Flying with a baby should not be expensive – by regulation, infants under two can share your seat. The real cost question is what happens the moment you fly internationally: lap infants are billed as a fraction of your fare, with taxes and surcharges on top, and every airline prices that fraction slightly differently. A family flying US domestic pays nothing; the same family crossing the Atlantic can quietly look at four hundred dollars or more in infant taxes on a long award ticket.
Here is exactly what to expect in 2026, airline by airline.
The Universal Rules First
- Domestic US, one infant per adult: free with your airline ticket; add the infant to the reservation at any point before the flight. An adult may hold only one infant simultaneously on the lap – a second baby needs their own seat by FAA regulation.
- International: a lap infant travels at roughly 10% of the adult fare plus taxes and surcharges – and the “plus” is where raw fees can be significant.
- Age cutoff: the infant must be under 2 for the entire trip – if they turn 2 mid-itinerary, they need their own seat (with its own fare class) for the remainder.
- Do not forget the ticket: even a free lap infant needs to be on your ticket; bring the birth certificate at check-in if asked to verify age.
The US Carriers: What They Charge
| Airline | Domestic lap infant | International lap infant |
|---|---|---|
| American Airlines | Free (add during or after booking on aa.com) | ~10% of adult fare + taxes; same terminal bag benefit |
| Delta | Free | ~10% of adult fare + international taxes/fees |
| United | Free (plus US-Canada and Mexico routes) | Flat fee, generally $20-$250 by route and cabin – not a fare percentage |
Credit United with a real improvement: Previously, lap-infant fees on international awards used to be computed as a percentage of the award’s cash price – United refactored to a flat fee capped between $20 and $250 by route and cabin, a genuinely cheaper structure on long international flight portions that many families still have not caught up on. American is the inverse: the system is simple but requires a phone call for international additions, with one checked bag (up to 25 lbs, 22x14x9 in) free from an airport agent.
The Miles Game: How Award Tickets Price the Lap Infant
If the adult is sitting on an award ticket, the lap infant pricing changes with your loyalty program – the fee is usually computed against the cash fare that the award replaced, not the freed-up miles:
- Air France-KLM Flying Blue: 10% of the cash fare, but only on Flying Blue-operated flights – no lap infants on partner-award routings
- British Airways: 10% of the Avios count for the adult award
- Air France: a 90% discount on the applicable fare (effectively 10%), with certain cabins requiring phone assistance; flights from Paris to Corsica skip the fee for infants altogether
Practical consequence: when you plan a creative international award trip with a baby, price the infant fee before the departure. Some multi-segment itineraries loaded with heavy surcharges charge more for the infant portion than the adult paid for the award.
One Day Old? Seat or Lap: The Real Question
Lap travel is legal; it is not the safest way to fly. The FAA recommends a seat with a tested and approved child restraint system for travel with an infant, and some airlines make the upsell story work better: an infant with their own seat pays a child fare, which brings its own baggage allowance and priority bassinet access. Air France and Air Canada both structure a child with an FAA-approved car seat in the cabin at the child fare, and bassinets are free at the bulkhead (request early; expect gate staff to re-verify age at your connection).
What to Pack and Request
- One carry-on diaper bag per child (allowed beyond your regular allowance).
- Stroller check at the gate for free; strollers ride through to the gate and wait cage-side for boarding or connection.
- Request the bassinet when you book – bulkhead rows sell early.
- For a lap child, add a birth certificate to carry-on for age verification.
- Formula, breast milk, and frozen packs are permitted in quantities beyond the usual liquid limit – declare at security and expect a separate screening step.
The Bottom Line
The math is friendly domestically and only friendly-with-fine-print internationally. Book the lap infant onto the same ticket (warning: call ahead for international or complex routings), calculate an approximate 10% of fares on most international carriers, and check whether United’s flat-fee structure makes it the best airline for your route. And if a full-price child fare can strengthen your budget rather than break it, the FAA-favored way to fly with an infant under two is in their own seat.
Looking after the bigger kids? Our guide to flying with toddlers walks the 2-to-4 age group, and the naptime itinerary shows how to build travel days around a toddler’s sleep. More family strategies sit in our family travel section.