How we know: Sources are listed at the end. This post explains fare structure and gives a method to compare; it does not quote fare figures, because the answer differs by route and date and you should check yours.
The old rule that a round trip is always cheaper than two one-ways is no longer reliable, but it has not flipped either. On domestic US routes and many short-haul international ones, one-way fares are now commonly priced at roughly half the return. On long-haul international routes, especially on legacy carriers, a one-way can still cost nearly as much as a round trip. The only way to know for your trip is to price both, and it takes two minutes.
Why the two can differ
Airlines file fares as sets of prices and rules attached to booking classes (Airline reservations system). Historically, cheap international fares carried rules requiring a round trip and a minimum stay, because that separated leisure travellers from business travellers who would pay more for a one-way. Low-cost carriers never priced that way; they sell each direction independently. As legacy airlines matched them on competitive routes, one-way pricing spread, but plenty of long-haul fare rules still penalise a one-way.
When two one-ways win
- Different airlines each direction. The cheapest outbound and the cheapest return are often on different carriers. Two one-ways let you take both.
- Open-jaw trips. Fly into Paris, home from Rome. Some airlines price this as a single ticket; often two one-ways or a "multi-city" search is cheaper.
- Mixing a low-cost carrier with a legacy carrier. Out on a budget airline, back on a full-service one when the budget return is inconvenient.
- Unsettled return dates. Book the outbound now, the return when your plans firm up, without paying a change fee.
- Domestic and short-haul routes, where per-direction pricing is the norm.
When the round trip wins
- Long-haul on legacy carriers, where fare rules still make the one-way disproportionately expensive.
- When you want a single record for protection. On one ticket, a delay on the outbound that causes a missed connection is the airline's problem; on two separate tickets, it is yours.
- Checked-bag economics. Two one-way tickets on a budget carrier mean paying baggage fees twice, and the fees can exceed the fare difference.
- Corporate or award bookings that price as returns.
The protection trade-off
Separate tickets are separate contracts. If the first flight is late and you miss the second, the second airline owes you nothing; you are buying a new ticket at walk-up prices. Leave generous connection time (ideally an overnight) between separately ticketed flights, and remember that the US 24-hour rule lets you cancel a ticket bought at least seven days out within 24 hours for a full refund when booked directly with the airline (Thrifty Traveler), which gives you a day to reconsider a two-ticket plan.
How to check in two minutes
- Search the round trip on Google Flights for your dates and note the total.
- Search the outbound as a one-way and the return as a one-way (reverse the airports) and add them up.
- Repeat step 2 with "multi-city" if the trip is open-jaw.
- Add baggage fees for whichever option involves a low-cost carrier.
- Compare. Check the route's current fare on its page, for example JFK to Paris, LAX to Barcelona or San Francisco to Tokyo, and set a price alert on the direction that is currently expensive.
Points and miles
Most loyalty programmes now price awards one-way at half the round-trip rate, which makes a cash outbound plus an award return (or the reverse) a common way to use a modest points balance on the expensive direction.
FAQ
Are one-way international flights more expensive? Often on legacy carriers for long-haul routes, rarely on low-cost carriers, and increasingly not on competitive routes. Price both.
Is it risky to book two one-way tickets? The risk is a missed connection between separately ticketed flights. Build in time, or avoid tight connections between them.
Can I book a one-way and decide the return later? Yes. Watch the return with an alert; if the one-way return stays expensive, a round trip from the other end is sometimes cheaper and you simply throw away the unused segment. Check the airline's rules before you do that.