Free Saju (Four Pillars / BaZi) reader in 20 languages — open source, solar-term correct

[hoonsikim]

Hi everyone, sharing what Ive been building for the past month.

TL;DR: A free Korean Saju / Chinese BaZi / Japanese 四柱推命 reader that works in 20 languages, computes correctly (solar-term-aware), and ships as ~12KB of vanilla JS.

• Live: https://hoonsikim.github.io/saju
• Repo: https://github.com/hoonsikim/saju (MIT)

What it is

Four Pillars of Destiny (年/月/日/時柱) is an East Asian system that maps your birth moment to four heavenly-stem + earthly-branch pairs. From those, traditional masters infer day master, five elements balance, ten gods, hidden stems, 12 life stages, void branches, and 神煞 (auspicious stars).

Most online BaZi calculators silently get the month pillar wrong — they use solar months instead of the 24 solar terms (節氣). This one uses lunar-javascript and is tested against the 立春 (Lichun) boundary.

Why I built it

Most "premium" BaZi services charge $30+, sit behind Mandarin/Korean-only UIs, and obscure their methodology. Computing four pillars is math — it should be free. Interpretation is where the art lives — thats the optional $7 paid tier with Claude Opus 4.8.

Stack

• Vanilla HTML/CSS/JS (~12KB total)
• lunar-javascript for solar/lunar conversion + four pillars
• Cloudflare Worker for compute (/prompt endpoint)
• Anthropic Claude Opus 4.8 for the paid LLM reading
• Resend for delivery (verified domain saju.thehueviz.com)
• Gumroad for $7 checkout
• GitHub Pages hosting — $0/month at current scale

What I would love feedback on

1. Methodology: Are the 12 life stages (生長沐浴...) computed correctly in your tradition? There are regional variants.
2. Cultural localization: The paid reading injects per-culture hooks (workplace/dating/401k for EN, 회식/번아웃/n잡 for KO, 空気/忖度/推し活 for JA). What other cultural touchstones land for your region?
3. Day Master archetypes: Free guide here — https://hoonsikim.github.io/saju/blog/day-master-archetypes-en.html

Happy to answer methodology / stack questions. PRs welcome.