2026-04-11 · 4 min read · korea · brn · validation · b2b

Korean BRN (사업자등록번호) Validation — Checksum, Format, Free API

Why this matters\n\nIf you're selling SaaS to Korean businesses, you need the BRN (사업자등록번호) to issue a VAT invoice. Get the checksum wrong and your accounting system silently emits invalid invoices.\n\n## The format\n\n10 digits: `XXX-XX-XXXXX`\n\n- First 3: tax office code\n- Next 2: entity type\n- Next 4: serial\n- Last 1: checksum\n\n## The checksum algorithm\n\n```ts\nfunction validateBRN(brn) {\n const d = brn.replace(/\\D/g, '');\n if (d.length !== 10) return false;\n const w = [1,3,7,1,3,7,1,3,5];\n let sum = 0;\n for (let i = 0; i < 9; i++) sum += parseInt(d[i]) * w[i];\n sum += Math.floor(parseInt(d[8]) * 5 / 10); // 9th-digit adjustment\n const check = (10 - (sum % 10)) % 10;\n return check === parseInt(d[9]);\n}\n```\n\nThat 9th-digit adjustment is what most online implementations miss.\n\n## Test cases\n\n-` 220-81-62517 `→ valid (Samsung Electronics, public)\n-` 104-81-10221 `→ valid (Kakao, public)\n\n## Free API\n\n```bash\ncurl 'https://api.lazy-mac.com/korean-business-validator/api/v1/validate?brn=220-81-62517'\n```\n\nOr part of K-Privacy Enterprise $99/mo with unlimited validation + real-time company-name lookup.\n\n## Going beyond validation\n\nFull Korean compliance stack via @lazymac/k-mcp:` npx -y @lazymac/k-mcp` then ask Claude: Verify Korean business number 220-81-62517.\n\nRelated: Korean SaaS Launch Checklist.

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