Nearly 50% of UK adults don't know how much money they can earn before tax

With the 31 January deadline looming for online self-assessment tax returns, we asked thousands of people to complete our Great British Tax Quiz to test their knowledge on the UK tax system - and the results are worrying.

Almost 50% of the 4,572 respondents didn't know their correctpersonal allowance,the amount you can earn before you pay income tax, while more than a third were in the dark about how much they can pay into a tax-free Isa this year.

It's important to know how the tax system works - particularly if you have to submit a self-assessment tax return.

Which? dispels some common tax myths - but first, give our Great British Tax Quiz a go to see how you fare.

Take the Great British Tax Quiz

To find out whether you're a taxability liability or a basic-rate boffin, you can try out our Great British Tax Quiz below.

Know your tax-free allowances

If you submit a self-assessment tax return, you should make the most of all of your allowances, which can help to reduce your bill.

When asked whether someone could earn up to £7,500 tax-free by renting out a spare room to a lodger, just 31% of people said they could - and 50% said they didn't know.

The statement is true, and refers to the rent-a-room scheme.It's available to anyone who wants to rent out a spare room in their home, but you must live in the property to be eligible.

If you earn more than £7,500 a year, you'll have to declare this to HMRC through a tax return.

Other allowances that could reduce the amount of tax you'll need to pay include marriage allowance, dividend allowance and personal savings allowance.

• Find out more:tax-free income and allowances

How Isas actually work

When asked if you're allowed to pay £25,000 into an Isa each year, 68% of people said they thought the statement was true, or didn't know.

Only a third correctly said the statement was false. The annual Isa allowance is £20,000, which can be split between several types of Isas.

You can deposit up to £4,000 into a lifetime Isa, up to £2,400 into a Help to Buy Isa (with an additional £1,000 in the first month of opening the account), and up to £20,000 in either cash, stocks & shares or innovative finance Isas.

You can mix and match how to split the £20,000, or you can deposit the whole sum into one Isa.

You can only deposit money into one type of Isa in each tax year - ie. one cash Isa, one stocks & shares Isa, etc.

• Find out more:Isa rules and allowances

The real penalties for late tax returns

If you fail to hit the 31 January deadline, you may incur a late penalty.

More than half of survey respondents thought they'd be fined £50 for submitting a late tax return. In fact, fines are steeper than this - starting at £100 and increasing from there.

• Find out more: late tax returns and penalties for mistakes

Get help to submit your tax return

If you haven't yet submitted your tax return for the 2017/18 tax year, time is running out.

The Which? tax calculator is an online self-assessment tool that can calculate your tax return, and submit it directly to HMRC - helping to take the hassle out of the process.

You have until midnight on 31 January to send your return to HMRC, otherwise you could face a late penalty.