I built a penalty calculator because HMRC won't tell you what you owe until it's too late

Last month I missed my self-assessment deadline by 3 days. The £100 penalty was automatic. Fine. But then I started reading the small print. Turns out there's a whole cascade of penalties that stack up — and HMRC doesn't exactly make it obvious. The penalty structure nobody explains clearly When What happens Running total 1 day late £100 automatic £100 3 months £10/day for up to 90 days Up to £1,000 6 months £300 or 5% of tax (whichever's more) Up to £1,300+ 12 months Another £300 or 5% Up to £1,600+ Plus late payment penalties on top. Plus interest. So I built a calculator that shows you exactly where you stand. What it does You enter: When your return was due When you filed (or if you haven't yet) How much tax you owe It gives you: Your exact penalty amount right now What it'll be if you wait another week/month Whether you have grounds for appeal What to do next Try the Self-Assessment Penalty Calculator → The maths that made me build it Right now (mid-March 2026), anyone who missed