A guide to SIM cards, the small chips that connect your phone to a cellular network

• A SIM card, or subscriber identity module, is a small card in your cellphone that connects you to the network.
• Your SIM card contains your phone number, and lets you make phone calls, send text messages, and more.
• SIM cards have evolved over the years and come in several sizes, including mini-, micro-, and nano-SIMs.

Most mobile phones rely on a small card called a SIM (subscriber identity module) to make phone calls and send text messages. Without it, your phone isn't able to connect with the cellular network and can only function as an internet device (the lack of a SIM card doesn't prevent your phone from using Wi-Fi).

What to know about SIM cards

A SIM card is important because it stores your phone number and other important information. Generally, you can transfer one SIM card between two phones and the new phone will inherit your phone number without issue.

Most SIM cards are provided by phone companies, and contain some basic identifying information like serial numbers and identifying codes. These let the network know what your phone number is, and what phone carrier you use (T-Mobile, Verizon, etc.).

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SIM cards can carry a great deal of data, like your contact list and text messages. If you change phones, as long as you keep the SIM card, these contacts and text messages will come with you to the new phone.

Types of SIM cards

SIM cards need to fit in mobile phones, so as time has progressed, there has been a need for smaller cards as phones have become increasingly thin and filled with larger batteries and more sophisticated electronics.

Today there are several kinds of SIM cards:

• Full SIM: No longer in use, the original SIM card was developed in the 1990s and measured 86x54mm. Despite the large size, virtually all of it was a plastic card - the actual contact surface was the same as later cards.
• Mini-SIM: Because the full-size SIM is no longer used, these days this is often thought of as the full or standard SIM. It measures 25x15mm and the contact surface is surrounded by a large segment of plastic.
• Micro-SIM: A micro-SIM has the exact same contact surface but is smaller (15x12mm) because the plastic is trimmed away, leaving a card almost entirely of just the contact surface.
• Nano-SIM: This is the smallest SIM format in use today. It has a smaller contact surface, making it physically incompatible with older phones that expect a micro-SIM. It measures 12.3x8.8mm.
• eSIM: An embedded SIM, or eSIM, is a new, emerging format in which the SIM is embedded directly into the device and is therefore not removable. Although not widely used, the advantage of an eSIM is that it allows you to change phones without transferring SIM cards; the mobile operator transfers the account information remotely.

How to remove a SIM card

SIM cards vary in size, but they all work the same way - they're designed to be easily transferred between phones. Your phone probably came with a small SIM card removal tool (it looks like a small pin), but if you've misplaced yours, you can use a small paperclip.

Find the SIM tray on the side of your phone and insert the SIM tool or paperclip in the hole; the tray should pop out easily.

For more details on how to do this, see our articles on how to remove the SIM card from an iPhone or Samsung Galaxy phone. If you have a different phone, no worries: The technique is almost exactly the same. If you have any trouble, check out our article on how to troubleshoot an invalid SIM.