Do Bollinger Bands work? The same bands tell you to buy weakness and buy strength.

One tool, two opposite trades

A Bollinger Band wraps a 20-period simple moving average with an upper and a lower band set two standard deviations of price away — a channel that widens when the market gets volatile and narrows when it's calm. Traders use it two opposite ways, and both are real, testable strategies:

• • Fade the band (reversion). Buy when price closes below the lower band — stretched down, betting it snaps back — and exit when it reverts up to the middle band. A hard 10% stop bails out if the fall just keeps going.
• • Follow the band (breakout). The opposite bet: buy when price closes above the upper band, treating the break as the start of a move, and exit only when price falls back to the middle. There's no stop — so a false breakout that reverses rides all the way down to the middle before the rule lets go.

They can never fire on the same bar — one waits for a close below the lower band, the other for a close above the upper. It's the same tool sold two opposite ways, which is exactly why testing it honestly is interesting: at most one of these instincts can be the right read of a band, and maybe neither is.

So — do Bollinger Bands work?

On the honest evidence, whichever way you trade them: mostly, no. Put both rules together and the bands beat buy & hold on 22 of 94 runs — 23%. That's about what any rule manages by chance — across every strategy on every asset the base rate is 26%.

Split the two directions apart and they disagree with each other as much as with holding — each measured against that same 26% base rate, never against one another:

• • Fade the band (reversion): beat on 8 of 47 — 17%, below the 26% base rate — worse than a rule picked at random.
• • Follow the band (breakout): beat on 14 of 47 — 30%, a shade above the 26% base rate.

The breakout rule winning a little more is the one number here that could look like an edge — so read it carefully. These are famous names that mostly trended over the window (and a breakout rule especially flatters itself on a trend), and the fact that you had to try both directions to find whichever scored higher on this survivorship-selected panel is itself a second guess. A slightly higher rate here is a fact about the panel, not evidence that the rule works — a win isn't evidence for a rule the way a loss is against it.

So the honest answer is the combined one: whichever way you point the bands, neither reliably beats simply holding. A handful of real winners exist on each side — that's why the question is worth asking honestly — but the burden of proof is on the rule, and “it worked on one chart” isn't that proof.

These are famous names that still trade today. Assets that delisted, went bankrupt or (for crypto) died or were rug-pulled aren't in the set (survivorship bias) — and that history isn't available from the free public data this tool runs on — so this rate isn't a representative base rate for every asset that has ever traded.

Where each rule shone, where it died

The average hides the real story, which is the spread: the same rule, unchanged, swings from a big beat to a wreck depending purely on which asset you point it at — and that's true of both directions. The figures below are hypothetical, simulated backtests.

Why each one looks like it works

• • Fade the band, and a trend runs you over. Reversion assumes a stretched move snaps back — but in a strong trend, oversold stays oversold. The rule buys weakness that keeps weakening until the stop fires, and sits out the very run buy & hold rode. It shines only where price was already going to mean-revert, which you can't know in advance.
• • Follow the band, and a range chops you up. Breakout assumes the push above the upper band keeps going — but in a sideways range the break fails, price falls straight back through the middle, and with no stop the rule gives the whole move back, over and over, paying costs each time.
• • And you get shown the one chart where it lined up. Picking the asset, the window, and the direction after you already know which worked is hindsight three times over. The whole glass above is what you get when you can't choose.