Interactive, client-side calculator comparing Poland's Osobiste Konto Inwestycyjne (OKI, in force from 2027-01-01: no 19% Belka capital-gains tax, instead an annual fee on average asset value above a 100 000 zł exemption) with a regular brokerage account taxed under the Belka rules.
Live: https://okicalc.pl/
- Year-by-year simulation of both accounts (contributions, dividends, limit valorization, transitional 2027 rate, NBP-linked fee-rate presets), plus an optional deterministic crash stress test (a −d% year every N years).
- Payout phase: set a net monthly withdrawal and see how many years each account funds it (OKI keeps paying the fee; each regular-account sale pays Belka on its profit share). Optional "in today's money" display toggle.
- Hybrid third strategy: OKI capped at the exemption limit with the yearly overflow skimmed (tax-free) to a regular account — shown automatically on the growth chart, in the verdict and in the year table once the portfolio outgrows the limit.
- Verdict banner, capital/advantage/taxes charts (hand-rolled SVG), interactive break-even heatmap, plain-language methodology with a year-by-year breakdown, PDF report via the print dialog.
- Law-status box, localized news list (data-driven from
js/news.js) and an SEO-marked FAQ (FAQPage JSON-LD). - Languages: PL / EN / RU / BE / UK, with correct Slavic plural rules and locale number formatting. Light/dark themes. Scenario sharing via URL.
- No build step, no frameworks, no backend: static ES modules.
npm run serve # python3 http.server on :8321 (ES modules need http://)
npm test # node --test — golden-value suiteThe financial model is specified in detail in MODEL.md
(rules, recursions, closed form, assumptions and their bias, validation).
js/engine.js implements it; tests/engine.test.js pins the engine to a
precomputed golden dataset: a full 16×14 break-even table, advantage
curves, sensitivity tables and an analytical closed-form cross-check.
GitHub Actions runs the test suite on every push and pull request.
Analytics is enabled: the GA4 Measurement ID lives in index.html
(window.GA_MEASUREMENT_ID). A localized accept/decline cookie banner
gates it — GA loads only after the visitor accepts (GDPR/ePrivacy), and the
choice is remembered in localStorage. Set the ID to an empty string to
disable both the banner and GA; replace it with your own ID when forking.
GitHub Pages, "Deploy from a branch" → main / root. No build required
(.nojekyll included). All asset paths are relative, so the site works under
/okicalc/.
Calculations follow the act of 3 July 2026 as understood on the build date. Not tax or investment advice.