Business Number Validator

The 統一編號 is the eight-digit number Taiwan assigns to every registered business — the equivalent of a company registration number. It appears on invoices, contracts and tax filings, and it carries a checksum so that a mistyped digit can be caught before it reaches your records.

Each of the eight digits is multiplied by a fixed weight: 1, 2, 1, 2, 1, 2, 4, 1. Each product is reduced to the sum of its own digits, so 7 × 2 = 14 contributes 5 rather than 14. The number is valid when the total is divisible by 5.

1  2  1  2  1  2  4  1   ← weights
2  2  0  9  9  1  3  1   ← 22099131
2  4  0  9  9  2  3  1   ← digit sums
= 30, and 30 mod 5 = 0

There is one exception. When the seventh digit is 7, the product 7 × 4 = 28 may be counted as either 1 or 2, so the number is valid if either the total or the total plus one is divisible by 5.

Before April 2023 the total had to be divisible by 10. The Ministry of Finance relaxed this to divisible by 5, roughly doubling the pool of issuable numbers. Every number valid under the old rule is still valid, but a validator written before 2023 will reject numbers that are now legitimate — if an old system is rejecting new companies, this is why.

• Validating a number on an invoice before filing
• Checking a customer-supplied number at the point of entry
• Finding out whether an old validator is using the pre-2023 rule
• Auditing a supplier list for typos