A while back, a friend came to me with a printout from his company's HR portal. He'd been in the company's Share Incentive Plan for three years and was thinking about pulling out early.
The numbers looked decent on paper, but when we actually worked through the tax implications — income tax, National Insurance, the HMRC holding period rules — the picture looked very different. He nearly left thousands of pounds on the table because no calculator online handled UK SIP rules properly.
That conversation stuck with me.
Most equity calculators I found either treated UK SIPs the same as US RSUs (completely wrong), or they were buried inside expensive financial planning software. So I built one myself.
It is a free, browser-based tool that helps UK employees (and global employees at UK-listed companies) understand the real after-tax value of their equity — whether that's a UK SIP, RSUs, stock options (ISO/NSO), or an ESOP.
Here's what it does that I haven't seen elsewhere:
→ HMRC-specific SIP logic: it actually applies the 3-year and 5-year holding period rules that determine whether you pay full income tax, partial tax, or nothing at all — a distinction worth thousands for many employees.
→ Custom vesting schedules: instead of locking you into a 25%/year model, you can enter your exact split year by year.
→ Side-by-side scenario comparison: useful if you're comparing two job offers with different equity structures, growth assumptions, or vesting periods.
→ Multi-currency support: GBP, USD, EUR, and CAD — useful for anyone working for a US company while based in the UK.
→ PDF export: so you can share the projection with a financial adviser or employer without retyping everything.
Building it taught me a lot — mainly how poorly documented UK SIP tax rules are, even on official resources. I spent more time reading HMRC guidance documents than writing actual code.
Now I'd genuinely love your feedback:
- Is the interface intuitive, or does anything feel confusing on first use?
- Are there equity types or tax scenarios you'd want the calculator to support that it currently doesn't?
- How is the mobile experience? Does it feel usable on a phone, or does it need work?
- Would a comparison export (side-by-side PDF of Scenario A vs B) be useful to you?
- Are there edge cases in UK SIP rules — like leaving a company early or a share price drop — that you'd want modelled?
- Is the explanatory content clear enough, or does it assume too much prior knowledge?
No sign-up required, no data is stored — everything runs in your browser. I'm not trying to sell anything; I just want to make this genuinely useful for employees navigating equity compensation.
Thank you for taking the time to look at it. Any feedback, harsh or otherwise, is very welcome.
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