You calculate cost of equity using the Capital Asset Pricing Model (CAPM)
CAPM:
Cost of equity = risk-free rate + ( Beta * Equity Risk Premium )
The risk-free rate represents the yield on a 10-year or 20-year US Treasury bond.
Beta is calculated based on the “riskiness” of Comparable Companies.
The Equity Risk Premium is the % by which stocks are expected to out-perform “risk-less” assets.