Carrier route record

WNPHX→BOI

Southwest Airlines · Phoenix to Boise

Flights: 813
Period: July 2025 to June 2026
Share of route: 41%
Carriers: 5

Historical statistics for July 2025 to June 2026, not live status. For today’s flights, check with Southwest Airlines.

Southwest on PHX to BOI lands a B+, level with the national rate of 77.6%. Roughly one flight in four arrives 15 minutes late or worse, and those flights are late by about 45 minutes on average. That is a workable record, not a spotless one. That rests on 813 flights, with a median arrival 1 minute early.

The US average over the same twelve months is 77.6%, so there is little to separate Southwest on PHX to BOI from the country as a whole. The variation that matters here is internal: which carrier, and what time of day.

On time

77.3%

Arrivals under 15 minutes late, of 810 completed flights

Avg delay

45 min

Across delayed arrivals only, not all flights

Cancelled

0.25%

National rate is 1.81%

Whole route

80.4%

Every carrier on PHX to BOI, 1,986 flights

These figures cover 813 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 PHX to BOI

Southwest flew 813 of the 1,986 flights on PHX to BOI, about 41% of it. Its 77.3% on time is 3 points worse than the route as a whole, which runs 80.4%. Ranked by on-time rate it is number three of five carriers on the route, where the range runs from Alaska at 100.0% to American at 67.3%. Three of those carriers flew fewer than 365 flights here, so treat their positions as rough.

Only one carrier clears this site's 365-flight floor here, so there is no comparison to draw. The table below is the full picture.

On-time performance by carrier on PHX to BOI
Airline	Flights	On time	Avg delay	Cancelled	Grade
Alaska Airlinesthin sample	5	100.0%	—	0.00%	A+
SkyWest Airlines	1,001	84.0%	69 min	0.60%	B+
Southwest Airlines (this page)	813	77.3%	45 min	0.25%	B+
Envoy Airthin sample	115	77.2%	34 min	0.00%	B+
American Airlinesthin sample	52	67.3%	60 min	0.00%	C+

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 40 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–8am29 flights—
8am–11am300 flights86.0%
11am–2pm118 flights78.8%
2pm–5pm163 flights83.3%
5pm–8pm68 flights45.6%
8pm–5am135 flights61.2%

Scale runs 35% to 90%. Rows with fewer than 30 flights show a dash.

The 5pm–8pm window is where this operation falls apart: 45.6% on time, against 86.0% 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 8am–11am window is the busiest, with 300 flights scheduled in it.

Across the year

Southwest on PHX to BOI through the year

Darker means more delays. Figures are on-time percentages.August best, June worst

June is the worst month at 56.7% on time, 33 points below August, the best at 89.5%. 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. December was the busiest month here, with 91 flights.

Across the week, Saturday is the strongest day at 85.4% on time and Monday the weakest at 68.9%. 17 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 58% of the 8,285 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.

Late-arriving aircraft
58%
Carrier
35%
National Airspace System
7%
Extreme weather
0%
Security
0%

58% 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 8,285 reported delay minutes here, that single category outweighs weather by 58 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 8,285 reported delay minutes.

The largest share of delay minutes, 35%, 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 2,901 of the 8,285 delay minutes reported here.

Cancellations are rare at 0.25%, 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

Badly late arrivals are rare here: only 4.9% 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 735 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 82.8% on time across 813 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. 813 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 PHX to BOI

Put together: treat all of this as provisional. 813 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 77.3% headline is the most reliable number on the page; the hour and month splits below it are built on a few dozen flights each. 810 completed flights over 12 months sit behind that.

Where to go next

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.

Key figures