Calculating your preliminary tax liability is about as much fun as cleaning the bathroom, but it really has to be done - and soon.

What sets this particular task apart from domestic chores is that you may have to pay interest or penalties if you don't get it done on time.

Some 230,000 self-assessed taxpayers have less than five weeks to pay preliminary tax before the deadline of October 31st. The calculation is made more complicated this year by the change to the calendar tax year.

It's a busy time for tax practitioners, who say the system is approaching meltdown as they struggle to cope with new deadlines and fundamental changes. But at least they're getting paid for their trouble. Three out of four self-assessed taxpayers use a tax professional to help with their tax return and work out their preliminary tax.

So what is preliminary income tax and who has to pay it? Put simply, it's an estimate of your income tax liability for the current tax year.

It includes payments for PRSI and the health levy. If you are self-employed, have unearned income, have income from non-insurable employment or are a proprietary director, you pay class S1 contributions.

The PRSI rate for the short tax year is 3 per cent on gross income, with a minimum contribution of £148 (€188). The health levy is 2 per cent of gross income.

Last year, the Revenue collected £556 million in preliminary tax and £121 million in PRSI from self-assessed taxpayers.

According to Mr John Bradley of KPMG, the self-employed, company directors and those in receipt of non-PAYE income are all classed as "chargeable persons" and are responsible for complying with the preliminary tax deadline.

The group includes those who carry out their own trades, farmers and some professions.

The Revenue Commissioners is in the process of sending out preliminary tax notices to all those on their records.

"Until last year, the notice included an amount due - what the Inspector of Taxes considered the taxpayer's preliminary tax liability to be. The notice will no longer be issued as a bill due and will just be a reminder of the October 31st due date," Mr Bradley said.

Just because you don't receive a notice in the post doesn't mean you don't need to think about your potential tax liability.

If you are not on the Revenue's records or if your circumstances have changed and you have a tax liability for any tax year, the onus is on you to make arrangements to pay preliminary tax for that year.

To avoid interest, the amount of preliminary income tax paid must be equal to either:

100 per cent of the income tax liability for the preceding tax year (2000/2001) or;

90 per cent of the income tax liability of the current shortened tax year (April to December 2001).

Because of the short tax year, there are special rules for the preliminary tax payment for tax years 2001 and 2002.

When calculating the amount of preliminary tax due, self-assessed taxpayers and the Revenue generally use the previous year's liability as a guide.

This October, the amount to be paid is 74 per cent of the tax for 2000/2001, to take into account the three-quarter length tax "year".

Preliminary tax can also be paid at a remove of two tax years - currently 1999/2000 - under a direct debit system.

For this to apply, the person must authorise the Collector General to collect the required amount of tax by direct debits from the person's bank account over the year of assessment. Only a small percentage of taxpayers avail of this option.

If you don't pay your preliminary tax on time or if the amount you pay is too low, you will have to pay an interest charge.

The rate of interest is 1 per cent per month or part of a month. In the case of fraud or neglect, the interest rate can be 2 per cent per month.

Where the correct amount of preliminary tax is paid, any balance of income tax payable is due no later than the date of filing the return for 2001, which is October 31st next year. If you pay too much, the excess will be refunded plus interest at a rate of 0.5 per cent per month.

A margin of error is allowed where a computational error arises, following the calculation of the taxpayer of his or her own liability in the absence of a Revenue assessment.

The shortfall of payment cannot exceed 5 per cent of the actual liability, subject to a maximum error of £2,500. If the taxpayer's liability for the year is £10,000 or less, then the margin of error permitted is £500.

Anyone with access to the internet has the option of paying preliminary tax through the Revenue Online Service, which is located at www.ros.ie.

This is the first year that the online service has been extended to include preliminary tax.