I built a real-time penalty calculator because HMRC won't tell you what you owe

#freelancing #javascript #webdev #beginners

Quick background: I'm building free tools for UK freelancers at landolio.com. This one came from personal frustration.

HMRC will send you a penalty notice weeks after the fact. Meanwhile, the daily £10 charges are silently racking up. There's no dashboard, no counter, no "hey, you currently owe £X" message anywhere.

So I built one.

What it does

The calculator shows:

Your exact penalty amount right now (live counter)
How the penalties break down (initial £100 + daily charges)
What happens at 3 months, 6 months, 12 months
How much you save by filing today vs waiting

The maths

For anyone who missed the 31 January 2026 deadline:

Day 1 (1 Feb): £100 fixed penalty
Day 30+ (1 Mar): £10/day begins
Day 120 (31 May): Daily penalties cap at £900
                  Total: £1,000
Day 183 (1 Aug): +£300 or 5% of tax due
Day 365 (1 Feb 2027): +£300 or 5% of tax due

As of today (17 March), that's £270 if you haven't filed.

The tech

Vanilla JS. No framework, no backend. Date arithmetic against known HMRC penalty dates. Runs entirely in the browser.

The hardest part wasn't the code — it was finding HMRC's actual penalty rules in plain English. Schedule 55 of the Finance Act 2009, if you're curious. Light reading.