Bollinger Bands calculator
Rolling middle band (SMA) with upper and lower bands at ±N standard deviations, plus %B and bandwidth. Runs in your browser, no upload.
About this tool
Bollinger Bands are a volatility indicator built from a moving average and the standard deviation of a price series:
- Middle band — the simple moving average (SMA) over the last
periodvalues (commonly 20). - Upper / lower bands — the middle band plus and minus
num_stdstandard deviations (commonly 2). The bands widen when the market is volatile and contract when it is calm.
For the most recent point the tool also reports:
- %B — where the price sits within the bands:
(price − lower) / (upper − lower). 0 means the price is on the lower band, 1 means it is on the upper band. - Bandwidth —
(upper − lower) / middle, a measure of how wide the bands are.
Enter the prices oldest-first, separated by spaces, commas, semicolons, or newlines. The standard deviation is the population standard deviation (÷N) of each window, matching the classic Bollinger definition.
Privacy
Everything runs in your browser via WebAssembly — your data is never uploaded. Also available from the gizza CLI and in chat (which return the bands as structured JSON).
FAQ
Why do I get fewer band rows than input values?
A band needs a full window of period values, so the first period − 1
points have no band. With N prices and a period of P you get N − P + 1
rows, aligned to the input by their index. If you supply fewer than period
values the tool returns an error telling you the minimum required.
Does this use the sample or the population standard deviation?
The population standard deviation (divide by N), which is the classic
Bollinger Bands definition. Spreadsheet functions like STDEV/STDEV.S use
the sample deviation (divide by N−1), so their bands come out slightly wider —
compare against STDEV.P instead if you're cross-checking.
What do %B and bandwidth tell me, and when are they missing?
Both describe the most recent point. %B is (price − lower) / (upper − lower): 0 means the price sits on the lower band, 1 on the upper band, and
values outside 0–1 mean it closed outside the bands. Bandwidth is
(upper − lower) / middle. %B is omitted when the bands have zero width (a
constant series), and bandwidth is omitted when the middle band is 0.
How should I format the price list?
Paste the values oldest first, separated by spaces, commas, semicolons, or
newlines — a column copied from a spreadsheet works as-is. Every token must be
a finite number; anything else (like a header cell) is reported back as
'…' is not a number.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool bollinger-bands "10, 11, 12, 13, 12, 14, 15, 16, 15, 17"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/bollinger-bands/?prices=10%2C%2011%2C%2012%2C%2013%2C%2012%2C%2014%2C%2015%2C%2016%2C%2015%2C%2017&period=20&num_std=2Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.