In this guide
- What is a SIM card?
- What SIM card sizes are available?
- What is an eSIM?
- How to find out which SIM your phone needs
- How to insert or change a physical SIM card
- Can I keep my number when I get a new SIM?
- FAQs
- What if I order the wrong SIM size?
- Can I cut a larger SIM down to fit?
- Does the SIM size affect which deals I can get?
- My phone is eSIM only. Can I still compare deals on SIM Only Finder?
- Editorial policy
The SIM card you need depends on your phone model and whether it supports eSIM. For most modern smartphones, a nano SIM is the right physical size, and many phones now support eSIM as an alternative. This guide explains the different SIM card sizes, how eSIMs work, and how to find out which your phone needs.
Reviewed by Phil Brown, founder of SIM Only Finder. Before launching SIM Only Finder, Phil worked in UK mobile retail at Three UK, advising customers on SIM plans, contracts, and network choices. He has spent over a decade running consumer comparison platforms. About the author
Last reviewed: March 2026.
What is a SIM card?
A SIM card (Subscriber Identity Module) is a small chip that connects your phone to a mobile network. It stores your phone number and account details and allows your phone to make calls, send texts, and use mobile data. Every phone that connects to a mobile network needs either a physical SIM card or an eSIM.
What SIM card sizes are available?
Physical SIM cards come in three sizes:
- Standard SIM: 25 x 15mm. Used in older handsets. Rarely required for modern smartphones.
- Micro SIM: 15 x 12mm. Introduced widely around 2010 to 2013. Less common now but still used in some devices.
- Nano SIM: 12.3 x 8.8mm. The standard size for most smartphones released in the last ten years.
If you are buying a SIM for a current smartphone, it will almost certainly need a nano SIM. Most networks send SIM cards in a multi-format card that includes all three sizes punched out together, so you can pop out whichever size fits your phone.
If you are unsure which size your phone needs, check the manufacturer's specifications on their website or look at the SIM tray on your existing phone.
What is an eSIM?
An eSIM is a digital SIM built directly into your phone. Instead of inserting a physical card, you activate the eSIM by scanning a QR code or following an in-app process, and your phone is connected to the network within minutes. There is no waiting for a card to arrive in the post.
eSIM is now supported by most modern smartphones, including all iPhones from the XS onwards, Samsung Galaxy S and Z series from 2020 onwards, Google Pixel phones from the Pixel 3 onwards, and many other recent Android devices. Some phones support both a physical SIM and an eSIM simultaneously, which is useful if you want to run two numbers on one device.
Most UK networks now offer eSIM activation. You can compare eSIM SIM-only deals on our deals page. For a full guide to eSIMs and which networks support them, see our eSIM guide.
How to find out which SIM your phone needs
The quickest way is to check the SIM tray on your current phone. The tray will show the shape and size of SIM it accepts. If your phone supports eSIM only (some newer iPhones sold in the US are eSIM-only, for example), there will be no physical tray at all.
You can also look up your phone's model number on the manufacturer's website, which will list the SIM type in the specifications.
How to insert or change a physical SIM card
On most modern phones, the SIM tray is on the side of the phone. Use the SIM ejector tool that came with your phone, or a straightened paperclip, to push into the small hole next to the tray. The tray will pop out. Remove the old SIM if there is one, place the new SIM in the tray with the chip facing down and the notched corner aligned with the tray, and push the tray back in until it clicks.
On some older phones, the SIM slot is under the back cover or beneath the battery. If your phone has a removable back cover, remove it to access the slot.
Can I keep my number when I get a new SIM?
Yes. Text PAC to 65075 to get a free PAC code from your current network. Give it to your new network when you sign up and your number will transfer automatically, usually within one working day. For full step-by-step instructions, see our guide on how to keep your number when switching to SIM only.
FAQs
What if I order the wrong SIM size?
Most networks send multi-format SIM cards that include nano, micro, and standard sizes in one card. You simply pop out the size you need. If you receive a single-size SIM that is too large, contact your network and request a replacement.
Can I cut a larger SIM down to fit?
This is not recommended. Cutting a SIM risks damaging the chip and rendering it unusable. Contact your network for a free replacement SIM in the correct size instead.
Does the SIM size affect which deals I can get?
No. The size of your SIM card has no effect on which plans or networks are available to you. All SIM sizes access the same plans.
My phone is eSIM only. Can I still compare deals on SIM Only Finder?
Yes. You can filter for eSIM-compatible deals on our deals page. Most major UK networks and many MVNOs now support eSIM activation.
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