If you missed the 31 January self-assessment deadline — and 860,000 UK taxpayers did — working out your actual penalty is surprisingly difficult.
HMRC sends you a £100 notice. Then silence. Then months later, a much larger bill arrives with daily penalties you didn't know about.
So I built a calculator that shows you exactly what you owe, in real time.
The penalty structure nobody explains clearly
| How late | Penalty |
|---|---|
| 1 day | £100 flat |
| 3 months | £10/day for up to 90 days |
| 6 months | £300 or 5% of tax due |
| 12 months | Another £300 or 5% |
The daily penalties started 1 March 2026. Right now, they're ticking at £10/day. By the time most people notice, they're already hundreds of pounds deep.
The calculator
Self-Assessment Penalty Calculator →
Enter your filing deadline and when you actually filed (or haven't yet). It calculates:
- Initial penalty
- Daily penalties accrued
- 6-month and 12-month penalties
- Total amount owed
- Live daily counter if you still haven't filed
No sign-up, no email required, runs entirely in your browser.
How I built it
Pure vanilla JS. No framework, no build step. The logic is straightforward date arithmetic:
const daysSinceDeadline = Math.floor((now - deadline) / 86400000);
const dailyDays = Math.min(90, Math.max(0, daysSinceDeadline - 90));
const dailyPenalty = dailyDays * 10;
The harder part was getting the penalty thresholds right. HMRC's guidance is spread across multiple pages and the 5%-of-tax-due calculations depend on whether you use estimated or actual figures.
Other tools in the set
This is part of a collection of 25+ free tools for UK freelancers:
- Day Rate Calculator — work out your ideal rate from target salary
- Late Payment Interest Calculator — statutory interest under UK law
- MTD Readiness Checker — does Making Tax Digital apply to you?
- Business Expense Tracker — HMRC categories, CSV export
- Freelancer Client Tracker — clients, projects, invoices in your browser
All free, no sign-ups, localStorage-based. Built with vanilla HTML/CSS/JS.
Why free?
I sell toolkits and templates for UK freelancers — the free tools bring people in, the paid products go deeper (contract templates, invoice recovery systems, tax checklists).
But the tools genuinely stand on their own. Use them, don't buy anything, that's fine.
If you're a UK freelancer who missed the deadline — check the penalty calculator first. Then file your return. Every day you wait is another £10.
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