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2025 flights

San Francisco to San Diego

Between San Francisco and San Diego, 3.5 million passengers flew nonstop in 2025, on 31 thousand flights of about 436 miles and 1.1 hours in the air. The average passenger accounted for about 78 kg of CO2 from the fuel burned, from 56 kg on Frontier Airlines to 107 kg on SkyWest Airlines.

  • 78 kgCO2 per passenger, average
  • 436 milesabout 1.1 hours in the air
  • 3.5 millionpassengers in 2025
  • 80%of seats filled

CO2 per passenger, airline by airline

CO2 per passenger by airline, San Francisco to San DiegoBar chart of kilograms of CO2 per passenger on each airline between San Francisco and San Diego in 2025.0255075100Frontier Airlines56 kgSpirit Airlines64 kgUnited Airlines68 kgAlaska Airlines72 kgSouthwest Airlines77 kgHorizon Air102 kgSkyWest Airlines107 kg
CO2 per passenger by airline, San Francisco to San DiegoBar chart of kilograms of CO2 per passenger on each airline between San Francisco and San Diego in 2025.050100Frontier Airlines56 kgSpirit Airlines64 kgUnited Airlines68 kgAlaska Airlines72 kgSouthwest Airlines77 kgHorizon Air102 kgSkyWest Airlines107 kg
Kilograms of CO2 per passenger, one way, 2025. Fuel burned only. Less is better.
San Francisco to San Diego by airline, 2025
AirlineShare of passengersFlights (thousands)Main aircraftSeats per flightSeats filledkg CO2 per passenger
Southwest Airlines57%17.0Boeing 737-700, Boeing 737 MAX 815476%77
United Airlines22%5.0Boeing 737-800, Boeing 737 MAX 817088%68
SkyWest Airlines11%6.1Embraer 1757685%107
Alaska Airlines3%0.8Boeing 737-900ER, Boeing 737 MAX 917477%72
Spirit Airlines3%0.7Airbus A320neo, Airbus A320-20017679%64
Frontier Airlines3%0.5Airbus A321neo, Airbus A320neo21584%56
Horizon Air2%1.0Embraer 1757688%102

What explains the differences

It's number 19 of the 100 busiest city pairs by passengers, and number 78 for the lowest CO2 per mile flown (0.18 kg per passenger-mile, number 1 being the lowest). A typical route of 436 miles in this set comes out at about 82 kg, so this one is 5.2% below it.

The flights use more than one pair of airports: SFO to SAN (48%), SJC to SAN (31%), OAK to SAN (21%). The figures here cover all of them.

The airlines on this route differ by 92% per passenger. Frontier Airlines burned 1,029 gallons per flight and 181 passengers on each, and SkyWest Airlines 705 gallons and 65 passengers. The gap comes from how many passengers share each flight, not from fuel burned per flight. Frontier Airlines flew 84% full here against 85% for SkyWest Airlines, flying mostly Airbus A321neo against Embraer 175.

For comparison, the EPA default factor for a flight of this length (0.129 kg per passenger-mile) would give 56 kg. The EPA figure is a national average for flights of that length. The measured figure for this route is 38% higher.

2019 compared with 2025

In 2019 the route carried 3.9 million passengers on 35 thousand flights. In 2025 it carried 3.5 million, down 11%, on 31 thousand flights, down 11%. The average flight had 145 seats then and 141 now, and 77% of them were filled against 80%.

CO2 per passenger was 84 kg in 2019 and 78 kg in 2025, down 7.2%. Fuel burned per flight was down 7.0% (967 gallons to 899) and passengers per flight were up 0.2% (112 to 113). The distance flown was about the same (436 miles then, 436 now).

Southwest Airlines led in both years, with 65% of passengers in 2019 and 57% in 2025. SkyWest Airlines gained the most share, 3% to 11%. Alaska Airlines lost the most, 14% to 3%. Spirit Airlines and Frontier Airlines and Horizon Air didn't have 1% of the passengers in 2019 and now together carry 7%.

Airline by airline, Alaska Airlines went from 94 kg to 72 kg per passenger (139 seats per flight to 174, main aircraft Airbus A319 then and Boeing 737-900ER now); SkyWest Airlines went from 108 kg to 107 kg per passenger (76 seats per flight to 76, main aircraft Embraer 175 in both years).

The airports changed too: SFO to SAN handled 43% of the passengers in 2019 and 48% in 2025.

San Francisco to San Diego, 2019 and 2025
Measure20192025
Passengers (thousands)3,9463,511
Flights (thousands)35.131.2
Seats filled77%80%
Gallons of fuel per flight967899
kg CO2 per passenger8478
CO2 per passenger by airline, 2019 and 2025, San Francisco to San DiegoBar chart of kilograms of CO2 per passenger on San Francisco to San Diego for each major airline in 2019 and in 2025.0255075100Southwest Airlines 201981 kgSouthwest Airlines 202577 kgUnited Airlines 201982 kgUnited Airlines 202568 kgSkyWest Airlines 2019108 kgSkyWest Airlines 2025107 kgAlaska Airlines 201994 kgAlaska Airlines 202572 kg
CO2 per passenger by airline, 2019 and 2025, San Francisco to San DiegoBar chart of kilograms of CO2 per passenger on San Francisco to San Diego for each major airline in 2019 and in 2025.050100Southwest Airlines 201981 kgSouthwest Airlines 202577 kgUnited Airlines 201982 kgUnited Airlines 202568 kgSkyWest Airlines 2019108 kgSkyWest Airlines 2025107 kgAlaska Airlines 201994 kgAlaska Airlines 202572 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 San Diego and back: time in the air

The time in the air is almost the same each way: 68 minutes on average from San Francisco to San Diego and 65 minutes back. A difference of 2 minutes is within what routing and congestion alone produce.

Time in the air by direction, 2025
DirectionAverage minutes in the airFlights
San Francisco to San Diego67.715,591
San Diego to San Francisco65.315,616

Fly or drive

Driving is about 510 miles by road, which we estimate as 1.17 times the straight-line distance. An average car emits about 152 kg of CO2 on the trip, at the EPA figure of 0.297 kg per vehicle-mile. Alone, that's 95% more than the 78 kg for a passenger on the flight.

With 2 people in the car, driving comes out lower per person than the flight (76 kg each, and 38 kg each for four). A larger SUV or pickup (0.394 kg per mile) would emit about 201 kg per vehicle.

CO2 per person by way of travel, San Francisco to San DiegoBar chart of kilograms of CO2 per person flying, driving or taking the train between San Francisco and San Diego.050100150Flight, average78 kgCar, driver alone152 kgCar, 2 people76 kgCar, 4 people38 kg
CO2 per person by way of travel, San Francisco to San DiegoBar chart of kilograms of CO2 per person flying, driving or taking the train between San Francisco and San Diego.050100150Flight, average78 kgCar, driver alone152 kgCar, 2 people76 kgCar, 4 people38 kg
CO2 per person, San Francisco to San Diego. 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 milesSan Francisco to Las Vegas71 kg CO2 per passenger over 404 milesNew York to San Francisco414 kg CO2 per passenger over 2,578 milesSan Francisco to Seattle119 kg CO2 per passenger over 683 milesChicago to Atlanta107 kg CO2 per passenger over 601 milesAtlanta to Tampa73 kg CO2 per passenger over 406 milesLos Angeles to Salt Lake City106 kg CO2 per passenger over 583 milesSan Francisco to Portland103 kg CO2 per passenger over 554 miles

Airports on this route: 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.