The stability of the banking sector has long been a matter of concern for public policy. The likelihood of bank failure depends on two factors: (i) the variability of bank income (which primarily reflects the variability of the rate of return on bank assets), and (ii) the capacity of a bank to absorb losses in the short run (which depends on bank capital). Accounting measures of the volatility of the rate of return on bank assets and bank capital ratios may not reflect the appropriate economic concepts. In this paper, we use share price data to calculate the economic values of Australian bank asset volatilities, capital ratios and the potential public sector liability which might arise as a result of claims by depositors of a failed bank. The public sector liability is found to be extremely small. We find that the estimated capital ratio for the Australian banking sector has risen over the past decade, while there has been no noticeable increase in the riskiness of banks. A preliminary investigation of the relation between asset volatility and bank capital is conducted which suggests that riskier banks do tend to maintain higher capital ratios, and that there is a positive relation between the two variables across time.