Whether it’s watching cats on YouTube, checking email or updating Facebook, if you go online while you’re out and about on your mobile or tablet, you’ll be using data.

Whether you are on a pay monthly or SIM-only offering, every month you’ll have a data allowance. BT Mobile Family SIMs currently have data allowances of 3GB, 10GB (instead of 6GB, that's 4GBextra, until November 16th!) and 15GB. 

[Read more: 7 reasons your family needs BT Mobile's Family SIM]

Some things use more data than others, for instance downloading a song uses around 4MB or data, while downloading a 45-minute TV show uses around 600MB. Use data to streaming video on Netflix or downloading a TV programme and you could quickly reach your limit and get a big surprise at bill time.

A 10-year old girl from London ran up a bill of over £1,700 watching loom band videos on her smartphone, while a mother from Edinburgh received a £718.69 bill after her eight-year-old son used her tablet to play Minecraft.

BT Mobile has some features to help you and your family avoid similar unexpected bills.

Set a spend cap

BT Mobile lets you set a spend cap on your bill for anything outside your included allowance. This means that even if you go over your allowance, you’ll know exactly how much you are spending.

To set a spend cap, log into bt.com/mymobile. Scroll down to ‘Tools to manage your spend’ and chose an amount using the drop-down arrow next to Monthly spend cap, choose a spend cap from £5.

Family SIM

According to Ofcom* just 54% of parents of children aged 12-15 and 40% of those of children aged 8-11 put a limit on the amount of money children can spend.

With Family SIM, however, parents don’t have to worry about their children (or other family members) running up high bills. You can set a spend cap for each Family SIM.  

Family with phones

What’s more, different family members can have different spend caps, for instance with young children who might not really understand how much things cost, you may want them to have a lower cap.

Family SIM spend can be set at bt.com/mymobile.

Text message alert

BT Mobile will send you a text message when you are close to your spend cap and another once you reach it. With BT Family SIM the SIM user gets a text when they are near their limit and when the cap is reached, the user and account holder both get a text.

If you find you are using more data than you expected, you can adjust your use.

Keep an eye on your spend by logging into bt.com/mymobile to check how much data you and any Family SIMs have used.

Roam Like Home

BT Mobile recently introduced Roam Like Home, which lets you use your phone, text and call allowance in over 40 EU countries. So you can use your phone like you do at home, without worrying about an unexpected bill.

Read more in our article: Roam Like Home - discover how using your phone in the EU is much cheaper

Use free wi-fi

BT Mobile customers can access the internet at six million BT Wi-Fi hotspots around the UK via the BT Wi-Fi app.

If you’ve got a Family SIM plan, each person can use the BT Wi-Fi app to get online away from home, so they can connect to the internet, check email or update Facebook, without using their data allowance.

Brighten up your Autumn with our 30-day money-back guarantee

Try BT Mobile for 30-days and we promise to give you your money back if you are not delighted.

Just tell us within 30 calendar days of placing your SIM only or Family SIM order and we’ll refund your first monthly plan charge and cancel your contract. You won’t have to pay a fee for cancelling either. You won’t be refunded for any extra services you’ve used outside your monthly plan allowances.**

BT Family SIM is available for BT Broadband and non-BT Broadband customers, find out more.

*Children and parents media use and attitudes report 2015

**Terms and conditions: 30 day money back offer: Offer available to new BT Mobile SIM only and Family SIM customers and existing BT Mobile customers purchasing an additional SIM. Offer limited to one SIM only or Family SIM plan per customer. This offer does not apply to handset plans, and does not apply to regrades. If you regrade your SIM bought during the 30 day offer period, the start date of the offer period will remain the same (the date at which you placed your order for the original SIM). A 12-month minimum term applies.  If you’re not happy with BT Mobile for any reason, just call us on 0800 800 150 within 30 calendar days of placing your order and ask us to refund your first monthly plan charge and cancel your contract. Your refund will be processed on your next bill date. You won’t be refunded for services you have used outside your monthly plan allowance and you’ll be charged for these at the rates shown in the Tariff Guide at bt.com/tariffguide. We’ll confirm when your contract has been cancelled and you won't have to pay a fee for cancelling early. You won’t be entitled to a refund if you’ve acted in breach of the BT Mobile terms and conditions at bt.com/legalstuff, for example if we suspect fraud or any other unauthorised activity. This offer doesn’t affect your statutory rights/any other legal rights that you may have.