Money Flow Index (MFI)

The money flow index (MFI) is a momentum oscillator that incorporates both price and volume to measure buying and selling pressure. It ranges from 0 to 100 and is often called the "volume-weighted RSI" because it uses the same overbought/oversold framework (above 80 and below 20) but adds volume to the calculation. When price rises on heavy volume, MFI increases more sharply than RSI would. When price rises on light volume, MFI barely moves. This volume component makes MFI a more reliable gauge of conviction behind price moves.

Why It Matters

Price without volume is a rumor. Volume confirms conviction. MFI combines both into a single number. For options traders, this distinction is critical because options premiums reflect expected moves, and moves backed by volume are more likely to follow through. An overbought signal on RSI tells you momentum is extended; an overbought signal on MFI tells you momentum is extended with heavy money behind it — a stronger signal.

MFI divergences are particularly powerful. When a stock makes a new high but MFI fails to confirm with a new high, it means the rally is happening on declining volume-weighted momentum. Institutions may be distributing shares. This early warning gives options traders time to adjust positions before the reversal hits.

How It Works

Calculation:

1. Typical Price = (High + Low + Close) / 3
2. Raw Money Flow = Typical Price x Volume
3. Separate positive money flow (up days) from negative money flow (down days) over 14 periods
4. Money Flow Ratio = Positive Money Flow / Negative Money Flow
5. MFI = 100 - (100 / (1 + Money Flow Ratio))

Key signals:

• MFI above 80: Overbought. Heavy volume has pushed price high. A pullback may be imminent.
• MFI below 20: Oversold. Heavy selling pressure may be exhausted. Watch for a bounce.
• Bullish divergence: Price makes a lower low but MFI makes a higher low. Selling pressure is weakening despite lower prices — bullish.
• Bearish divergence: Price makes a higher high but MFI makes a lower high. The rally is losing volume-weighted momentum — bearish.

MFI vs. RSI:

• RSI uses only price. MFI uses price and volume.
• MFI is generally more reliable on liquid, high-volume stocks where volume data is meaningful.
• On thinly traded stocks, MFI can be distorted by a single large block trade.

Limitations: MFI can stay overbought or oversold for extended periods during strong trends, just like RSI. It works best as a confirmation tool rather than a standalone signal. Pair it with price action and trend analysis.

Quick Example

A tech stock rallies from $100 to $115 over three weeks. RSI hits 72 — overbought by RSI standards. But MFI is only at 65 because recent up days occurred on declining volume. MFI is not confirming the overbought reading. You interpret this as a rally losing steam and sell a call credit spread at the $118/$120 strikes. The stock stalls at $116 and pulls back, and your spread expires out of the money for a full profit.