SIM cards come in three different sizes: Standard SIM, Micro SIM and Nano SIM. The right size of SIM card to fit in your handset will depend on the manufacturer and the model of your smartphone.
As of 2015, most new smartphones are either using Micro SIM or Nano SIM.
Handsets requiring a Micro SIM include the Galaxy S5, Galaxy Note 4, LG G4 and Moto G. If you’re changing to a smartphone which requires Micro SIM, you might need to change the size of your SIM card. You can order a micro-sized SIM card directly from your mobile network.
Handsets requiring a Nano SIM include the latest flagship smartphones from Apple and Samsung (e.g. the iPhone 6s, iPhone 6s Plus, Galaxy S6 and iPhone 5s). The Xperia Z5 and HTC One M9 also use nano-sized SIM cards. You might need to change the size of your SIM card when upgrading to a handset which uses Nano SIM. You can order a nano SIM directly from your network.
All three SIM card types work in the same way from a technical perspective: they only differ in the amount of plastic housing that surrounds the metallic chip.
If you’re willing to take the risk, it’s possible to cut your Standard SIM down to Micro SIM dimensions (e.g. with a Micro SIM cutting tool). However, we’d normally advise against doing this: instead, it’s easier and safer to order a Micro SIM directly from your mobile network. It’s normally free and you won’t risk invalidating your phone’s warranty with a badly-cut Micro SIM.
It isn’t possible to cut your own Nano SIM. This is due to the differences in thickness between Nano SIM and other types of SIM card.