Denver to Seattle

Between Denver and Seattle, 1.8 million passengers flew nonstop in 2025, on 13 thousand flights of about 1,024 miles and 2.3 hours in the air. The average passenger accounted for about 135 kg of CO2 from the fuel burned, from 105 kg on Frontier Airlines to 174 kg on Delta Air Lines.

135 kgCO2 per passenger, average
1,024 milesabout 2.3 hours in the air
1.8 millionpassengers in 2025
87%of seats filled

CO2 per passenger, airline by airline

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

Denver to Seattle by airline, 2025

Airline	Share of passengers	Flights (thousands)	Main aircraft	Seats per flight	Seats filled	kg CO2 per passenger
United Airlines	31%	3.9	Boeing 737-800, Airbus A320-200	169	88%	141
Alaska Airlines	26%	3.1	Boeing 737-900ER, Boeing 737 MAX 9	175	89%	127
Southwest Airlines	23%	2.7	Boeing 737 MAX 8, Boeing 737-800	170	89%	125
Delta Air Lines	10%	1.8	Airbus A220-300	130	77%	174
Frontier Airlines	8%	0.8	Airbus A321neo, Airbus A320neo	215	86%	105

What explains the differences

It's number 79 of the 100 busiest city pairs by passengers, and number 16 for the lowest CO2 per mile flown (0.13 kg per passenger-mile, number 1 being the lowest). A typical route of 1,024 miles in this set comes out at about 154 kg, so this one is 13% below it.

The airlines on this route differ by 65% per passenger. Frontier Airlines burned 2,005 gallons per flight and 186 passengers on each, and Delta Air Lines 1,773 gallons and 100 passengers. The gap comes from how many passengers share each flight, not from fuel burned per flight. Frontier Airlines flew 86% full here against 77% for Delta Air Lines, flying mostly Airbus A321neo against Airbus A220-300.

For comparison, the EPA default factor for a flight of this length (0.129 kg per passenger-mile) would give 132 kg. The EPA figure is a national average for flights of that length. The measured figure for this route is 2.1% higher.

2019 compared with 2025

In 2019 the route carried 1.9 million passengers on 14 thousand flights. In 2025 it carried 1.8 million, down 2%, on 13 thousand flights, down 7%. The average flight had 155 seats then and 165 now, and 89% of them were filled against 87%.

CO2 per passenger was 150 kg in 2019 and 135 kg in 2025, down 10.0%. Fuel burned per flight was down 5.7% (2,112 gallons to 1,991) and passengers per flight were up 4.7% (137 to 144). The distance flown was about the same (1,024 miles then, 1,024 now).

Alaska Airlines had the most passengers in 2019 (29%) and United Airlines in 2025 (31%). Delta Air Lines lost the most, 15% to 10%.

Airline by airline, Alaska Airlines went from 146 kg to 127 kg per passenger (163 seats per flight to 175, main aircraft Boeing 737-900ER in both years); United Airlines went from 146 kg to 141 kg per passenger (166 seats per flight to 169, main aircraft Boeing 737-900ER then and Boeing 737-800 now).

Denver to Seattle, 2019 and 2025

Measure	2019	2025
Passengers (thousands)	1,866	1,826
Flights (thousands)	13.6	12.7
Seats filled	89%	87%
Gallons of fuel per flight	2,112	1,991
kg CO2 per passenger	150	135

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.

Denver to Seattle and back: time in the air

Flights from Denver to Seattle spent 146 minutes in the air on average in 2025, against 130 minutes from Seattle to Denver: 16 minutes longer, 13% 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 865 gallons per flight hour, those extra minutes cost about 235 gallons per flight, which is about 1.6 gallons or 16 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
Denver to Seattle	146.2	6,364
Seattle to Denver	129.9	6,319

Fly or drive

Driving is about 1,198 miles by road, which we estimate as 1.17 times the straight-line distance. An average car emits about 356 kg of CO2 on the trip, at the EPA figure of 0.297 kg per vehicle-mile. Alone, that's 164% more than the 135 kg for a passenger on the flight.

With 3 people in the car, driving comes out lower per person than the flight (119 kg each, and 89 kg each for four). A larger SUV or pickup (0.394 kg per mile) would emit about 472 kg per vehicle.

CO2 per person, Denver to Seattle. 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, Seattle-Tacoma 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.