Hey Product Hunt 👋 I'm HY, the maker of CalcIsland.

CalcIsland is a set of free money calculators that show their working. Next to each result you'll see the formula, the assumptions, and (for tax tools) the official source and tax year it follows. Everything runs in your browser. There's no account, and nothing you type is sent to us.

A few things that make it a bit different:

• Languages: every page in English and Traditional Chinese (Hong Kong usage), plus Japanese and Korean pages for the Japan/Korea tools and core calculators

• Regional tools done carefully: Hong Kong salaries tax and MPF, US dividend withholding tax, and take-home pay for the US (2026), UK (2026/27), Japan (令和8年/2026) and Korea (2026), each checked against official worked examples

• Embeddable: put any calculator on your blog or class page with one line of code

• Short guides that explain the maths in plain language

There are 12 calculators today: compound interest, mortgage, FIRE, DCA, FX conversion cost, Hong Kong salaries tax, Hong Kong MPF, US dividend withholding tax, and US, UK, Japan and Korea take-home pay. Which tool or region should come next? And if you spot a number that looks wrong, please tell me. Accuracy matters more to me than features.

(Estimates for learning only, not financial advice.)