The deadline for online self-assessed tax returns is 31 January 2019. Miss it, and you could be fined £100.

But fear not. Here's our handy guide to who is eligible, with links to tax calculators, tax rules and the online form.

Self-assessment is a system HM Revenue and Customs (HMRC) uses to collect Income Tax.

This money is usually deducted automatically from wages, pensions and savings, however self-employed people and those with additional income must report it to the tax-man.

If you're worried you've made a mistake, here's how to amend it and for help completing your form, see our self-assessment step-by-step guide.

For everything else, have a scroll below.

Do I need to file a tax return?

You must fill it in if you are:

  • Self-employed and work alone (a sole trader)

  • A partner in a business

  • Got £2,500 or more in untaxed income in the past year, for example from tips or renting out a property

  • Your income in the past year from savings or investments was £10,000 or more (before tax)

  • Your income (or your partner’s) was over £50,000 and one of you claimed Child Benefit

  • A company director (unless it’s an unpaid position for a non-profit organisation like charity)

You can check here if you are still not sure.

The tax return applies to the tax year 6 April 2017 to 5 April 2018. You must send your tax return, even if you don’t have any tax to pay.

How much can I earn before paying tax?

Most people had a tax-free personal allowance for 2017-18 of £11,500. After that, 20% of your pay must be taxed. If you earn more than £43,000, 40% of the income above that threshold is taxed.

You can check out exactly what you will pay using this tax calculator.

Do pensioners need to file a tax return?

Pensioners who earn just their private and state pension do not have to fill in a tax return unless they were born before 6 April 1948.

Tax-free allowances for people born before this date are affected by annual income over a certain amount, so a tax return may sometimes be necessary.

If you're a pensioner whose only source of taxable income is your state pension, you have to fill in an annual tax return if the amount you receive is higher than your tax-free personal allowance.

Does a company director need to file a tax return?

Yes - unless your sole income has already been taxed through your salary, you work for a charity or you have no income at all.

Could I be due for a rebate?

If you’re self-employed, you can deduct allowable expenses. This means you can shrink your taxable profit by including the costs of running your business. To check what counts as an allowable expense, click here.

In previous years, unusual excuses for late tax returns included a pet goldfish dying, a 'run-in' with a cow and a distracting volcanic explosion on TV.

The deadline for paper tax returns has already passed. So you will need to fill in your tax return online using the Government website.