Monthly contracts are great, letting you take home a high-end smartphone without handing over any cash. But if you stop to work out just how much you are paying over the lifespan of the contract, you'll probably find you could save a lot of money buying the phone upfront and getting a SIM-only deal. Here we look at the best SIM-only deals and best Data SIM deals in the UK in 2017.

It's worth pointing out that changing your SIM deal does not need to mean losing your number.

Compare all SIM deals

Here, you can use our SIM deals comparison tool to help you find a contract that suits you. Use the sliders and drop down menus to adjust your preferences and see which networks offer a contract that'll suit you and your budget.

If you're looking for more advice on how to choose a contract, or want to see some of the top contracts picked out by our editors for light users, moderate users, heavy users and tablet users, click here.

How to choose the best SIM deal for you

Unlike smartphone contracts, which typically stretch to 24 months, SIM-only deals are much shorter. Most SIM-only deals come with 12-month contracts, but you can also get rolling monthly contracts – sometimes at the same price or just a few pounds more expensive – that are ideal for visitors to the UK.

The price you pay per month will largely depend on how many minutes and texts you need, and how much data you require. Plus, you'll need to decide whether you need 3G or 4G data and whether your SIM is to go in a tablet (in which case you'll only need data) or a smartphone (in which case you'll want minutes, texts and data).

It's worth noting that most operators don't allow you to simply take your SIM out of your smartphone in a tablet. They also often frown upon tethering.

While 4G is much faster than 3G connectivity, 4G data is sometimes more expensive than 3G data. However, just because your phone or tablet might support 4G, it doesn't mean you have to take advantage of that connectivity if it can save you money not to. Some carriers charge the same amount for 3G and 4G data, though.

Pinning down what is the best SIM-only smartphone deal is tricky due to the huge number of combinations of minutes, texts and data available.

However, most people will fall into one of three camps that we'll focus on here: those who simply want to make the odd call or text, and use their PC to go online; those who make calls and texts, go online, check emails, use apps and perhaps watch the odd video; and those who are permanently glued to their smartphone.

Or, to put that more simply, light-, moderate- and heavy users.

SIM deals for light use

There are plans available for just £5 per month if you plan on using your SIM for light use.

We'd recommend considering iD, which is Carphone Warehouse's mobile network. It offers a capped SIM for just £5 on its ShockProof plan type, which offers 500 minutes, 5,000 texts and 1GB data on a one-month rolling contract.

This SIM is designed to cut you off if you go over your minutes, texts or data to make sure you don't have any nasty surprises when you get the bill through at the end of the month. You will also get Data Rollover that will save unused data for you to use in the next month.

  • 1GB data, 500 minutes, 5,000 texts, 4G, - £5 per month. View deal here.

SIM deals for moderate use

For moderate users, the best deals tend to be around the £10 mark for SIM-only plans.

iD is a good option for moderate users. One of the best is the £10 per month rolling contract that offers 500 minutes, 5,000 texts and 5GB of 4G data.

  • 5GB data, 500 minutes, 5,000 texts, 4G - £10 per month. View deal here.

Both of the contracts described above will be capped at your agreed price, stopping you from accidentally racking up a big bill if you go over your minutes, texts or data.

There are other moderate deals at decent prices, as listed below, but none of them can quite match iD's current pricing, so it'll depend whether you've got a particular preference for a network you've been on before or have been recommended.

For example, Giffgaff's £10 Goodybag is good for moderate users, offering 500 minutes, unlimited texts and 2GB of 4G internet, with free Giffgaff to Giffgaff calls on top of that. This is a one-month contract that you can set to auto-renew, but with Giffgaff you also get the flexibility to swap to a different Goodybag if you feel like you need more minutes, data or texts, or if you need less.

This is a one-month contract that you can set to auto-renew, but with Giffgaff you also get the flexibility to swap to a different Goodybag if you feel like you need more minutes, data or texts, or if you need less.

  • 2GB data, 500 minutes, unlimited texts, 4G - £10 per month. View deal here.

Three also offers good moderate use SIM-only deals. For example, for £10 per month you can get 4GB data, 200 minutes and unlimited texts on a 4G, 12-month contract.

  • 4GB data, 200 minutes, unlimited texts, 4G - £10 per month. View deal here.

SIM deals for heavy use

For anyone who wants the freedom to text and use the internet as much as they like, our favourite deal is Three's 30GB data, unlimited texts and unlimited minutes contract, which is a very reasonable £18 per month.

  • 30GB data, unlimited minutes, unlimited texts, 4G - £18 per month. View deal here.

A good, affordable option for heavy users is iD's £15 deal, which offers 2,000 minutes, 5000 texts and 6GB data.

  • 6GB data, 1000 minutes, 5,000 texts, 4G - £12 per month. View deal here.

Tablet plans (Data SIMs)

You can't simply take the SIM out of your phone and stick it in your tablet. You need to get a SIM that uses a data-only plan (and don't forget that not all tablets offer SIM-card slots, in which case you'll only be able to connect to WiFi).

How much data you need will very much depend on the activities you intend to enjoy on your tablet, plus whether you spend any of your time within range of a Wi-Fi hotspot. If you want a SIM just to cover you on the odd occasion that you use your tablet away from home and need to continue to access your email, you certainly don't need to pay out extra for a 4G plan.

Some of the best SIM-only deals for tablets come from Three, which offers 1GB for £7.50 per month, 2GB for £8 per month, 20GB for £15 per month. You can view those deals here.