Module 7 · Lesson 5
Bollinger Bands
Bollinger Bands place a changing envelope around a moving average. The width reflects recent price variation. A touch of an upper or lower band is a description of price relative to that envelope, not a prediction.
Quick answer
What do Bollinger Bands measure?
A common chart setting plots a 20-period simple moving average with upper and lower bands two standard deviations away. The bands widen when recent price variation grows and narrow when it shrinks. These parameters can be changed, so check the chart settings before interpreting a band.
What do the middle and outer lines mean?
The middle line is an average of the chosen price series. The outer lines are calculated distances above and below it, often using standard deviation. They move when the price window changes. The lines are not fixed support, resistance, or valuation levels.
Does a touch of the lower band mean “buy”?
No. It shows that price reached a lower relative position under the selected calculation. In a sustained decline price can track near the lower band; in a strong rise it can track near the upper band. Check the trend and relevant company news rather than assuming a bounce or reversal.
What is a “squeeze”?
A squeeze is a period when the bands become relatively narrow because recent price variation has declined. A later larger move can widen them, but the squeeze alone does not identify direction or timing. Define the comparison period when you describe one.
How do you compare bands fairly?
Use the same security, chart interval, price adjustment, moving-average period, and standard-deviation setting. A short intraday window can behave very differently from a daily chart. Compare a band-width change with actual price bars.
What are the limits of this indicator?
A band touch or narrow envelope can look like a precise instruction, but both depend on historical price variation. The stock can keep moving beyond a band; trading repeatedly on touches adds cost and risk.
A real-data Bollinger Bands calculation: March 2, 2020
Sources: FRED: S&P 500 daily closing observations (S&P Dow Jones Indices data); Fidelity: Bollinger Bands calculation. Prices checked October 8, 2026. Calculations below use these dated observations, not invented prices. The index series excludes dividends.
Use the 20 trading-day closes from February 3 through March 2, 2020. This exercise sets the middle band to the 20-day simple moving average and the outer bands to two standard deviations above and below it. It uses the population standard deviation (divide squared deviations by 20, not 19); another chart’s settings can produce different bands.
Show the dated closing-price inputs
| Date | Close |
|---|---|
| 2020-02-03 | 3,248.92 |
| 2020-02-04 | 3,297.59 |
| 2020-02-05 | 3,334.69 |
| 2020-02-06 | 3,345.78 |
| 2020-02-07 | 3,327.71 |
| 2020-02-10 | 3,352.09 |
| 2020-02-11 | 3,357.75 |
| 2020-02-12 | 3,379.45 |
| 2020-02-13 | 3,373.94 |
| 2020-02-14 | 3,380.16 |
| 2020-02-18 | 3,370.29 |
| 2020-02-19 | 3,386.15 |
| 2020-02-20 | 3,373.23 |
| 2020-02-21 | 3,337.75 |
| 2020-02-24 | 3,225.89 |
| 2020-02-25 | 3,128.21 |
| 2020-02-26 | 3,116.39 |
| 2020-02-27 | 2,978.76 |
| 2020-02-28 | 2,954.22 |
| 2020-03-02 | 3,090.23 |
| Calculation | Result |
|---|---|
| Middle band = sum of 20 closes / 20 | 3,267.9600 |
| Population standard deviation | 135.0988 |
| Upper band = middle + 2 × standard deviation | 3,538.1576 |
| Lower band = middle − 2 × standard deviation | 2,997.7624 |
| Observed March 2 close | 3,090.23 |
The March 2 close was inside these bands. The preceding daily closes include a sharp decline, which increases dispersion around their average. The table describes that historical price window; it does not say the index is cheap, that a reversal is due, or that future prices must remain inside the bands.
Key takeaways
- Bollinger Bands combine a moving average with a measure of recent price dispersion.
- Lookback, deviation multiplier and standard-deviation convention affect the result.
- A band touch does not prove value or guarantee that a price reversal will follow.
Check your understanding
1. What is commonly used as the middle band?
A. A moving average. B. Company revenue. C. The bid-ask spread.
Show answer and feedback
A. The common setup centers the bands on a moving average.
2. What does a narrow band usually describe?
A. Certain upward direction. B. Relatively lower recent price variation. C. A guaranteed price floor.
Show answer and feedback
B. Width reflects recent variation under the chosen formula.
3. What does a lower-band touch guarantee?
A. A bounce. B. A profitable trade. C. Nothing about the next move.
Show answer and feedback
C. Price can continue along or beyond a band.
Why question: Why can a price remain near an outer band for several periods?
Show a sample explanation
A persistent trend can keep new prices toward one side of the rolling average while the bands adjust to incoming data.
Observe AAPL with this indicator
Open StockEducation’s chart tool with AAPL loaded. Open the AAPL daily chart. If Bollinger Bands are available, record a period when the bands were relatively narrow and a later period when they widened; note the dates and what price actually did. If the overlay is unavailable, compare a quiet and a volatile stretch of the price chart and explain how the band width would tend to differ.
Where can you check the definition and risk context?
- Fidelity: Bollinger Bands
- FINRA: What Is Market Timing?
- FINRA: Stocks and Price Volatility
- SEC Investor.gov: Introduction to Investing
End of Lesson · Continue to the Next Topic
AI responses are for educational purposes. Always verify with official course materials.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
Sources and reference material
Primary sources and original reference material for the concepts discussed on this page.
- FRED: S&P 500 daily closing observations
- Fidelity: Bollinger Bands
- FINRA: market timing and buy-and-hold strategies
- FINRA: Stocks
- SEC Investor.gov: Introduction to Investing
References added 14 October 2026.
AI responses are for educational purposes. Always verify with official course materials.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
