Your smartphone is a highly advanced piece of modern technology, jam-packed with wi-fi, GPS, a brilliant camera, Bluetooth, motion sensors and much, much more.

However, to make calls, use mobile data and send cellular text messages, you’ll still need to insert a SIM card to connect with your network operator.

The SIM card is that small, fingernail-sized piece of plastic that contains a small microchip which holds your account information. Its full name is Subscriber Identification Module, and it informs the phone that you have an active network service.

Three BT Sim cards

If you’re buying a phone directly from a network or a third-party retailer, the chances are they’ll hook you up with the right SIM card there and then.

However, if you are opting for a SIM-only deal such as with BT Mobile, you’ll need your own handset. Whether you are buying a second-hand phone, inheriting one from a friend or picking one up SIM-free from a retailer in order to choose, you’ll need to ensure you have pick up the corresponding card.

Here’s a guide to the type of SIM cards available and how to suss out which you need.

[Read more: The basics of buying a smartphone]

Types of SIM card

There are generally three types of SIM card still in use today. There’s a Standard SIM, a smaller Micro SIM and an even smaller Nano SIM. The type you’ll need depends entirely on the handset you’ve bought, although there are ways you can make a SIM work with whatever phone you have.

Standard SIM

The oldest of the three types of SIM is generally used in all handsets prior to 2010 and plenty issued thereafter. It is the largest of the three SIMs types, measuring 25x15mm.

It’s now rare to see a Standard SIM card slot in new devices, although it is sometimes found on basic feature phones.

Micro SIM

The only real difference between the SIM standards is the size of the plastic around the microchip. The Micro SIM was developed as part of the perennial quest to make smartphones slimmer and lighter and measures 15x12mm.

Micro SIMs, introduced in 2010, still appear in some new handsets.

Nano SIM

Many newer smartphones now use the Nano SIM standard introduced in 2012. The newest devices from Samsung and others all feature this slot which measures 12.3x8.8mm

Combi SIM

The Combi SIM is a Micro SIM card within a Standard SIM-sized unit, so if you need the smaller size of card you simply break the additional plastic away from around the Micro SIM card. This type of card allows networks to cater for more phone users with a single card. Most of the pay-as-you-go SIM cards you buy from convenience stores operate under this format as they’re more useful to more users.

How do I know which SIM to buy? 

Standard, Micro and Nano SIM slots

Make sure you order the right SIM for the mobile network you intend to use.

If the phone comes with a box and instructions, you’ll be told which SIM card you need to use, and some phones have the type of SIM card they use written on the slot.

If you’re not sure, a quick Google search for your model of smartphone will tell you which card you need for your phone - the manufacturer’s website is a good place to start.

[Read more: Never lose your phone again]

How do I know which SIM I have?

If you already have SIM cards knocking around and want to know which you have, compare your cards with the image below. With Standard and Micro SIMs the chip is always the same size, so it’s the amount of plastic surrounding it that determines which type of SIM you have. Nano SIMs are slightly smaller.

Standard, Micro and Nano SIMs

Modifying your SIM

SIM adapters

If you have a Micro SIM card and wish to use it in an older handset, buy an inexpensive adapter, which can be picked up from retailers like Amazon for around £1. You can convert a Nano SIM to a Micro SIM and create a Standard SIM from a Nano or Micro SIM.

There’s very little science to these adapters. Effectively, they provide the extra plastic necessary to make the card sit securely in your phone’s SIM card slot.

Chopping a SIM down to size

If your SIM card is too big for your phone’s SIM card slot, a bit of DIY can come to the rescue. While you need to be careful, you can turn a Standard SIM into a Micro or Nano SIM simply by trimming it. This can save you getting a new SIM when you upgrade phones to a model which uses a different card standard.  There are a host of tutorials you can follow online and on YouTube, but do so at your own risk - if you damage the chip, the SIM won’t work at all.

Thinking about moving to BT Mobile? Discover how easy it is

BT Mobile has a range of SIM only deals, with up to 50% off for BT Broadband customers.