Value at Risk (VaR) is the most commonly used measure of the amount that could be lost from a position or a portfolio.
VaR is understood to be the maximum loss which could occur at a given confidence level over a specified time horizon. More precisely, it is the threshold value such that the probability that the marktomarket loss on the portfolio over the given time horizon exceeds this value is the given probability level. We assume normal markets and no trading in the portfolio.
VaR calculations often assume that returns are normally distributed over the time horizon. Common parameters are 1% and 5% probabilities and a one day or 10 days (2 weeks) horizon. The inputs for VaR calculations include details of the portfolio, the time horizon and the parameters which drive the distribution of the underlyings (the average growth rate, volatilities and correlations). For short time horizons the growth rate is negligible.
If you are working with standard deviations only then you can go from one confidence level to another by adding the corresponding difference of their distances of standard deviations from the mean:
