SIM cards: Everything you need to know

8 August 2020


We should all be familiar with the little SIM cards that accompany every new mobile phone contract and PAYG package by now. These little plastic chips have been a constant of mobile phone ownership ever since the ’90s.

But what exactly are these SIM cards, and what do they do? Why do they come in different sizes, and do you need another new SIM card to connect to the 5G network?

For answers to these questions and much more, read on.

What is a SIM card?

SIM cards are the tiny strips of plastic that each mobile network operator provides you with, and which must be inserted into your mobile phone in order to get your unique phone number working on that device.

The term SIM is an acronym that stands for Subscriber Identity Module. Each SIM card contains a unique serial number (ICCID), an international mobile subscriber identity (IMSI) number, several security authentication methods, and any temporary information needed to connect your phone to your chosen mobile network.

SIM Card deals compared

SIM card types

SIM card sizes compared

SIM cards haven’t always been the same shape and side. In fact, the very first SIM released in 1991 was about the size of a credit card. Today’s Nano SIMs, by contrast, are much smaller than a postage stamp.

Here are all of the different varieties of SIM that you may have encountered in recent years.

Standard SIM

Though it’s called the Standard SIM, it’s no longer anything of the sort. Released in 1996, this is the original 25mm x 15mm SIM that most of us will have used in our first feature phones. The Standard SIM remained a staple right up into the smartphone era, including the first few iPhones.

Micro SIM

Measuring 15mm x 12mm, the Micro SIM is the next size down from the Standard SIM. Having launched in 2003, the Micro SIM is fairly close to the current Nano SIM in both shape and size. But as the Nano SIM managed to shave the chip housing down even further, it’s all but obsolete in 2020. If your smartphone is newer than five years old, then it probably won’t use this any more.

Nano SIM

The SIM variety that virtually all of us use today. Debuting in 2012, the Nano SIM measures just 12.3mm x 8.8mm, making it around half the size of the Standard SIM. At this point, the plastic housing is only slightly larger than the chip itself, so this is about as small as the traditional SIM format is likely to get.

Do I need a special SIM for 5G?

No, you don’t need a special 5G SIM. Your existing SIM card will enable you to access the UK’s 5G networks just fine.

SIM cards are not what determines your phone’s technical ability to connect to a network. This facility is instead dependent on the type of modem that’s hard-wired to each individual smartphone’s circuit board.

What are eSIMs then?

The brutal truth about SIMs is that they’re a creaky piece of legacy technology. Indeed, they’re technically unnecessary. Electronic SIMs, or eSIMs, can now be embedded within a device, and any subsequent network or number change can then be handled through a straight forward software update.

Modern iPhones and Pixel phones contain eSIMs (though they also contain normal SIM slots), and the likes of EE and Vodafone already support their usage. You’ll also find them in Apple’s cellular iPads.

Many expect the wider smartphone industry to eventually adopt them and to phase out physical SIMs altogether, as a further space and materials-saving move. It’s also more practical to run multiple numbers from a single phone with eSIMs.

Why do different sized SIM cards exist?

While the modern SIM as we know it has come in three basic sizes - the Standard, Micro and Nano trio mentioned above - the actual chip part of the card that holds your data remains the same size across all three. All that’s different is the piece of plastic that frames it.

This plastic has been whittled down over the years as mobile phone design has grown more sophisticated and internal space more precious. Manufacturers like Apple and Samsung require every spare millimetre for increased battery capacity, more sophisticated cameras, and other important features.

Is there any other difference between them?

No, just the size that they come in. Everything else about them is identical.

How do I know which SIM card I require?

If you’re buying a new smartphone, it will almost certainly be a Nano SIM. If you want to make sure, though, you will find information on your phone’s specific SIM type in the included paperwork.

You can also find your model’s specification information on the manufacturer’s website, where SIM type should be included.

How do I access my phone’s SIM?

Most phones have a dedicated SIM tray accessed with a SIM tool that comes bundled with the phone. Look for a little pin-shaped implement in the box of your phone.

The SIM tray itself will typically look like a little flat hatch on one of the edges of your device. There should be a little hole on or immediately to the side of this hatch. Take the SIM tool - or an equivalent long thin piece of metal, like an unrolled paperclip - and insert it into this hole. Push firmly until the tray pops out.

Your SIM should slot closely into one side of this SIM tray. Use the angled corner as a guide, and make sure the SIM card is lying flat when you reinsert the tray.

Make sure your phone is turned off before you remove or insert a SIM.

What are SIM adaptors?

Because the chip part of the SIM is uniform across all three sizes, it’s possible to use a Nano SIM in a phone that takes a Micro or Standard SIM. It’s also possible to use a Micro SIM in a phone that takes a Standard SIM.

All you need is a SIM adaptor. This is simply a piece of plastic that the Nano or Micro SIM slot into to form the shape of a larger SIM. Head into your local network shop, and they should be able to supply you with one of these SIM adaptors, quite possibly for free. Alternatively, you can buy one from all good phone accessory outlets, both physical and online.

Is it possible to physically downsize a SIM card?

Because the key chip part of the three SIM types is essentially identical, it is technically possible to cut one of the two larger types down to fit into a Micro or Nano-sized smartphone. But we wouldn’t recommend it.

The fit is quite tight in the SIM tray of a modern phones, so you can’t afford to be off by even a fraction with your cutting. It would be far easier and more secure to ask your network for a new SIM card and to switch your number to it.

SIM Card deals compared


What exactly are the different SIM card sizes?

In order of size and age, starting with the oldest: Standard (25mm x 15mm), Micro (15mm x 12mm), and Nano (12.3mm x 8.8mm).

Which is the most common sized SIM card?

Nano SIMs are by far the most common these days. Any smartphone released over the past five or six years will almost certainly use this SIM standard.

How do I know if I have a 5G SIM?

All SIM cards are 5G-ready. What determines whether you can access 5G or not is your network operator’s coverage, the type of smartphone you’re using, and the plan or contract you’re on.

What is an unlocked SIM free phone?

An unlocked SIM free phone is a phone that you buy outright, without a SIM (hence SIM-free) rather than as part of a contract deal. Because it’s not locked to any one network, you can insert your own SIM from any network provider, and it will work.

Will any SIM fit in my phone or device?

No. If your phone was released any time over the past five years, it will almost certainly take a Nano SIM. That being the case, either of the larger types won’t fit.

If your phone is older and accepts one of the larger SIM types, you could fit a smaller SIM using a SIM adaptor.

What is a Combi SIM?

Networks will often supply Combi SIMs when supplying a new SIM. These are SIMs that come in a nested format with perforated edges, allowing you whittle them down according to the size of SIM card required.

How do I convert a standard SIM to a Nano SIM?

It is possible to cut a Standard SIM down to Nano size, but we wouldn’t recommend it. Instead, visit or contact your network operator and ask for a replacement Nano SIM with your phone number. They should be able to arrange for a swift change over.

What is a Mini SIM card?

The Mini SIM was an in-betweener SIM card size that sat in between the familiar old Standard SIM and the current Nano SIM, though it’s much closer to the latter in shape and size. It’s currently all but obsolete.

What happens if I put my SIM in another phone?

If a phone is unlocked, your SIM should work fine in it. If it’s locked to a different network, it won’t work. If you purchased the phone outright rather than as part of a contract, it should be unlocked. But these days, even many phones that were obtained on a contract remain unlocked. Check with the operator.

What is a SIM card adaptor and where can I get one?

A SIM adaptor is a piece of plastic the shape of a larger, older SIM size, which you can slot a smaller SIM type into. This effectively converts the smaller SIM for use on older phones.

Your network operator should be able to supply you with one of these, quite possible for free. Alternatively, you’ll be able to find them in any good phone accessory provider.

Are SIM cards universal or not?

The SIM standard is a global one, so should fit into a phone from any country. However, network operators can and often do lock rival network SIM cards out, so it’s important to check before you buy.

Jon Mundy
About Jon Mundy

Technical Writer at

Jon has nine years experience of writing and editing copy for leading publications, as well as attending technology shows and events and conducting interviews. Currently working with, TechRadar, Trusted Reviews, T3, Digital Spy, What Mobile, Pocket Gamer, and The Gadget Show.

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