On most legacy international routes a round-trip is cheaper than two one-ways. On low-cost carriers and many US domestic routes, two one-ways cost the same and give more flexibility. Always price both, and consider mixing airlines with two one-ways.
It depends on the airline model:
Legacy/full-service, especially international: round-trip fares are often significantly cheaper than buying two one-way tickets on the same airline. Historically one-ways were priced punitively; that’s eased but round-trips still usually win.
Low-cost carriers (Ryanair, easyJet, Southwest, Spirit) and much of US domestic: fares are priced per direction, so two one-ways cost the same as a round-trip — and let you fly out and back on different airlines to get the best of each.
When one-ways win: open-jaw trips (fly into one city, out of another), uncertain return dates, or mixing a cheap outbound on one carrier with a cheap return on another.
The only way to know is to price it both ways in the search — it takes 30 seconds and occasionally saves a lot.
Related questions
How do I find cheap flights?
Search flexible dates on Google Flights and Skyscanner, check nearby airports, set price alerts, consider one-stop routings and stopover programs, and compare the total cost with bags and seats — not the headline fare. Full answer →
When is the best time to book a flight?
There is no magic day. For most routes, book 1–3 months ahead for domestic and 2–6 months for international, earlier for peak dates. Track prices with alerts and buy when the fare hits a target you set in advance. Full answer →
Airline, airport and government rules change without notice. Treat this as a starting point and confirm with the airline or authority for your specific trip. Request a correction if something is out of date.