Why Your January Tax Bill Is Half Again Bigger Than You Calculated

Everyone doing a UK tax return knows about 31 January. Far fewer know that the bill arriving that day is often two payments stapled together, or that there is a second deadline on 31 July that nobody mentions at parties.

This is the single most common shock in a first year of self-employment: a tax bill roughly half again as large as the arithmetic suggested. The reason is not a mistake. It is a system called payments on account, and it is entirely predictable once you have seen it.

What is a payment on account?

A prepayment towards next year's tax, collected in two instalments. GOV.UK defines them as "'Payments on account' are payments towards your next tax bill (including Class 4 National Insurance if you're self-employed)."

The dates: "These payments are due by midnight on 31 January and 31 July."

And the sizing, which is the part worth committing to memory: "Each payment is half of the tax you owed last year."

So HMRC is not guessing at your future income in any sophisticated way. It takes last year's tax, halves it, and asks for one half in January and the other in July. The calculation is "based on your estimated earnings (usually the amount you earned the previous year)."

Why is the January bill so much bigger than expected?

Because two different things fall due on the same date. In January you may be settling last year's shortfall and making the first prepayment towards this year.

The shortfall has a name — a balancing payment — and GOV.UK describes it as what happens when your actual earnings exceeded the estimate: it "is worked out by deducting the payments on account you've made from the total tax you owe", and it is "due by midnight on 31 January the following year."

Sit with the arithmetic in a first year. You owe last year's tax in full, because there were no payments on account for a year you had not yet filed for. On the same day, you owe the first payment on account for the current year, which is half of that same figure. The bill is therefore around one and a half times the number you had been mentally preparing for.

Nothing has gone wrong. But if you budgeted for the tax you calculated, you are now short by 50% of it, with no notice period.

Who does not have to make them?

Two exemptions, and both are worth checking before assuming this applies to you. GOV.UK: you do not need to make payments on account if "the amount of tax you owed last year was less than £1,000", or if "last year you paid more than 80% of the tax you owed outside of Self Assessment."

That second one catches more people than they realise. Someone who is mostly employed — with tax collected through PAYE — and has a modest side income may well be over the 80% threshold, and therefore outside the payments-on-account system entirely. Which means the July date is irrelevant to them, and the January bill is just the January bill.

It also means the position can change between years. A freelancer who takes a salaried job, or an employee whose side income grows, can move in or out of this system without anyone flagging it.

What about 31 July?

It is a real deadline with a real payment attached, and it is the one that gets missed, because it has none of January's cultural presence. No news coverage, no accountant's reminder emails, no colleagues talking about it.

It is also the more dangerous of the two. January arrives right after the filing deadline, when tax is on your mind and the figures are in front of you. July arrives mid-summer, six months after you last thought about any of this, for an amount you were told about half a year earlier.

Which reminders are worth setting?

Four, and the first two are the ones that prevent the shock rather than just recording it:

• Early December, every year. The point of this one is not to pay — it is to find out how much January will be, and whether it includes a balancing payment as well as a payment on account. Knowing the number seven weeks out is the entire difference between a planned payment and a scramble.
• Mid-January. Enough margin before the 31st for a bank transfer to clear and for anything unexpected in the figures.
• Mid-July. For the second payment on account. Set this the same day you set the January one, because in July you will have no other prompt at all.
• The day you file, a one-off to write down next year's two amounts. They are knowable as soon as this year's tax is settled — half of it each, on two dates you already know.

In ReminderIt you can set these by message — "remind me every year on 10 July to pay the second payment on account" — and it reads the schedule back before saving. All four suit annual recurring reminders, because unlike most deadlines these dates genuinely never move.

If you are in your first year of self-employment, the December reminder is the important one. It is the only thing standing between you and discovering the one-and-a-half-times problem in the last week of January.

What this page is not

Tax advice, and not a calculation. Whether payments on account apply to you, how much they are, whether you can ask to reduce them if your income has fallen, and what happens if you pay late are all questions with real consequences and real nuance — and they belong to HMRC or an accountant, not to a blog.

The figures and rules here are GOV.UK's own, and tax rules change with each Finance Act, so check the current year's guidance before acting. Your HMRC account shows your actual amounts and dates, which beats any general description.

What the rules do tell you plainly is the shape: two dates that never move, each payment half of last year's tax, a January that may carry two obligations at once, and a July deadline with nothing in the culture to remind you of it.

Source: GOV.UK, Understand your Self Assessment tax bill — Payments on account — the definition and inclusion of Class 4 National Insurance, the 31 January and 31 July deadlines, each payment being half of the previous year's tax, the £1,000 and 80% exemptions, and the balancing payment and its 31 January due date.