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.

Continue reading for the best SIM deals, and if you're looking for other tech bargains from phones and laptops to games and audio, visit Tech Advisor Deals here.

SIM-only deals explained

If you're new to the idea of a SIM-only deal, we've got everything covered below, but if you already know what type of user you are and have a good idea of what you're looking for, you can skip ahead using the following links.

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.

And then there are pay-as-you-go plans, so you can pay just for what you need when you need it, although you can expect these to work out more expensive.

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.

How to choose the best SIM deal for you

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 several 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 250 minutes, 5,000 texts and 1.5GB 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.

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 can also choose Data Rollover for this plan instead of ShockProof, which will save unused data and roll it over to the next month.

  • 1.5GB data, 250 minutes, 5,000 texts, 4G, - £5 per month. View deal here.

BT Mobile offers 400 minutes, unlimited texts and 500MB of data for £5, but that's only for existing BT Broadband customers. It's a 12-month contract and includes 4G, so it's a really good, cheap deal that's worth considering if you're a customer already and don't need loads of minutes and only occasionally need to use data.

  • 500MB data, 400 minutes, unlimited texts, 4G - £5 per month. View deal here

Alternatively, Virgin Media offers a 30-day rolling contract that isn't exclusively for its broadband customers for just £6. It offers 300 minutes, unlimited texts and 300MB data.

  • 300MB data, 300 minutes, unlimited texts, 4G - £6 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 Data Rollover is a good option for moderate users. One of the best Data Rollover contracts is the £8 per month rolling contract that offers 500 minutes, 5,000 texts and 3GB of 4G data.

  • 3GB data, 500 minutes, 5,000 texts, 4G - £8 per month. View deal here.

For a bit more data, you can get the £10 per month contract from iD which offers 5GB.

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

Both of the contracts described above also have a ShockProof alternative for the same price. This means that your spending is 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, and with Three you get the added bonus of being able to use your phone abroad at no extra cost with the Feel At Home feature.

For example, for £12 per month you can get 4GB data, unlimited minutes and unlimited texts on a 4G, 12-month contract.

  • 4GB data, unlimited minutes, unlimited texts, 4G - £12 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 200 minutes contract, which is a very reasonable £20 per month.

  • 30GB data, 600 minutes, unlimited texts, 4G - £19 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, 2000 minutes, 5,000 texts, 4G - £15 per month. View deal here.

Tablet plans (Data SIMs)

As we mentioned earlier, 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.