83.3% on time across 2,222 flights

These figures cover 4,108 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 SLC to SEA

The choice of carrier is not cosmetic here. Delta Air Lines arrives on time 83.3% of the time; Alaska Airlines manages 76.7%. Sample sizes sit next to every figure, because a two-point gap across 1,369 flights means less than a ten-point one.

On-time performance by carrier on SLC to SEA
Airline	Flights	On time	Avg delay	Cancelled	Grade
Delta Air Lines	2,222	83.3%	54 min	0.54%	B+
SkyWest Airlines	419	77.3%	73 min	0.00%	B+
Alaska Airlines	1,369	76.7%	46 min	1.83%	B
Frontier Airlinesthin sample	98	69.5%	62 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 20 points, against 7 points between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.

5am–8am516 flights92.4%
8am–11am469 flights73.5%
11am–2pm1,051 flights85.6%
2pm–5pm877 flights74.9%
5pm–8pm529 flights81.3%
8pm–5am666 flights72.8%

Scale runs 65% to 100%. Rows with fewer than 30 flights show a dash.

Book the early departure. Flights leaving between 5am–8am arrive on time 92.4% of the time, against 72.8% for the overnight window. That is 20 points of difference for no extra money, and it comes from the same aircraft having had no earlier flight to fall behind on. The 11am–2pm window is the busiest, with 1,051 flights scheduled in it.

Across the year

When delays peak on this route

This route's year swings by 19 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.

Darker means more delays. Figures are on-time percentages.April best, December worst

December is the weak month at 70.7% on time, against 89.2% 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 409 flights.

Across the week, Tuesday is the strongest day at 83.1% on time and Thursday the weakest at 78.4%. 5 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 43,000 reported delay minutes.

Carrier
40%
Late-arriving aircraft
33%
National Airspace System
22%
Extreme weather
6%
Security
0%

The largest share of delay minutes, 40%, 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 17,000 of the 43,000 delay minutes reported here. The split is drawn from 43,041 reported delay minutes.

The practical consequence: the 5am–8am departure is the one that starts clean. Its aircraft has no earlier leg to be late from, which is exactly why that window shows 92.4% on time here.

Cancellations sit at 0.97% 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 4.7% 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 83.6% on time across 4,107 flights, 3 points above 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 4,108 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 89.2% on time and December 70.7%, a swing of 19 points. If the dates move at all, move them out of December before you think about anything else. 4,064 completed flights over 12 months sit behind that.

Common questions

SLC to SEA questions

How often is SLC to SEA delayed?

About 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 19.8% of the 4,108 flights in this analysis, against 22.4% nationally. Delayed flights are late by roughly 55 minutes on average.

Which airline is most reliable on SLC to SEA?

Delta Air Lines leads at 83.3% on time across 2,222 flights. Alaska Airlines is at the other end on 76.7%, 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 SLC to SEA?

The 5am–8am departure window is the strongest at 92.4% on time. The 8pm–5am window is the weakest at 72.8%, 20 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 SLC to SEA?

December is the weakest month at 70.7% on time, compared with 89.2% in April. Across the year that is 19 points between the best and worst months. Moving flexible dates out of that month is one of the few things that reliably helps.

Where to go next

Direction matters. SLC to SEA and SEA to SLC 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.

Key figures