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2025 flights

San Francisco to Denver

Between San Francisco and Denver, 2.9 million passengers flew nonstop in 2025, on 20 thousand flights of about 960 miles and 2.1 hours in the air. The average passenger accounted for about 136 kg of CO2 from the fuel burned, from 109 kg on Frontier Airlines to 140 kg on United Airlines.

  • 136 kgCO2 per passenger, average
  • 960 milesabout 2.1 hours in the air
  • 2.9 millionpassengers in 2025
  • 82%of seats filled

CO2 per passenger, airline by airline

CO2 per passenger by airline, San Francisco to DenverBar chart of kilograms of CO2 per passenger on each airline between San Francisco and Denver in 2025.050100Frontier Airlines109 kgSouthwest Airlines134 kgUnited Airlines140 kg
CO2 per passenger by airline, San Francisco to DenverBar chart of kilograms of CO2 per passenger on each airline between San Francisco and Denver in 2025.050100Frontier Airlines109 kgSouthwest Airlines134 kgUnited Airlines140 kg
Kilograms of CO2 per passenger, one way, 2025. Fuel burned only. Less is better.
San Francisco to Denver by airline, 2025
AirlineShare of passengersFlights (thousands)Main aircraftSeats per flightSeats filledkg CO2 per passenger
United Airlines49%9.1Airbus A321neo, Boeing 737-80018984%140
Southwest Airlines43%9.6Boeing 737 MAX 8, Boeing 737-70016579%134
Frontier Airlines8%1.4Airbus A320neo, Airbus A321neo19782%109

What explains the differences

It's number 25 of the 100 busiest city pairs by passengers, and number 33 for the lowest CO2 per mile flown (0.14 kg per passenger-mile, number 1 being the lowest). A typical route of 960 miles in this set comes out at about 147 kg, so this one is 7.5% below it.

The flights use more than one pair of airports: DEN to SFO (61%), DEN to SJC (24%), DEN to OAK (16%). The figures here cover all of them.

The airlines on this route differ by 29% per passenger. Frontier Airlines burned 1,802 gallons per flight and 161 passengers on each, and United Airlines 2,269 gallons and 158 passengers. Fuel burned per flight explains about 92% of the gap and the number of passengers on each flight the other 8%. Frontier Airlines flew 82% full here against 84% for United Airlines, flying mostly Airbus A320neo against Airbus A321neo.

For comparison, the EPA default factor for a flight of this length (0.129 kg per passenger-mile) would give 124 kg. The EPA figure is a national average for flights of that length. The measured figure for this route is 9.4% higher.

2019 compared with 2025

In 2019 the route carried 3.0 million passengers on 21 thousand flights. In 2025 it carried 2.9 million, down 4%, on 20 thousand flights, down 2%. The average flight had 172 seats then and 177 now, and 85% of them were filled against 82%.

CO2 per passenger was 140 kg in 2019 and 136 kg in 2025, down 3.4%. Fuel burned per flight was down 5.1% (2,115 gallons to 2,006) and passengers per flight were down 1.8% (147 to 144). The distance flown was about the same (960 miles then, 960 now).

United Airlines led in both years, with 49% of passengers in 2019 and 49% in 2025. Southwest Airlines gained the most share, 39% to 43%. GoJet Airlines no longer reaches 1% of passengers, having had 2% in 2019.

Airline by airline, Southwest Airlines went from 138 kg to 134 kg per passenger (157 seats per flight to 165, main aircraft Boeing 737-700 then and Boeing 737 MAX 8 now); Frontier Airlines went from 110 kg to 109 kg per passenger (190 seats per flight to 197, main aircraft Airbus A320neo in both years).

San Francisco to Denver, 2019 and 2025
Measure20192025
Passengers (thousands)3,0432,927
Flights (thousands)20.720.3
Seats filled85%82%
Gallons of fuel per flight2,1152,006
kg CO2 per passenger140136
CO2 per passenger by airline, 2019 and 2025, San Francisco to DenverBar chart of kilograms of CO2 per passenger on San Francisco to Denver for each major airline in 2019 and in 2025.050100150United Airlines 2019144 kgUnited Airlines 2025140 kgSouthwest Airlines 2019138 kgSouthwest Airlines 2025134 kgFrontier Airlines 2019110 kgFrontier Airlines 2025109 kg
CO2 per passenger by airline, 2019 and 2025, San Francisco to DenverBar chart of kilograms of CO2 per passenger on San Francisco to Denver for each major airline in 2019 and in 2025.050100150United Airlines 2019144 kgUnited Airlines 2025140 kgSouthwest Airlines 2019138 kgSouthwest Airlines 2025134 kgFrontier Airlines 2019110 kgFrontier Airlines 2025109 kg
Kilograms of CO2 per passenger on the airlines that flew the route in both years. 2019 figures use that year's filings and estimated aircraft fuel rates, with type-average replacements for missing or outlying filings as described in the methodology. Less is better.

San Francisco to Denver and back: time in the air

Flights from Denver to San Francisco spent 132 minutes in the air on average in 2025, against 124 minutes from San Francisco to Denver: 8 minutes longer, 6% more. That's the westbound direction, into the prevailing west-to-east winds, which at flight levels often blow at well over 50 miles an hour.

At this route's average burn of about 939 gallons per flight hour, those extra minutes cost about 117 gallons per flight, which is about 0.8 gallons or 8 kg of CO2 for each passenger on the slower direction. The route figure above averages both directions.

Time in the air by direction, 2025
DirectionAverage minutes in the airFlights
Denver to San Francisco131.910,181
San Francisco to Denver124.410,097

Fly or drive

Driving is about 1,123 miles by road, which we estimate as 1.17 times the straight-line distance. An average car emits about 334 kg of CO2 on the trip, at the EPA figure of 0.297 kg per vehicle-mile. Alone, that's 146% more than the 136 kg for a passenger on the flight.

With 3 people in the car, driving comes out lower per person than the flight (111 kg each, and 83 kg each for four). A larger SUV or pickup (0.394 kg per mile) would emit about 443 kg per vehicle.

CO2 per person by way of travel, San Francisco to DenverBar chart of kilograms of CO2 per person flying, driving or taking the train between San Francisco and Denver.0100200300Flight, average136 kgCar, driver alone334 kgCar, 2 people167 kgCar, 4 people83 kg
CO2 per person by way of travel, San Francisco to DenverBar chart of kilograms of CO2 per person flying, driving or taking the train between San Francisco and Denver.0100200300Flight, average136 kgCar, driver alone334 kgCar, 2 people167 kgCar, 4 people83 kg
CO2 per person, San Francisco to Denver. Driving figures divide one car's emissions between the people in it. See flying vs driving for other trip lengths.

Flights are listed under the airline that operates them, so regional flying appears under the regional. How the CO2 and drive figures are worked out is in the methodology.

Routes with shared cities or nearby CO2 per mile

Los Angeles to San Francisco75 kg CO2 per passenger over 337 milesLos Angeles to Denver130 kg CO2 per passenger over 850 milesSan Francisco to Las Vegas71 kg CO2 per passenger over 404 milesNew York to San Francisco414 kg CO2 per passenger over 2,578 milesSeattle to Anchorage203 kg CO2 per passenger over 1,448 milesDallas-Fort Worth to Orlando138 kg CO2 per passenger over 981 milesDallas-Fort Worth to Miami160 kg CO2 per passenger over 1,119 milesChicago to Houston134 kg CO2 per passenger over 928 miles

Airports on this route: Denver International Airport, San Francisco International Airport, San Jose International Airport, Oakland San Francisco Bay Airport.

Airport and airline references

Source: US Bureau of Transportation Statistics (BTS), T-100 traffic and Schedule T-2 fuel filings; US EPA transport emission factors. See the methodology.

See all routes, or how the airlines compare overall on the airline pages.