Everything you need to know about SIM cards

SIM stands for subscriber identity module or subscriber identification module . It follows that a SIM card contains unique information that identifies it to a specific mobile network, allowing a subscriber (like you) to use the device’s communication features.

Without the SIM card inserted and working properly, some phones cannot make calls, send SMS messages or connect to mobile Internet services (3G, 4G, etc.)


SIM also means ‘simulation’, and could refer to a video game that simulates real life.

What is a SIM card used for?

Some phones need a SIM card to identify the owner and communicate with the mobile phone network. So if you have, say, an iPhone on Verizon’s network, you need a SIM card so Verizon knows the phone belongs to you and you’re paying for the subscription, but also for certain features to work.

The information in this document should apply to both iPhone phones and Android phones (it doesn’t matter who made your Android phone: Samsung, Google, Huawei, Xiaomi, etc.).

You may have found yourself in a situation where you receive a used phone that is missing a SIM card and soon find that it functions as nothing more than an expensive iPod. While you may be able to use the device on Wi-Fi and take photos, you cannot connect to any carrier’s mobile Internet network, send text messages, or make phone calls.

Some SIM cards are mobile, meaning that if you put them into an upgraded phone you just bought, the phone number and carrier plan details will “magically” start working on that phone. In that sense, if your phone runs out of battery and you desperately need to make a phone call, and you have a spare, just put the SIM card in the other phone and use it immediately.

The SIM card also contains a small amount of memory that can store up to 250 contacts, some SMS messages, and other information used by the carrier that supplied the card.

In many countries, SIM cards and devices are locked to the carrier from which they are purchased. This means that while a SIM card from one carrier will work in any device sold by that same carrier, it will not work in a device sold by another carrier. Unlocking a cell phone is usually possible with the help of the carrier.

Does my phone need a SIM card?

You may have heard the terms GSM and CDMA in connection with your smartphone. GSM phones use SIM cards, while CDMA phones do not.

If you’re on a CDMA network like Verizon Wireless, Virgin Mobile, or Sprint, your phone may use a SIM card, but the identification features explained above are not stored on the SIM card. This means that if you have a new Verizon phone that you want to start using, you can’t just put your current SIM card in the phone and expect it to work.

So, for example, putting the SIM card from your broken Verizon iPhone into a working iPhone doesn’t mean you can start using the new iPhone with Verizon. To do so, you have to activate the device from your Verizon account.

In these cases, with CDMA phones, the SIM card is most likely used because the LTE standard requires it, or because the SIM slot can be used with foreign GSM networks.

However, the SIM card of GSM phones can be swapped with other GSM phones without problem, and the phone will work just fine on that GSM network that the SIM card is linked to, such as T-Mobile or AT&T.

This means that you can remove the SIM card from one of your GSM phones and put it in another and still use the data, phone number, etc. from your phone, all without having to get approval from the carrier like you have to when you use Verizon, Virgin Mobile or Sprint.

Originally, cell phones that used the CDMA network instead of the GSM network did not use a removable SIM card. Instead, the device itself would contain the identification numbers and other information. This meant that the CDMA device could not be easily changed from one carrier network to another, and could not be used in many countries outside of the United States.

More recently, CDMA phones have begun to feature a removable user identity module (R-UIM). This card looks almost identical to a SIM card and will work in most GSM devices.

How is a SIM card?

A SIM card looks like a small piece of plastic. The important part is a small embedded chip that can be read by the mobile device it is inserted into, and contains a unique identification number, phone number, and other data specific to the user it is registered for.

Early SIM cards were about the size of a credit card and were the same shape all around. Both Mini and Micro SIM cards now have a cut corner to help prevent incorrect insertion into the phone or tablet.

Here are the dimensions of the different types of SIM cards.

  • Full SIM – 85mm x 53mm
  • Mini SIM – 25mm x 15mm
  • Micro SIM – 15mm x 12mm
  • Nano SIM – 12.3mm x 8.8mm
  • Embedded SIM – 6mm x 5mm

If you have an iPhone 5 or later, your phone uses a Nano SIM card. The iPhone 4 and 4S use the larger Micro SIM card.

Samsung Galaxy S4 and S5 phones use Micro SIM cards, while Nano SIM is required for Samsung Galaxy S6 and S7 devices.

See the SIM Card Sizes table for Local SIM to find out what type of SIM card your phone uses.

A Mini SIM card can be cut to make it a Micro SIM, as long as only the surrounding plastic is cut.

Despite the differences in size, all SIM cards contain the same types of identification numbers and information on the small chip. Different cards contain different amounts of memory space, but this has nothing to do with the physical size of the card.

Where can I get a SIM card?

You can get a SIM card for your phone from the company you subscribe to. This is usually done through customer service.

For example, if you have a Verizon phone and need a Verizon SIM card, you can order one at a Verizon store or order a new one online when you add a phone to your account.

How can I remove or insert a SIM card?

The process to replace a SIM card varies depending on your device. It can be stored behind the battery, which is only accessible through a panel on the back. However, some SIM cards are accessible on the side of the phone.

The SIM card for your specific phone may be one where you have to pry it out of its slot with something sharp like a paper clip, but others may be easier to remove where you can slide it with your finger.

If you need help changing the SIM card on your iPhone or iPad, Apple has instructions here. Otherwise, check your phone’s support pages for specific instructions.