Late Filing Penalty Calculator
Missing the 31 January Self Assessment deadline triggers a fixed £100 penalty immediately, rising by £10 a day after 3 months (capped at £900), with further charges at 6 and 12 months (gov.uk). This calculator estimates your total filing and payment penalties plus interest.
Two separate penalty regimes
HMRC charges penalties for a late RETURN and penalties for late PAYMENT separately — filing on time doesn't excuse late payment, and paying on time doesn't excuse a late return. This calculator works out both regimes side by side from your filing date, payment date, and the tax you still owe.
The late-filing ladder
Miss the filing deadline (31 January online, 31 October on paper) by even a day and HMRC charges a fixed £100 penalty — regardless of whether you owe any tax at all. Stay late past 3 months and a further £10 a day applies, up to 90 days (£900 max). Past 6 months, another penalty of the greater of £300 or 5% of the tax due is added, and the same again past 12 months.
The late-payment penalties
Separately, if the tax itself isn't paid by 31 January, HMRC charges 5% of what's unpaid at 30 days late, another 5% at 6 months, and a further 5% at 12 months — three penalties that can stack to 15% of the unpaid tax on top of the filing penalties above.
Interest is an assumption, not a live rate
This calculator applies simple daily interest to unpaid tax from the payment due date, at a rate you can adjust. HMRC's actual late-payment interest rate is the Bank of England base rate plus 4 percentage points, and it changes whenever the base rate does — the figure this calculator defaults to is illustrative, not a guarantee of today's published rate, so check gov.uk's current rate before relying on the interest figure for anything more than planning.
Why the milestones matter more than the calendar month
The 3-, 6- and 12-month filing milestones and the 30-day, 6- and 12-month payment milestones are measured from the deadline itself — a return filed on day 91 pays the full £900 daily-penalty cap even though it isn't yet 6 months late, while day 89 pays £890. Each milestone is a cliff-edge, not a gradual taper.
When penalties can go higher than the standard rates
In cases where HMRC finds that a taxpayer deliberately withheld information, the 6- and 12-month penalties can rise to 70% or 100% of the tax due instead of the standard 5%-or-£300 minimum. That's a finding HMRC makes, not something this calculator computes — it always assumes the standard rates unless you have reason to think otherwise.
A worked example
A return due 31 January is filed 4 months late, with £3,000 of tax still unpaid at the time of filing. The £100 fixed penalty applies, plus £900 for the full 90 days of the £10 daily penalty once 3 months late — £1,000 in filing penalties so far, with the 6-month penalty still to come if it stays unfiled. On the payment side, being over 30 days late on £3,000 unpaid adds a 5% penalty of £150, plus simple daily interest accruing on the £3,000 from 31 January.
If you have a reasonable excuse
HMRC can cancel a penalty if you had a reasonable excuse for missing the deadline — a serious illness, a bereavement, a system failure while you were filing, or a genuine unforeseen disaster, among others. Insufficient funds, forgetting, or finding the process difficult don't qualify. Appeals go through HMRC's formal process, and the sooner you appeal after the penalty notice, the smoother it tends to go.
Filing now, even if you can't pay
Because the £100 filing penalty applies whether or not you owe tax, and mounts further at each milestone, filing your return on time — even if you can't pay the bill straight away — avoids the filing-penalty ladder entirely and leaves only the separate, generally smaller late-payment penalties and interest to deal with. ac-co's Self Assessment service can get a return filed well ahead of the deadline.
Questions about the late filing penalty calculator
Is the £100 penalty charged even if I don't owe any tax?
Yes — the fixed £100 penalty applies purely for filing late, regardless of whether you have a tax bill or have already paid what you owe in full.
Is the interest rate in this calculator HMRC's actual current rate?
No — it's an adjustable assumption, defaulted to an illustrative figure. HMRC's real rate is the Bank of England base rate plus 4 percentage points and changes when the base rate does — check gov.uk for the current published rate.
Can the late-filing and late-payment penalties both apply to me?
Yes — they're separate regimes. Filing 4 months late with tax still unpaid triggers both the filing-penalty ladder and the late-payment penalties at the same time, on top of each other.
What counts as a reasonable excuse?
Things like serious illness, a bereavement, or a system failure while you were trying to file — not simply forgetting, finding the process difficult, or not having the money to pay. Appeal through HMRC's formal process.
Do the 6- and 12-month penalties ever exceed 5% of the tax due?
Only if HMRC finds you deliberately withheld information, in which case those two penalties can rise to 70% or 100% of the tax due — this calculator always assumes the standard 5%-or-£300 rates.