This whitepaper proposes the Brand Capital Index (BCI), an open methodology that extends brand valuation from a single financial dimension to a three-dimensional time-series indicator. The BCI formula is: BCI = α·FBV + β·SCV + γ·AIV, where α + β + γ = 1 and BCI ∈ [0,100]. The three components are: FBV (Financial Brand Value) — brand contribution to corporate earnings, calculated using the ISO 10668:2010 income approach. SCV (Sustainability Compliance Value) — regulatory-risk-adjusted value integrating ISSB S1/S2, TNFD, EU CSRD, and Taiwan FSC phased ESG disclosure rules. AIV (AI Visibility Value) — brand citation rate and position score across ChatGPT, Gemini, Perplexity, and Claude. BCI addresses two structural blind spots of legacy valuation: (1) mandatory sustainability regulation has become a market-access determinant, and (2) generative AI platforms have become a primary brand-discovery channel (>30% consumer penetration as of 2025). Preliminary validation on 30 Taiwan-listed companies shows BCI explains 15–20% more subsequent revenue variation than single-dimension FBV alone, and detects brand capital impairment signals 1–2 quarters earlier. The methodology is published under CC BY-NC-SA 4.0, is designed to be globally applicable and regulation-neutral, and ships with a governance scaffold (weight calibration, audit trail, version control).