Chicago to Denver

Between Chicago and Denver, 2.8 million passengers flew nonstop in 2025, on 18 thousand flights of about 890 miles and 2.0 hours in the air. The average passenger accounted for about 129 kg of CO2 from the fuel burned, from 101 kg on Frontier Airlines to 152 kg on American Airlines.

129 kgCO2 per passenger, average
890 milesabout 2.0 hours in the air
2.8 millionpassengers in 2025
83%of seats filled

CO2 per passenger, airline by airline

Kilograms of CO2 per passenger, one way, 2025. Fuel burned only. Less is better.

Chicago to Denver by airline, 2025

Airline	Share of passengers	Flights (thousands)	Main aircraft	Seats per flight	Seats filled	kg CO2 per passenger
United Airlines	49%	7.1	Airbus A321neo, Boeing 757-300	218	86%	126
Southwest Airlines	31%	6.6	Boeing 737 MAX 8, Boeing 737-800	166	78%	129
American Airlines	13%	2.7	Boeing 737-800, Airbus A319	153	84%	152
Frontier Airlines	7%	1.2	Airbus A320neo, Airbus A321neo	208	82%	101

What explains the differences

It's number 27 of the 100 busiest city pairs by passengers, and number 36 for the lowest CO2 per mile flown (0.14 kg per passenger-mile, number 1 being the lowest). A typical route of 890 miles in this set comes out at about 138 kg, so this one is 6.5% below it.

The flights use more than one pair of airports: DEN to ORD (75%), DEN to MDW (25%). The figures here cover all of them.

The airlines on this route differ by 51% per passenger. Frontier Airlines burned 1,762 gallons per flight and 171 passengers on each, and American Airlines 2,006 gallons and 129 passengers. Fuel burned per flight explains about 31% of the gap and the number of passengers on each flight the other 69%. Frontier Airlines flew 82% full here against 84% for American Airlines, flying mostly Airbus A320neo against Boeing 737-800.

For comparison, the EPA default factor for a flight of this length (0.129 kg per passenger-mile) would give 115 kg. The EPA figure is a national average for flights of that length. The measured figure for this route is 12% higher.

2019 compared with 2025

In 2019 the route carried 2.9 million passengers on 18 thousand flights. In 2025 it carried 2.8 million, down 6%, on 18 thousand flights, down 4%. The average flight had 181 seats then and 187 now, and 87% of them were filled against 83%.

CO2 per passenger was 130 kg in 2019 and 129 kg in 2025, down 0.7%. Fuel burned per flight was down 2.9% (2,111 gallons to 2,050) and passengers per flight were down 2.2% (158 to 155). The distance flown was about the same (890 miles then, 890 now).

United Airlines led in both years, with 43% of passengers in 2019 and 49% in 2025. Spirit Airlines no longer reaches 1% of passengers, having had 7% in 2019.

Airline by airline, Frontier Airlines went from 111 kg to 101 kg per passenger (175 seats per flight to 208, main aircraft Airbus A319 then and Airbus A320neo now); American Airlines went from 139 kg to 152 kg per passenger (162 seats per flight to 153, main aircraft Boeing 737-800 in both years).

Chicago to Denver, 2019 and 2025

Measure	2019	2025
Passengers (thousands)	2,925	2,756
Flights (thousands)	18.5	17.8
Seats filled	87%	83%
Gallons of fuel per flight	2,111	2,050
kg CO2 per passenger	130	129

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.

Chicago to Denver and back: time in the air

Flights from Chicago to Denver spent 130 minutes in the air on average in 2025, against 114 minutes from Denver to Chicago: 16 minutes longer, 14% 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 1,007 gallons per flight hour, those extra minutes cost about 269 gallons per flight, which is about 1.7 gallons or 17 kg of CO2 for each passenger on the slower direction. The route figure above averages both directions.

Time in the air by direction, 2025

Direction	Average minutes in the air	Flights
Chicago to Denver	130.1	8,944
Denver to Chicago	114.1	8,841

Fly or drive

Driving is about 1,041 miles by road, which we estimate as 1.17 times the straight-line distance. An average car emits about 309 kg of CO2 on the trip, at the EPA figure of 0.297 kg per vehicle-mile. Alone, that's 140% more than the 129 kg for a passenger on the flight.

With 3 people in the car, driving comes out lower per person than the flight (103 kg each, and 77 kg each for four). A larger SUV or pickup (0.394 kg per mile) would emit about 410 kg per vehicle.

CO2 per person, Chicago 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

Airports on this route: Denver International Airport, O'Hare International Airport, Midway International 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.