Short answer. Fares are constructed, not calculated from distance. Full-service airlines build international pricing around the round trip, so a one-way often fails to qualify for the discounted fare and lands on a higher one. The fix is usually a return or an open-jaw ticket, not a cheaper one-way.
Key points
- A one-way is not half a return. It is a different fare with different conditions.
- Some carriers do price per leg, so this pattern is not universal.
- An open-jaw ticket lets you fly in to one city and home from another on one ticket.
- Airlines price by market, so adding a connection can lower the fare rather than raise it.
Why is a one-way sometimes more expensive?
Discounted international fares carry conditions, and many of those conditions assume a return journey. A one-way frequently does not qualify for the cheapest fare in the market and falls back to a less restricted, more expensive one. The result is the odd situation where adding a return leg lowers the total price.
Demand reinforces it. The travellers who most often need a genuine one-way tend to be less able to shop around, and fares reflect that.
Does every airline do this?
No, and it is worth checking rather than assuming. Some carriers price each leg independently, so a return is simply two one-ways added together. That is common among low-cost carriers. The pattern is strongest on long-haul international routes flown by full-service airlines, and weakest where a low-cost carrier competes directly.
What is an open-jaw ticket?
An open-jaw is a single ticket that flies you into one city and home from another. The gap in the middle is yours to cross however you like. Airlines sell these as multi-city itineraries, and because the ticket is still constructed as a round trip, it often prices far closer to a return than to two separate one-ways.
Most flight search tools expose this behind a multi-city or multi-stop option rather than the default return search, which is the main reason travellers never find it.
Why does distance not decide the price?
Because airlines price by market, not by miles flown. A city pair with little competition can cost more than a longer one with several carriers fighting over it. This is why an itinerary with a connection sometimes prices below the non-stop on the same route, and why comparing fares by flight time is unreliable.
Why the same seat can carry two prices
Airlines in an alliance sell one another’s flights, and each sets its own fares for what it sells. The same seat, on the same aircraft, on the same day, can price differently depending on which airline issues the ticket. Checking the operating carrier and the alliance partners is a straightforward way to find that difference.
Why this matters if you need an onward booking
People who need an onward or return booking often reach for a one-way, find it expensive, and start looking for a way around the requirement. The fare structure above is the reason it looked expensive, and it usually points at a cheaper legitimate answer: a return, an open-jaw, or a refundable fare that can be cancelled if plans change.
See proof of onward travel for what airlines actually check and why, and held and refundable bookings for the mechanisms that let you hold a real booking you can undo.