MONEY FLOW INDEX
Money Flow Index (MFI) is an oscillator calculated over an N-day period, ranging from 0 to 100, showing money flow on up days as a percentage of the total of up and down days. Money flow in technical analysis is typical price multiplied by volume, a kind of approximation to the dollar value of a day's trading.
MFI is used as an oscillator. A value of 80 is generally considered overbought, or a value of 20 oversold. 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. It will be noted the MFI is constructed in a similar fashion to the relative strength index. Both look at up days against total up plus down days, but the scale, i.e. what is accumulated on those days, is volume (or dollar volume approximation rather) for the MFI, as opposed to price change amounts for the RSI.
It's important to be clear about what "money flow" means. It refers to dollar volume, i.e. the total value of shares traded. Sometimes finance commentators speak of money "flowing into" a stock, but that expression only refers to the enthusiasm of buyers (obviously there's never any net money in or out, because for every buyer there's a seller of the same amount).
For the purposes of the MFI, "money flow", i.e. dollar volume, on an up day is taken to represent the enthusiasm of buyers, and on a down day to represent the enthusiasm of sellers. An excessive proportion in one direction or the other is interpreted as an extreme, likely to result in a price reversal.