4 airlines fly nonstop from San Francisco to Seoul, 5,645 miles: United Airlines, Korean Air, Asiana Airlines, and Air Premia. The figures below come from the records every airline files with the U.S. government for each flight into or out of the United States.
Emptiest months on this route, all its airlines together: January (70 percent full), April (72 percent full), and September (73 percent full). Fullest: June (93 percent), May (89 percent), and October (88 percent). Months every airline here usually runs 80 percent full or less: January, April, and September.
| Month | United Airlines | Korean Air | Asiana Airlines | Air Premia |
|---|---|---|---|---|
| January | 73 percent full, usually open | 76 percent full, usually open | 64 percent full, usually open | 55 percent full, usually open |
| February | 81 percent full, tight | 79 percent full, usually open | 90 percent full, tight | 79 percent full, usually open |
| March | 88 percent full, tight | 81 percent full, tight | 60 percent full, usually open | 78 percent full, usually open |
| April | 71 percent full, usually open | 71 percent full, usually open | 78 percent full, usually open | 62 percent full, usually open |
| May | 89 percent full, tight | 89 percent full, tight | 90 percent full, tight | 83 percent full, tight |
| June | 94 percent full, usually full | 93 percent full, usually full | 96 percent full, usually full | 80 percent full, usually open |
| July | 78 percent full, usually open | 80 percent full, usually open | 85 percent full, tight | 62 percent full, usually open |
| August | 73 percent full, usually open | 76 percent full, usually open | 82 percent full, tight | 67 percent full, usually open |
| September | 74 percent full, usually open | 69 percent full, usually open | 78 percent full, usually open | 62 percent full, usually open |
| October | 88 percent full, tight | 88 percent full, tight | 94 percent full, usually full | 72 percent full, usually open |
| November | 78 percent full, usually open | 78 percent full, usually open | 94 percent full, usually full | 58 percent full, usually open |
| December | 87 percent full, tight | 84 percent full, tight | 93 percent full, usually full | 78 percent full, usually open |
Each figure is the average for that month across the recent years on record: usually open is 80 percent full or less, tight is 81 to 90 percent, usually full is over 90 percent. These are how full flights have run, not live seats, standby lists, or fares. Check with your airline before you travel.