Spirit does not publish a static, across-the-board fee sheet for checked luggage. Instead, the carrier operates the Spirit Bag-O-Tron calculator, an algorithm that prices luggage according to seasonal demand, specific city pairs, and route popularity. A short nonstop flight between Orlando and Atlanta during a Tuesday in September yields the lowest possible rate. An itinerary from Chicago to Las Vegas during peak spring holiday periods commands baggage prices near the top of the spectrum.
The variable algorithm creates massive price swings. An advance online baggage purchase completed during initial flight selection yields the steepest discount available to the public. If you wait to add luggage through the airline's mobile app after the initial ticket purchase, the rate ticks up by $5 to $10. Postponing payment until online check-in opens 24 hours before departure increases the charge again. The software is calibrated to extract increasing revenue as departure draws closer.
Route density dictates these baseline prices. Transcontinental flights and Caribbean international routes consistently show higher luggage charges across all booking stages. The algorithm recalculates ancillary charges continuously, meaning two travelers on the same flight can pay entirely different rates for the exact same piece of luggage.
