Gist: When your trip naturally ends in a different city, compare flying into A and home from B against a same-city return, then price the overland middle before you celebrate any “cheap” fare.
Prices checked: 3 September 2026
Planning a long weekend on a lunch break? It is tempting to search “London to Lisbon return”, book the cheapest tile, and force the holiday to match the airports. That habit is the trap. If your plan already runs Lisbon to Porto, a same-city return quietly adds a second journey: Porto back to Lisbon, plus transfers, luggage, and often an early hotel night or a painful pre-dawn start.
For thrifty travellers, that backtrack is not free. It costs money, hours, and the one thing a short trip cannot spare: a usable day. The better first question is not “What is the cheapest flight?” It is “Where should this trip start and finish?”
Think in shapes
An open-jaw trip arrives in one city and leaves from another. In our worked example that is the UK into Lisbon, then Porto back to the UK. The two airports are the open ends of the journey.
A multi-city search usually puts both flights into one search and, if the airline offers it, one booking: UK–Lisbon and Porto–UK. It is a neat way to compare the whole shape at once. It is not mandatory. If multi-city is pricey, awkward, or unavailable, two separate one-ways with the same airports and dates are a valid Plan B.
Two one-ways give more choice of airline, airport and date. They also mean separate-change risk: if the outbound airline shifts its timetable, the inbound booking will not automatically protect you. Check bags, rules and connection plans separately too.
Open-jaw is not always cheaper. Sometimes the same-city return still wins on the flight total alone. The point is to compare the full door-to-door shape (flights, middle leg, and the backtrack you would otherwise buy) before you lock anything.
A worked shape
Let’s use Monday 5 to Sunday 11 October 2026 as a planning window: fly from London to Lisbon, spend a few days there, take the train to Porto, then fly from Porto back to the UK.
Do not begin with a single headline airfare. Search these two shapes for the same dates:
- Return: London–Lisbon–London.
- Open-jaw or multi-city: London–Lisbon, then Porto–London.
Now price the middle: Lisbon to Porto. CP’s Alfa Pendular fare table lists €35.70 for a one-way Tourist-class ticket from Lisbon (Santa Apolónia or Oriente) to Porto Campanhã, or about £30.66 at the 3 September 2026 Xe rate. Its published timetable examples show the journey taking about 2 hours 50 minutes.
CP also advertises limited Promo seats, with discounts of up to 65% when bought at least eight days ahead, but the cheapest places can sell out. Treat €35.70 as the clear reference price, not a promise of what your particular train will cost.
| Trip shape | What you compare | Cost to remember |
|---|---|---|
| Return to Lisbon | Two flights, then Porto–Lisbon | €35.70 train reference, plus the backtrack |
| Lisbon in, Porto out | Two flights and Lisbon–Porto | €35.70 train reference |
The table is deliberately incomplete: airfares move, and we are not pretending a live checkout price is fixed. Exact London–Lisbon or Porto–UK fares are omitted on purpose. The useful comparison is the return ticket’s saving against the cost of returning to Lisbon.
Add the train or coach, getting across the city, luggage, a possible overnight stay and the value of several hours. A headline fare can stop looking cheap once it has a door-to-door price.
Search it properly
First, search the return to one city. Then search the open-jaw or multi-city version. If the multi-city result is expensive or unavailable, price two one-ways with the same airports and dates.
Next, price the middle leg on its own. Compare a direct train with a coach or a low-cost flight, but include station or airport transfers, cabin-bag rules and time.
Rail often wins when it takes you from a city centre to a city centre and avoids a second security queue. A low-cost flight may win on the ticket alone, but not after transfers and add-ons.
Finally, price the avoided backtrack. Would you need to leave Porto before breakfast? Would you pay for a Lisbon night before a morning flight? Would you lose a useful day?
Put those costs beside the two flight totals. You are buying a route through places, not two isolated tickets.
Keep entry rules boring
Lisbon and Porto are both in Portugal and in the Schengen area. Their domestic train journey does not normally involve a new international border check.
That does not remove entry rules. For a full British citizen passport, GOV.UK says Portugal follows Schengen passport-validity rules and visa-free tourist travel is generally limited to 90 days in any 180-day period.
Rules depend on your nationality, passport and purpose, so check the official advice before booking and carry the documents you need. Do not invent onward-ticket stories from forum posts. Use the government’s own pages for your passport.
The mistake to avoid
The common mistake is treating the return airport as the centre of the trip. A cheap return to Lisbon may be perfectly sensible if you are staying in Lisbon.
It is less convincing when your plan ends in Porto and the “saving” buys a long backtrack, extra transport, an early start and less time travelling well.
Rome in, Milan out
Use the same worksheet for northern Italy. Rome and Milan are both in Schengen, so the overland middle is usually a domestic rail or coach hop rather than a new border crossing. Still check current GOV.UK advice for Italy before you book.
Build three tabs for the same dates:
- Return into Rome and home from Rome.
- Open-jaw or multi-city: into Rome, home from Milan (or the reverse if your route runs north to south).
- Two one-ways if the bundled multi-city price looks odd.
Then price the middle on official rail or coach sites. Prefer centre-to-centre timings over “flight plus two airports”. We are not quoting live Rome–Milan fares or journey times here because they move with date, train type and sales.
Your job is the same door-to-door sum: flight difference + middle leg + the backtrack you avoid (extra hotel night, dawn transfer, lost day). If the open shape costs more after that maths, keep the Rome return. Open-jaw is a tool, not a rule.
Bangkok in, Singapore out
Southeast Asia loves this shape because many travellers want Thailand and then a short hop south. Bangkok in and Singapore out can work as a two-country plan, and neither example needs a round-the-world ticket.
Treat it as a planning framework, not a fare promise:
- Price a Bangkok return against Bangkok in / Singapore out (multi-city or two one-ways).
- Price the middle: train, bus, or a short flight, including transfers and baggage rules.
- Check entry for each country separately through official sources for your nationality. Thailand and Singapore are not one Schengen-style zone; visa rules, passport validity and any stay limits can differ.
- Confirm what each airline or border page actually asks for on exit or onward travel. Do not rely on invented “you must show a return ticket” folklore. If an official page asks for proof of onward travel, follow that page, not a screenshot from a forum.
Again, we omit invented airfares and bus prices. The thrifty win is the comparison method: full shape versus same-city return, with the middle and the avoided backtrack written down before you pay.
Nomad Tip
Search the route in three tabs: return, open-jaw or multi-city, and two one-ways. Save the total door-to-door cost, not just the flight headline, and check it again before paying.
Flexibility is useful only when you know which booking carries the risk.
The Thrifty Gist
- When your itinerary ends in a different city, compare flying into A and home from B before you book a same-city return.
- Open-jaw is not always cheaper; multi-city is handy but not mandatory. Two one-ways are fine if you accept separate-change risk.
- Lisbon–Porto reference (planning window 5–11 Oct 2026): CP Alfa Pendular Tourist €35.70 ≈ £30.66 (Xe, 3 Sep 2026), about 2h50. Treat as a clear rail floor, not a locked quote.
- Price the middle and the avoided backtrack (hotel night, dawn transfer, lost day) beside both flight totals.
- Rome–Milan and Bangkok–Singapore use the same worksheet; cross-border pairs need official entry checks per country. No invented onward-ticket rules.