Flight number record
UA 2123
United Airlines · Denver to San Francisco
- Route
- DEN → SFO
- Departs
- 21:10
- Flights
- 306
- Period
- September 2025 to August 2026
Historical statistics for September 2025 to August 2026, not live status. For today’s UA 2123, check with United Airlines.
UA 2123 earns a D+, which is as low as this site's scale goes for a reason. Only 56.6% of flights arrive inside the 15-minute window, 21 points worse than the national rate of 77.3%. Late arrivals average about an hour. Treat any tight connection here as a coin flip. That rests on 306 flights, with a median arrival 9 minutes late.
The national on-time rate over the same period is 77.3%, so UA 2123 runs 21 points behind the country as a whole. Across a thousand flights that is about 207 extra late arrivals.
Key figures
On time
56.6%
Arrivals under 15 minutes late, of 304 completed flights
Avg delay
63 min
Across delayed arrivals only, not all flights
Cancelled
0.33%
National rate is 1.89%
1 hour+ late
17.1%
National rate is 8.3%
These figures cover 306 flights between September 2025 and August 2026. Beyond the headline rate, 17.1% of arrivals land a full hour or more behind schedule, against 8.3% nationally.
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Schedule
When UA 2123 flies
UA 2123 flies DEN to SFO without a single fixed slot. Its most common scheduled departure was 21:10, used on 82 of 306 flights, with 21:05 (70 flights) and 21:02 (32 flights) next.
- Departs 21:1082 flights26.8%
- Departs 21:0570 flights22.9%
- Departs 21:0232 flights10.5%
Scale runs 0% to 100%.
Share of scheduled departures at each local time. Times used on fewer than one flight in ten are not listed.
A 21:10 departure sits in the 8pm–5am window. Across every US flight scheduled in that window the on-time rate is 70.1%, and UA 2123 runs 13 points worse at 56.6%. By that time of day most aircraft have flown several legs already, and whatever went wrong earlier is still in the schedule.
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Against the route
UA 2123 next to DEN to SFO
Every carrier on DEN to SFO combined arrived on time 63.2% of the time across 5,488 flights, so UA 2123 is 7 points behind it. United flew 3,429 flights on the same city pair under all its flight numbers, including this one, and those ran 69.0% on time. Across its whole network United Airlines managed 78.8%, which this flight trails by 22 points.
| Compared with | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| UA 2123thin sample | 306 | 56.6% | 63 min | 0.33% | D+ |
| All United Airlines on DEN–SFO | 3,429 | 69.0% | 63 min | 0.23% | C+ |
| All carriers on DEN–SFO | 5,488 | 63.2% | 62 min | 0.40% | D+ |
| United Airlines, whole network | 830,825 | 78.8% | 73 min | 1.02% | B |
Across the year
UA 2123 quarter by quarter
UA 2123 did not run the same all year. Its best quarter was Jan–Mar 2026 at 80.5% on time and its weakest Jul–Sep 2026 at 36.1%, a swing of 44 points on roughly 77 flights a quarter. The twelve-month rate averages over that, so if you are flying in a weak quarter the quarter's own figure is the better guide.
- Oct–Dec 202566 flights59.1%
- Jan–Mar 202687 flights80.5%
- Apr–Jun 202691 flights45.6%
- Jul–Sep 202662 flights36.1%
Scale runs 30% to 85%.
By weekday, UA 2123 did best on Tuesday at 82.5% on time and worst on Sunday at 40.0%. Each day rests on about 44 flights, so a gap of 43 points is worth noticing but could still narrow in another year.
Arrival by arrival
How late UA 2123 runs when it is late
The 132 late arrivals split 44 under half an hour, 36 between 30 and 60 minutes, and 52 an hour or more.
- 15+ min early34 flights11.2%
- Within 15 min138 flights45.4%
- 15–29 min late44 flights14.5%
- 30–59 min late36 flights11.8%
- 1–2 hours late39 flights12.8%
- 2+ hours late13 flights4.3%
Scale runs 0% to 100%.
Share of 304 completed flights by arrival against schedule.
16 of 119 late departures made up the time and arrived on time, 13% of them. In the other direction, 29 flights left on time yet arrived 15 minutes late or more, so that delay built up after pushback: in the air, in a holding pattern or on the ground at SFO.
The worst days
Worst days for UA 2123
The latest arrival of the period was on 29 July 2026, 11 hours 2 minutes behind schedule; most of its delay minutes were filed as carrier-caused (crew, maintenance, fuelling, baggage). The five worst days are spread across July, August, January and February, with no single month standing out.
| Date | Arrived late by | Largest filed cause |
|---|---|---|
| 29 July 2026 | 11 hours 2 minutes | Carrier |
| 3 August 2026 | 3 hours 31 minutes | Late-arriving aircraft |
| 28 July 2026 | 3 hours 18 minutes | Late-arriving aircraft |
| 2 January 2026 | 3 hours 13 minutes | National Airspace System |
| 15 February 2026 | 2 hours 59 minutes | National Airspace System |
Delay causes
Why UA 2123 runs late
National Airspace System accounts for 50% of the 8,312 delay minutes carriers reported here, and the remaining four split what is left.
- 50%National Airspace System
- 27%Late-arriving aircraft
- 23%Carrier
- 0%Extreme weather
50% 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 4,157 minutes out of 8,312, against a national NAS share of 21%. The split is drawn from 8,312 delay minutes reported on UA 2123.
Cancellations are rare at 0.33%, below the national 1.89% 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 routes
What a delay on this flight costs
17.1% of arrivals land a full hour or more behind schedule, against 8.3% nationally. Ninety minutes is the honest minimum for a connection here, and two hours if the onward flight is on a separate ticket.
The 967 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.
UA 2123 is not only a DEN to SFO flight. It also flew MCI to DEN (272 flights, 65.1% on time) often enough to be scored on its own. Airlines attach one number to several legs of an aircraft's day, and each leg keeps its own record here, so check which city pair is on your booking before reading a rate.
One caution: this is a thin sample. 306 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.
| Also flown as | Flights | On time | Avg delay | Cancelled | Grade |
|---|---|---|---|---|---|
| MCI to DENthin sample | 272 | 65.1% | 67 min | 0.37% | C+ |
What to do with this
Booking UA 2123
For a booking, UA 2123 is a flight to put a long connection behind. 17.1% of its completed flights arrived an hour or more late, which a short layover will not absorb. If the onward flight is on a separate ticket, the risk sits with you.
Related
Where to go next
- DEN to SFO, every carrier and departure
- SFO to DEN, the return direction
- United Airlines 1812 on the same route73.4%
- United Airlines 2633 on the same route68.7%
- United Airlines 2391 on the same route71.7%
- United Airlines 2367 on the same route95.3%
- United Airlines 1007 on the same route66.2%
- United Airlines 1561 on the same route66.8%
- Denver International (DEN) delay statistics
- San Francisco International (SFO) delay statistics
- United Airlines on-time record
- Every United Airlines flight number with a record
- The most and least reliable flight numbers in the US
- How often a connection fails at the major US hubs
A flight number here means United Airlines’s own number as it reports to the DOT. How the figures are computed is on the methodology page.
Source: US DOT Bureau of Transportation Statistics, on-time performance data, September 2025 to August 2026. Last updated ; recomputed monthly. How this data is collected.