Carrier route record
WNDEN→BOI
Southwest Airlines · Denver to Boise
- Flights
- 1,093
- Period
- July 2025 to June 2026
- Share of route
- 36%
- Carriers
- 4
Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.
Southwest on DEN to BOI sits in the middle of the pack with a C+. About seven flights in ten arrive on time by the DOT's definition, 6 points worse than the national rate of 77.6%. The late ones land roughly 45 minutes behind schedule. That rests on 1,093 flights. The strongest carrier on the route with a full sample, United Airlines, flew 946 at 79.2% on time.
The national on-time rate over the same period is 77.6%, so Southwest on DEN to BOI runs 6 points behind the country as a whole. Across a thousand flights that is about 61 extra late arrivals.
Key figures
On time
71.5%
Arrivals under 15 minutes late, of 1,089 completed flights
Avg delay
46 min
Across delayed arrivals only, not all flights
Cancelled
0.18%
National rate is 1.81%
Whole route
76.3%
Every carrier on DEN to BOI, 3,039 flights
These figures cover 1,093 flights between July 2025 and June 2026. On-time means an arrival less than 15 minutes after the scheduled gate time, the same threshold the Department of Transportation uses in its own reports. Cancelled and diverted flights are excluded from the on-time rate and counted separately, which is why the cancellation column matters on its own.
Carrier by carrier
Southwest against every carrier on DEN to BOI
Southwest flew 1,093 of the 3,039 flights on DEN to BOI, about 36% of it. Its 71.5% on time is 5 points worse than the route as a whole, which runs 76.3%. Ranked by on-time rate it is last of four carriers on the route, where the range runs from United at 79.2% to Southwest at 71.5%. One of those carriers flew fewer than 365 flights here, so treat their positions as rough.
The two carriers here are closely matched: half 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 | 946 | 79.2% | 57 min | 0.32% | B+ |
| SkyWest Airlines | 874 | 78.7% | 71 min | 0.11% | B+ |
| Frontier Airlinesthin sample | 126 | 78.6% | 39 min | 0.00% | B+ |
| Southwest Airlines (this page) | 1,093 | 71.5% | 46 min | 0.18% | C+ |
No carrier stands out. United tops the table at 79.2%, half a point clear of the bottom, which is inside the range this kind of sample moves year to year.
Departure window
When Southwest runs best on this route
Departure time matters more here than the choice of carrier. The gap between the best and worst window is 29 points, against half a point between the best and worst carrier. Each bar is the on-time rate for flights scheduled inside that window.
- 5am–8am59 flights84.7%
- 8am–11am306 flights87.5%
- 11am–2pm149 flights69.1%
- 2pm–5pm215 flights67.3%
- 5pm–8pm24 flights—
- 8pm–5am340 flights58.9%
Scale runs 50% to 95%. Rows with fewer than 30 flights show a dash.
The 8pm–5am window is where this operation falls apart: 58.9% on time, against 87.5% 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 8pm–5am window is the busiest, with 340 flights scheduled in it.
Across the year
Southwest on DEN to BOI through the year
June is the worst month at 52.2% on time, 35 points below January, the best at 87.6%. 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. May was the busiest month here, with 97 flights.
Across the week, Wednesday is the strongest day at 78.8% on time and Friday the weakest at 61.3%. 18 points is a smaller lever than departure hour, but it is free.
Delay causes
Why Southwest runs late on this route
One category carries most of the weight here. Late-arriving aircraft accounts for 54% of the 14,000 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.
- 54%Late-arriving aircraft
- 33%Carrier
- 11%National Airspace System
- 2%Extreme weather
54% 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 14,000 reported delay minutes here, that single category outweighs weather by 25 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 14,349 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 87.5% on time here.
Cancellations are rare at 0.18%, below the national 1.81% and low enough to earn the plus on the grade. Roughly one flight in 100 is called off; the risk here is arriving late, not not arriving.
Connections and stage length
What a delay here costs
6.2% 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 roughly 45 minutes, which a normal layover absorbs.
The 649 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 77.8% on time across 1,092 flights, 6 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 6 points of difference.
One caution: this is a thin sample. 1,093 flights over twelve months is enough to clear this site's floor but not enough to pin the rate down tightly, and a single bad week moves it by more than a point. Treat the grade as a rough reading.
What to do with this
Booking Southwest on DEN to BOI
Put together: treat all of this as provisional. 1,093 flights over twelve months is enough to clear this site's floor and not enough to be confident to better than a couple of points. The 71.5% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 1,089 completed flights over 12 months sit behind that.
Related
Where to go next
- DEN to BOI, every carrier
- Southwest on BOI to DEN, the return leg
- United Airlines on DEN to BOI79.2%
- SkyWest Airlines on DEN to BOI78.7%
- Frontier Airlines on DEN to BOI78.6%
- Southwest Airlines network-wide record
- Denver International (DEN) delay statistics
- Why the first flight of the morning is statistically safest
- The best time of day to fly, according to 7.04 million flight records
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.