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 correctthe 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.
Which? dispels some common tax myths - but first, give our Great British Tax Quiz a go to see how you fare.
To find out whether you're a taxability liability or a basic-rate boffin, you can try out our Great British Tax Quiz below.
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.
If you earn more than £7,500 a year, you'll have to declare this to HMRC through a tax return.
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 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.
If you fail to hit the 31 January deadline, you may incur a late penalty.
If you haven't yet submitted your tax return for the 2017/18 tax year, time is running out.