The Money Flow Index (MFI) is a volume-weighted Relative Strength Index (RSI). It is calculated using the following formula:
Money Flow Index = 100 - (100/ (1 + Money Ratio))
where Money Ratio = Positive Money Flow/Negative Money Flow
and Money Flow = Typical price * Volume
and Typical Price = (High + Low + Close) / 3
Many traders use the MFI as an oscillator. A value of 80 is generally considered overbought, while a value of 20 is considered oversold. Some traders use more extreme thresholds (eg. 90 as the overbought threshold, or 10 as oversold threshold). Divergences between MFI and price action are also considered significant, for instance if price makes a new rally high but the MFI high is less than its previous high then that may indicate a weak advance, likely to reverse.