If you’re a sole trader in Ireland, preliminary tax is one of the parts of self-assessment that catches people out most often, not because it’s complicated, but because it asks you to pay tax on a year that isn’t finished yet. Understanding how it works removes most of the stress around it.
What Is Preliminary Tax?
Preliminary tax is an estimated payment towards your income tax bill for the current tax year, paid in advance of Revenue confirming your actual liability. Ireland’s self-assessment system requires you to pay tax as you go, rather than waiting until your accounts are finalised.
Every year, as a sole trader, you’re dealing with two tax payments at once:
- Preliminary tax for the current year (an estimate)
- Balance of tax for the previous year (once your actual figures are known)
Both are typically due on the same date, which is why the October tax deadline often feels like a larger bill than expected.
How Much Do You Need to Pay?
Revenue gives you three ways to calculate a preliminary tax payment that avoids interest charges. You only need to meet the lowest of the three.
The 90% Rule
Pay at least 90% of your final liability for the current tax year. This requires a reasonably accurate estimate of what you’ll owe, which isn’t always straightforward if your income varies.
The 100% Rule
Pay 100% of your actual liability for the previous tax year. This is the more predictable option for most sole traders, since it’s based on a figure you already know rather than a forecast.
The 105% Rule (Direct Debit Only)
Pay 105% of your liability from two years prior. This option is only available if you pay by direct debit, and it doesn’t apply if that earlier year’s liability was nil.
Most sole traders with steady or growing income find the 100% rule easiest to work with, since it removes the guesswork. If your income is falling year on year, the 90% rule may work out cheaper, and if you’re already set up on direct debit, it’s worth checking whether the 105% rule beats both.
When Is Preliminary Tax Due?
Preliminary tax for the current year and the balance of tax for the previous year are both due on 31 October, alongside your Form 11 return. If you file and pay through Revenue’s ROS system, this deadline is usually extended by a couple of weeks, though the exact date is confirmed by Revenue each year.
What Happens If You Underpay?
If your preliminary tax payment falls short of whichever threshold applies to you, Revenue charges interest on the shortfall from the original due date, not from when you eventually pay it. The longer the gap goes unaddressed, the more it costs.
Underpaying doesn’t just create an interest charge. It can also mean a larger, less predictable bill the following October, since you’re settling last year’s shortfall on top of the new year’s preliminary payment.
Paying by Direct Debit
Revenue offers a monthly or quarterly direct debit option for preliminary tax, which spreads the payment across the year instead of one lump sum in October. This doesn’t change the total amount owed, but it can make cash flow more manageable, particularly if your income is seasonal, and it’s the only route that unlocks the 105% rule above.
Common Mistakes to Avoid
- Treating preliminary tax as optional or “next year’s problem”
- Underestimating income when using the 90% rule
- Forgetting that preliminary tax and last year’s balance are usually due together
- Not adjusting the following year’s preliminary payment after a strong year of income growth
How RizFin Helps
We calculate your preliminary tax using whichever method results in the lower payment, factor it into your October filing alongside your Form 11, and flag it early enough that it’s never a last-minute surprise. Get in touch if you’d like help getting ahead of it.
Frequently Asked Questions
Do I need to pay preliminary tax in my first year as a sole trader? Often not, if you have no prior year’s liability to base the 100% rule on, though you may still need to make a reasonable estimate under the 90% rule depending on your circumstances.
Can I change my preliminary tax payment during the year? Yes, you can revise it before the deadline if your income estimate changes significantly.
What if I overpay? Any excess is credited against your final liability or refunded once your return is processed.
Final Thoughts
Preliminary tax feels unfamiliar mainly because it asks you to plan ahead of your own accounts. Once you know which rule applies to you, it becomes a predictable part of your year rather than an October surprise.