Three mechanisms, not three prices
Carrier roaming extends your existing plan onto a foreign network. Nothing changes on the phone, your number keeps working for calls and texts, and you pay a daily or package fee for the privilege. It is the option that requires no decisions and it is normally the most expensive.
A travel eSIM is a second cellular profile downloaded to the handset, usually as a QR code scanned before departure. It carries data on a local or regional network at prices set by a competitive market of resellers rather than by your carrier. On a phone that supports eSIM alongside its physical SIM, your home line can stay switched on for incoming texts while the eSIM carries the data, which is why it has become the default answer for most travellers.
A local prepaid SIM is bought on arrival and put in the tray. It is generally the cheapest way to buy a large amount of data and it gives you a local number, which is genuinely useful for restaurant bookings, delivery apps and taxi drivers. The cost is that your home SIM is in your pocket rather than in the phone, so for the duration of the trip your usual number reaches nobody.
Estimating the data honestly
The usual failure is guessing low. Maps navigation running while you walk a city, messaging with photos, a few searches and some social scrolling adds up to something like half a gigabyte to a gigabyte on a full day out. Video, video calls and uploading photo albums push it to two or three. Against that, hotel and cafe Wi-Fi removes a large share of it, and downloading offline maps for the city before you leave removes most of the navigation load.
Because packages are sold in fixed sizes, the practical approach is to estimate, then buy the next size up rather than the closest fit. Running out mid-trip means buying a second package at a worse rate per gigabyte, which is exactly the penalty this calculator flags when the package you entered is smaller than your estimate.
Sharing by hotspot
For a group, one data line shared by hotspot is usually far cheaper than one line each, and the calculator will show that if you compare a headcount of one against your real headcount. Two things temper it. Hotspotting drains the host phone quickly enough that a power bank stops being optional, and some travel data products restrict or throttle tethering, so it is worth checking the terms rather than assuming. The other consideration is that the group has to stay together to stay connected, which is fine for a family and unhelpful for people splitting up during the day.
The condition that decides it more often than price
Whether you must receive calls and texts on your home number is usually the deciding input, not the money. Bank verification codes, two-factor SMS, a work line and family who will call the number they know all point the same way. Roaming handles it by definition. An eSIM handles it as long as the home line stays enabled, which for received texts is generally free but for received calls may not be, and that is a rate to check with your carrier before you go rather than discover afterwards. A local SIM cannot handle it at all.
If codes are the whole reason, the durable fix is to move accounts off SMS verification and onto an authenticator app, which works over any data connection and does not care which SIM is in the phone. That is a job for the week before the trip, not the airport.
Before you leave, in order
Check that the handset supports eSIM if that is the plan, by looking for the option to add a cellular plan in settings rather than trusting a model number. Buy and install the travel eSIM at home while you still have a working connection, since installation needs data and airport Wi-Fi is an unreliable place to discover a problem. Turn off data roaming on the home line. Download offline maps. And set the travel line as the default for data but leave the home line active for texts if you need them. Doing all five takes twenty minutes and removes every one of the ways this goes wrong.
Questions people ask
How much data will I use on a week abroad?
For maps, messaging, searching and light social use while out most of the day, something in the range of half a gigabyte to a gigabyte a day is a reasonable planning figure, so five to seven for a week. Video streaming, video calls and uploading large photo sets can double or triple it. Wi-Fi at the hotel changes the picture more than anything else, since it takes the evening entirely off the cellular line. The reliable way to find your own number is to look at what your phone reported for cellular data on a recent week at home when you were out and about, and adjust for the fact that you will navigate far more in an unfamiliar city.
Will using a travel eSIM change my phone number?
No. An eSIM adds a second line with its own number for data; your existing number stays on the handset as long as you leave that line enabled, and messaging apps that are already signed in keep working over whichever connection has data. What does change is which line sends outgoing texts and calls if you are not paying attention, so set the default line deliberately. If an app asks you to re-verify a phone number while you are abroad, decline it and deal with it at home, because verifying against the wrong line is the one way this genuinely does cause trouble.
Why do I get a huge bill even though I barely used my phone abroad?
Because pay-as-you-go roaming is charged at a rate that assumes you did not mean to use it, and phones use data in the background constantly: mail syncing, cloud photo backup, app updates and location services all run without you touching the screen. If data roaming is left on and no pass is active, that background traffic meters at the standard rate. The defence is to turn data roaming off at the system level before you land rather than relying on being careful, and to switch it on only for a line that is covered by a pass or a travel plan.
Is an unlimited travel plan really unlimited?
Nearly always it means a daily allowance at full speed followed by reduced speeds for the rest of that day, with the allowance resetting the next day. That structure suits a traveller reasonably well, since the busy part of a day is usually navigation and messaging rather than sustained video. What it does not suit is tethering a laptop for work or streaming in the evening, both of which will hit the threshold early. Read the daily figure rather than the word unlimited, and if the plan is for working days rather than sightseeing, price a metered package with enough gigabytes instead.