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Money Flow Index: Volume-Weighted RSI Explained
The Money Flow Index is a momentum oscillator that works like the Relative Strength Index but with volume baked into the calculation. It runs from 0 to 100 and is often called a "volume-weighted RSI."
Key Takeaways
- MFI weights each period's money flow, typical price times volume, before running an RSI-style ratio, so larger-volume moves carry more influence.
- The conventional thresholds are 80 for overbought and 20 for oversold, stricter than RSI's 70/30 because volume-weighted extremes are rarer.
- MFI divergence from price, especially price making a new high while MFI makes a lower high on declining volume, is a more robust warning than price-only divergence.
- On illiquid stocks or during off-hours sessions, MFI becomes unreliable because a few large prints can swing the volume-weighted math dramatically.
Key Takeaways
- MFI weights each period's money flow, typical price times volume, before running an RSI-style ratio, so larger-volume moves carry more influence.
- The conventional thresholds are 80 for overbought and 20 for oversold, stricter than RSI's 70/30 because volume-weighted extremes are rarer.
- MFI divergence from price, especially price making a new high while MFI makes a lower high on declining volume, is a more robust warning than price-only divergence.
- On illiquid stocks or during off-hours sessions, MFI becomes unreliable because a few large prints can swing the volume-weighted math dramatically.
What It Is
MFI was introduced in 1989 by Gene Quong and Avrum Soudack in an article titled "Volume-Weighted RSI: Money Flow" in Technical Analysis of Stocks & Commodities. The two analysts, then at Shearson Lehman Brothers, wanted a momentum oscillator that reflected how much money was backing each move, not just whether price closed up or down.
The indicator is typically calculated over a 14-period lookback and is read on the same 0 to 100 scale as RSI. Traditional convention treats readings above 80 as overbought and below 20 as oversold. Those thresholds are stricter than the 70 / 30 defaults used for RSI because MFI tends to hit extremes less often.
The Intuition
RSI measures the balance of recent price gains against recent price losses. It ignores volume. A small up day on billions of shares and a small up day on a handful of shares count the same in RSI. MFI says that cannot be right. A move powered by heavy volume should matter more than the same move on a quiet tape.
The fix is to weight each period's money flow by volume before running a RSI-style calculation. A day's "money flow" becomes the typical price multiplied by volume. Gains and losses are then compared in dollar (or money-flow) terms rather than in raw price-change terms.
How It Works
The calculation has four steps.
Step 1. Compute the Typical Price for each bar:
Typical Price = (High + Low + Close) / 3
Step 2. Compute Raw Money Flow:
Raw Money Flow = Typical Price * Volume
Step 3. Classify each bar. If today's Typical Price is higher than yesterday's, the bar's Raw Money Flow is positive. If lower, it is negative. If unchanged, it is excluded. Sum the positive and negative flows separately over the lookback window (typically 14 periods):
Money Ratio = Positive Money Flow / Negative Money Flow
Step 4. Apply the RSI transform:
MFI = 100 - (100 / (1 + Money Ratio))
Variables:
- High, Low, Close are the bar's OHLC.
- Volume is shares or contracts traded.
- Positive / Negative Money Flow sums span the lookback period (14 by default).
When all periods in the window are positive, Negative Money Flow is zero, the ratio is undefined, and most platforms clip MFI at 100. The mirror case pins it at 0.
Worked Example
Assume four bars with these values.
| Bar | TP | Volume | Raw MF | Direction |
|---|---|---|---|---|
| 1 | 100.00 | 1,000,000 | 100,000,000 | start |
| 2 | 101.00 | 1,200,000 | 121,200,000 | up |
| 3 | 100.50 | 800,000 | 80,400,000 | down |
| 4 | 102.00 | 1,500,000 | 153,000,000 | up |
Positive Money Flow (bars 2 and 4): 121,200,000 + 153,000,000 = 274,200,000 Negative Money Flow (bar 3): 80,400,000 Money Ratio: 274,200,000 / 80,400,000 = 3.41 MFI: 100 - (100 / (1 + 3.41)) = 100 - 22.68 = 77.3
A reading of 77.3 is strong but not yet at the 80 overbought threshold. In a full 14-period window the logic is identical, just summed over more bars.
Common Mistakes
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Treating MFI like an RSI with the same thresholds. MFI and RSI share the 0 to 100 scale, but the volume weighting pushes the distribution around. MFI reaches 80 less often than RSI reaches 70, and a 20 reading is a genuinely lopsided tape, not just a routine pullback. Using RSI's 70 / 30 levels on MFI produces too many signals.
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Ignoring that thin volume creates noisy MFI. In low-volume periods, a handful of outlier prints can swing Raw Money Flow enough to dominate the window. The indicator looks precise but is really being driven by a few bars. Confirm extreme readings against whether volume was actually above average.
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Over-weighting MFI in thin markets. Small-cap stocks, after-hours sessions, and illiquid futures all share the same issue: volume is unreliable, so a volume-weighted oscillator built on it is also unreliable. Stick to MFI on liquid names and regular trading hours.
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Forgetting the indicator excludes unchanged periods. Days where Typical Price equals the prior day's Typical Price contribute nothing to either the positive or negative sum. In a quiet, range-bound market MFI can drift on relatively few qualifying data points, which makes it jumpier than a quick glance suggests.
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Using MFI alone for entries. Like RSI, MFI is best as a filter or a divergence tool, not a trigger. Strong trends can leave MFI above 80 or below 20 for weeks. A bearish divergence (price higher, MFI lower) is more useful than an absolute level, and even that signal benefits from a second confirmation.
Frequently Asked Questions
Q: What is the money flow index in simple terms? MFI is a 0–100 oscillator that measures buying and selling pressure, similar to RSI, but weights each period by volume. A day with heavy buying volume pushes MFI higher than the same price move on thin volume.
Q: How does the money flow index affect investment decisions? It confirms whether price moves have volume backing them. A stock rising with MFI above 60 and climbing shows heavy money flowing in. The same price rise with MFI flat or declining warns that volume is not supporting the advance.
Q: What is a real-world example of MFI divergence? A large-cap stock rallies to a new six-month high, but MFI makes a lower high compared to the previous peak three weeks prior. That bearish divergence, price up but money flow weakening, preceded a 10 percent pullback as institutional selling quietly offset retail buying.
Q: How can investors use MFI practically? Use MFI's 80 threshold as a more demanding overbought signal than RSI's 70, meaning it fires less often but with more significance. A simple rule: when MFI crosses above 80 and then crosses back below on declining volume, consider reducing exposure or tightening stops.
Q: How is MFI different from RSI? RSI measures the ratio of average price gains to average price losses, volume plays no role. MFI multiplies each period's typical price by volume before running the same ratio, so it reflects how much capital backed each move, not just whether price closed up or down.
Sources
- StockCharts ChartSchool. "Money Flow Index (MFI)." https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-indicators/money-flow-index-mfi
- Quong, G. & Soudack, A. (1989). "Volume-Weighted RSI: Money Flow." Technical Analysis of Stocks & Commodities, 7(3), 76-77. https://store.traders.com/-v07-c03-volumew-pdf.html
- Investopedia. "Money Flow Index (MFI)." https://www.investopedia.com/terms/m/mfi.asp
- Fidelity Learning Center. "Money Flow Index." https://www.fidelity.com/learning-center/trading-investing/technical-analysis/technical-indicator-guide/mfi
Disclaimer
This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.