Self Assessment countdown

Self Assessment has two deadlines a year, not one — 31 January, when online returns are filed and the balancing payment plus first payment on account fall due, and 31 July, when the second payment on account is due. This tool tracks whichever is coming up next, and uses last year's tax bill to estimate what each payment on account is likely to be, so the number on your calendar comes with a rough figure attached rather than just a date.

What a payment on account actually is

HMRC doesn't wait until the following January to collect tax on income it expects to keep arriving — payments on account are advance instalments toward next year's bill, each set at 50% of the previous year's tax and Class 4 NIC. They're only charged if last year's bill was over £1,000 and less than 80% of it was already collected at source through PAYE or similar — most self-employed trades don't have significant PAYE income, so this usually applies in full once a bill crosses that threshold.

A worked example

A sole trader whose Self Assessment bill last year came to £6,000 will typically owe two payments on account this year, each £3,000 — one alongside the 31 January balancing payment, one on 31 July. That's £6,000 set aside on top of whatever the current year's balancing payment turns out to be, which is exactly the kind of number that's far easier to plan for in August than to discover in January. Knowing which of the two deadlines is next, and roughly what's due, is most of the battle.

Estimating ahead of the actual bill

Because payments on account are based on last year's figure, not this year's, the estimate here is only as good as the number fed in. For working out what this year's bill is actually likely to be — which also feeds into what next year's payments on account will be set at — the tax & NI calculator is the better starting point. Anyone above the Making Tax Digital threshold is also juggling quarterly digital updates on top of these annual deadlines — the MTD readiness checker covers that obligation, which runs alongside Self Assessment rather than replacing it.

What this isn't

This doesn't check whether payments on account actually apply in your situation, doesn't account for a claim to reduce them if this year's income has genuinely dropped, and doesn't include any balancing payment owed for the current tax year on top of the estimate shown. Always confirm the exact figures HMRC has calculated through your own Self Assessment account or with an accountant before paying.