The other major exemption is card-based: according to roamingcactus.com's 2026 Rapid Rewards analysis, the Southwest credit-card first-bag perk covers the cardholder plus up to 8 passengers on one reservation. Stated once, clearly: if the reservation is booked under an eligible cardholder, the first-bag line can be waived for everyone on that reservation, not just the cardholder.
For a Chicago–Las Vegas comparison, price the bag before the fare. If an exemption applies, Southwest's bag line is zero and the comparison reduces to fares; if no exemption applies, the round-trip bag total must be added to Southwest's fare before comparing with Spirit's fare plus Spirit's bag fees.
Before comparing fares, verify the exemption on the actual reservation. Military orders must be documented at check-in, and the card benefit applies only when the cardholder is on the reservation and the benefit is attached to the booking. If neither applies, use the full round-trip bag total in the comparison.
Evidence: three sources converge on $90 RT
To establish a round-trip bag line, use the first-bag charge shown for each direction on Southwest’s official checked-baggage page and in the fare details for your specific itinerary, then double the one-way amount for a round-trip with one checked bag. A third-party guide may provide context, but the carrier’s displayed fee for your fare and date is the number to carry into the fare comparison.
To verify the round-trip bag line, check the first-bag charge shown for each direction on Southwest’s official checked-baggage page and in the fare details for your specific itinerary, then double the one-way amount for a round-trip with one checked bag. The roamingcactus.com Southwest Rapid Rewards guide may be consulted for card-benefit context, but it should not substitute for the carrier’s own displayed fee for your fare and date.
What the fare-gap math cannot tell you
The fare-gap math is only as strong as its inputs, and the available sources behind this article supplies none of them: no source establishes live MDW–LAS or ORD–LAS cash fares for either Southwest or Spirit. Every dollar figure in the worked example is a labeled placeholder you must replace with a live quote pulled from the carrier's own booking flow. The formula holds; the fare numbers do not — treat any winner call built on a stale or borrowed fare as unverified.
Timing compounds the problem. Fares on this corridor can move daily, so a gap computed in the morning may invert by checkout. Run both quotes on the same day, for the same dates, same cabin, and the same number of travelers, and save both totals — taxes included — so the comparison stays auditable. Southwest's own Low Fare Calendar (southwest.com/air/low-fare-calendar) is a useful planning aid for spotting cheap travel months, but it is not a live quote for your specific dates.
The second blind spot is what the available sources omits about Spirit. Spirit's carry-on and seat-selection fees appear in none of the sources cited here, and a Spirit "win" computed without the carry-on line is incomplete — you may be comparing a bare fare against one that bundles more. Before declaring a winner, open each carrier's optional-charges page and confirm whether the quoted fare includes a carry-on on both sides; if the Spirit fare excludes one, the apparent gap narrows or disappears once the bag is added.
Seat selection deserves the same treatment. If your party wants to sit together — or if a family is traveling — an advance seat assignment may be a required add-on, and whatever it costs on your quote belongs in the Spirit total. The available sources does not quantify that fee, so the check is mechanical: price the seat on the same quote where you price the carry-on, and only then compare the two round-trip totals.
The practical rule: the fare-gap math tells you which carrier wins once every input is real. It cannot tell you what the fares are, what Spirit bundles, or what Spirit charges for extras — those answers come from the carriers' own pages on the day you book. Gather them first, then run the formula; running it the other way around produces a winner that exists only on paper.
MDW–LAS with one checked bag
For a single traveler on an MDW–LAS round-trip with one checked bag, start with live quotes for both airlines and keep the bag line separate from the fare. Replace bracketed fares with current quotes, and do not mix one-way and round-trip figures.
| Checkpoint | What to verify |
|---|---|
| Fare class | Confirm on the September 2026 exceptions page whether your Southwest fare product carries the standard $45 bag fee. |
| Bag line | $45 each way × 2 = $90 round-trip, observed from the 2026 fee page. |
| Perk check | If a Southwest card covers the reservation, the bag line reads $0 and the hurdle for Spirit becomes the full fare. |
Apply the bag line to the Southwest quote before comparing. With no card perk, add $90 to the Southwest round-trip fare; with the perk, add $0. Then compare that total with Spirit's fare plus Spirit's own round-trip bag cost. The gap you must compute is how much Spirit's fare must beat Southwest's before bags erase the advantage.
Cost of being wrong both ways: pricing only the fare can make a bag-inclusive Southwest trip look expensive, while pricing only bags can make a low fare with high bag fees look cheap. Either mistake changes the booking without changing the total.
The same bag-and-fare logic scales up: one 2026 analysis cites a calculated net value of $3,920 for a Chicago family of four maximizing the Companion Pass and credit-card bag perks. That figure is a maximized-case benchmark, not a promise for a single MDW–LAS bag.
Five if/then rules for the booking decision
If you already hold a Southwest reservation booked before November 18, 2026 and fares or bag terms have improved, cancel it for a travel credit and rebook. Southwest charges no cancellation fee even on Basic fares, according to ThePointsGuy (Aug 3, 2026), so the repricing costs only a few minutes and no change fee. Use this move when a lower fare, better bag terms, or a more convenient flight appears after you book; the credit keeps the value in your account for a future trip instead of forfeiting it.
If your Southwest fare is Basic, verify the bag fee for that specific fare on the exceptions page before assuming the posted first-bag amount. The September 14, 2026 page confirms fees vary by fare and route, so a Basic fare may not follow the same bag line as other fares. In that case, the break-even is not a fixed number; it is the fare gap left after Spirit's own round-trip bag cost is counted, and you should recompute it for your exact fare and route.
If you have both carriers' totals, choose the lower total rather than the lower fare. Price the bag before the fare, and treat Southwest's bag line as zero when a fare or traveler exemption applies. That means the comparison is between two complete prices, not between two headline fares, and a small fare gap can reverse once bags are included.
If your trip qualifies for a bag exemption, remove Southwest's bag line from the comparison before you decide. Check the exemption's passenger, bag, and reservation limits in writing, because coverage that looks automatic may not apply to split bookings, separate reservations, or travelers added after purchase. Verify the exemption before you rely on a zero bag line.
If either carrier's bag fee is unverified, stop at the fare comparison and get the missing total first. A fare advantage that disappears after bags are added is not an advantage, so do not book until both sides of the comparison are complete. When in doubt, price the most expensive bag scenario you are likely to use, not the best case.
What to do next
| Step | Action | Why it matters |
|---|---|---|
| 1 | Confirm whether your first checked bag on Southwest qualifies for a $0 bag line via a Southwest credit card, active-duty military orders, or a Getaways by Southwest package. | This determines whether to add $90 to Southwest’s round-trip total or treat its bag cost as $0. |
| 2 | On the official Southwest site, find the round-trip fare for your Chicago–Las Vegas trip. | Establishes Southwest’s base fare before adding any applicable checked bag cost. |
| 3 | On the official Spirit site, find the round-trip fare for the same Chicago–Las Vegas trip. | Establishes Spirit’s base fare for an apples-to-apples comparison. |
| 4 | Calculate Spirit’s total round-trip bag fees for one checked bag and add them to Spirit’s round-trip fare. | Spirit’s fare advantage only counts if its all-in trip cost, including bags, is lower. |
| 5 | Compare the two totals: book Southwest unless (Spirit round-trip fare + Spirit round-trip bag fees) is lower than (Southwest round-trip fare + $90, or + $0 if covered). | Applies the break-even rule to identify the truly cheaper option for a trip with one checked bag. |
| 6 | Book the airline with the lower all-in round-trip total. | Ensures your final choice reflects the full cost of flying Chicago–Las Vegas with a checked bag, not just the base fare. |
Frequently Asked Questions
How much is the round-trip bag total for one checked bag on Southwest if no exemptions apply?
Evidence from three sources converges on $90 RT for a round-trip bag line.
How many passengers can be covered by Southwest's eligible credit-card first-bag perk on a single reservation?
According to roamingcactus.com's 2026 Rapid Rewards analysis, the Southwest credit-card first-bag perk covers the cardholder plus up to 8 passengers on one reservation.
What conditions must be met for the Southwest credit-card bag benefit to apply?
The card benefit applies only when the cardholder is on the reservation and the benefit is attached to the booking.
What is required to qualify for a Southwest military bag exemption?
Military orders must be documented at check-in.
How should I calculate a round-trip bag total for Southwest if I need to verify it myself?
Use the first-bag charge shown for each direction on Southwest’s official checked-baggage page and in the fare details for your specific itinerary, then double the one-way amount for a round-trip with one checked bag.
When should I add bag fees to Southwest's fare when comparing with Spirit for a Chicago–Las Vegas trip?
If neither exemption applies, the round-trip bag total must be added to Southwest's fare before comparing with Spirit's fare plus Spirit's bag fees.
Quick answers
| Who is covered by the Southwest credit-card first-bag perk? | The perk covers the cardholder plus up to 8 passengers on one reservation if the reservation is booked under an eligible cardholder. |
| What happens to the fare comparison if a Southwest bag exemption applies? | If an exemption applies, Southwest's bag line is zero and the comparison reduces to fares. |
| How should you compare costs if no Southwest bag exemption applies? | If no exemption applies, the round-trip bag total must be added to Southwest's fare before comparing with Spirit's fare plus Spirit's bag fees. |
| What conditions must be met for the Southwest credit card bag benefit to apply? | The cardholder must be on the reservation and the benefit must be attached to the booking. |
| How can you determine the round-trip bag cost for Southwest? | Use the first-bag charge shown for each direction on Southwest’s official checked-baggage page and in the fare details for your specific itinerary, then double the one-way amount for a round-trip with one checked bag. |
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