What is a SIM card? Types, sizes, and how it works

What is a SIM card?

A SIM (Subscriber Identity Module) card is a small, removable card used in mobile devices to connect to cellular networks. It’s a piece of plastic with a metallic chip at its core, but don’t be fooled by its humble appearance — this component securely stores important information, such as your phone number and carrier details.

The SIM card serves as your phone's connection to the carrier's network. Every time you make a call to a friend, send a message, or browse the web using mobile data, your SIM card is working behind the scenes to link you to your carrier’s network. Depending on your device, you may need different sizes of SIM cards, such as standard, micro, or nano, each progressively smaller than the last.

What does a SIM card do?

A SIM card connects your phone to a cellular network, storing key details like your phone number, account information, and authorization so you can make calls, send texts, and use mobile data. So basically it handles three big jobs: identification, connectivity, and storage.

Each SIM card carries unique identifiers — IMSI (International Mobile Subscriber Identity) and ICCID (Integrated Circuit Card Identifier). The IMSI is how your carrier knows who you are, ensuring your calls, texts, and data are linked to the correct account. For example, if you stop paying your bill, your IMSI is the code that allows your provider to block you from its network. The ICCID, on the other hand, tells mobile operators which network your SIM should connect to, directing your device to the right provider.

The SIM card is also responsible for network connectivity. It links your device to your carrier’s network, and it tells your phone where to connect. Whether you're using cellular data at home or roaming abroad, your SIM card ensures your phone can always find a signal.

Finally, even though SIM cards don’t offer much storage space, they’re efficient in what they keep. They hold your phone number, some settings, and even a few contacts, making sure you stay connected and organized.

What does a SIM card store?

Your SIM card stores a handful of information, but don't expect it to hold your vacation photos — it has specific tasks and sticks to them. 

• A SIM card holds some personal data. Before smartphones, SIM cards were the go-to spot for saving your contacts and text messages. While most of that data now lives in the cloud or on your phone’s internal storage, your SIM can still store a limited number of contacts and a few SMS messages.

• A SIM card is a secure vault for network data. It stores your IMSI, ICCID, and essential carrier settings, which allow your phone to connect to the network and ensure that your calls and texts stay protected.

But remember that SIM memory is not the same as your device’s storage or cloud backups. Your photos, apps, and most messages won’t touch the SIM — they’re kept on the phone or synced online.

Advantages and disadvantages of SIM cards

SIM cards offer several key benefits that have made them indispensable in mobile communication. However, like any technology, they come with both strengths and limitations:

One of the biggest perks of a SIM card is how easy it is to move around. Pop it out of one phone, slide it into another, and your number and plan can travel with you.

That said, SIM cards aren’t perfect. For example, a damaged SIM can disrupt your ability to connect to a network, leaving you without service. With the rise of eSIMs, many of these issues are disappearing, but physical SIMs still play an important role and remain a familiar, dependable choice for millions of people.

SIM card sizes and types — a complete breakdown

Since their debut in the early 1990s, SIM cards have steadily evolved — shrinking in size while keeping the same essential role. Each new version reflected the changing design of mobile phones, making room for sleeker shapes, bigger batteries, and more advanced features.

Full-size SIM card

The full-size SIM card (1FF) was the original format used in mobile devices. Roughly the size of a credit card (85.6 mm x 53.98 mm) and introduced in the early days of mobile phones, these cards could be considered the grandfather of all SIMs. Full-size SIM cards were anything but compact, but they were perfect for the first GSM mobile phones of the 1990s. While now a relic of the past, they paved the way for the SIM cards we use today.

Mini-SIM card 

The standard SIM card, also known as a mini-SIM and 2FF, marked the first step toward making SIM cards more practical. Introduced after the bulky full-size version, the mini-SIM measures 25 mm x 15 mm and became the go-to size during the late 1990s and early 2000s. This version balanced functionality and portability, fitting into most devices and leaving room for other internal components.

Micro-SIM card 

As phones became smaller, so did SIM cards. The micro-SIM (3FF), a smaller version measuring 15 mm x 12 mm, was introduced in 2003 by the European Telecommunications Standards Institute (ETSI). However, it only gained widespread use in the early 2010s — the iPhone 4 was one of the first major smartphones to use the micro-SIM, and it set a trend for other manufacturers to adopt this size. Offering the same functionality as the larger SIM formats, the micro SIM allowed manufacturers to free up phone space for other components, like larger batteries and improved cameras.

Nano-SIM card 

The nano-SIM (4FF) is the smallest and most advanced physical SIM card to date. Launched in 2012 and measuring just 12.3 mm x 8.8 mm, it's nearly all chip with minimal plastic. This ultra-compact design allows manufacturers to save valuable internal space and make the thinner smartphones we use today. Despite its tiny size, the nano SIM delivers the full capabilities to keep devices connected.

eSIM (embedded SIM)

An eSIM (embedded SIM) is a built-in chip in your device that works like a physical SIM. It’s the new kid on the block — unlike traditional SIMs, eSIMs are digital and allow you to switch carriers or plans without swapping a card. This technology saves space and makes setting up a new network connection as easy as scanning a QR code. Their adoption is growing, but eSIMs aren’t universal yet. However, their flexibility and convenience make them an exciting step forward for users who frequently travel internationally or switch networks.

iSIM (integrated SIM)

The newest step in SIM technology is the iSIM, or integrated SIM. Unlike a physical SIM or even an eSIM, an iSIM is built directly into the phone’s processor, taking up virtually no extra space. This design could open the door to even slimmer devices, longer battery life, and new applications in wearables and IoT devices. Right now, iSIMs are still in their early stages, with limited availability and carrier support, but they point to a future where connectivity is built into the core of our devices.

eSIM vs. physical SIM card

The eSIM vs. physical SIM card battle doesn’t have a clear winner — it depends on your lifestyle and mobile needs. While physical SIM cards have been the traditional choice, eSIMs are quickly taking the lead. Here’s how they compare: 

All in all, physical SIM cards are ideal if you prefer a tangible card or switch phones often or if your carrier doesn’t yet support eSIMs.

However, eSIMs are a lot better for frequent travelers, digital nomads, or anyone who wants flexibility without the hassle of swapping cards. With just a few taps, you can add a new plan, update your carrier, or activate service abroad.

eSIM apps like Saily make staying connected easier than ever. With an intuitive platform, activating an eSIM is fast and simple, taking only a few taps. If you’re traveling internationally, Saily ensures you stay seamlessly connected no matter where you are.