David Bellairian on William O'Neil's 7-8% Rule: The Discipline of Cutting Losses Short

By David Bellairian, Founder of Glencore Associates

Of everything William O'Neil wrote across his career — the CAN SLIM framework, decades of Investor's Business Daily columns, his study of the biggest stock winners in market history — the single rule he's probably best known for is also the simplest: sell, without exception, once a position falls 7-8% below your purchase price. Not a suggestion. A rule. I think it's worth understanding why he insisted on it, because the reasoning behind it applies well beyond any one stock pick.

The rule itself

O'Neil's own summary of the logic is one of the more quoted lines in investing writing: the whole secret to winning big is not being right all the time, but losing the least amount possible when you're wrong. The 7-8% threshold isn't derived from a complicated model — it's a practical line drawn to keep a normal, expected mistake from becoming a portfolio-threatening one.

Why the math behind small losses matters

The reasoning behind the specific number comes down to a math asymmetry that's easy to state but easy to underestimate: a 10% loss only needs an 11% gain to break even, but a 50% loss needs a 100% gain just to get back to even. Losses don't grow in a straight line relative to what it takes to recover from them — they grow faster. A discipline that caps losses at 7-8% keeps an investor almost entirely inside the shallow, easy-to-recover-from end of that curve. A discipline that lets losses run to 25%, 40%, or worse pushes into territory where recovery requires an outsized win just to get back to flat.

Why this is a discipline problem, not a knowledge problem

O'Neil's underlying point wasn't really about stock-picking accuracy — it was about behavior under pressure. Every investor, no matter how good their process, is going to be wrong some fraction of the time; O'Neil himself was direct about this, having built his career studying market history rather than claiming immunity from it. The real determinant of long-term results, in his framework, isn't how often you're right. It's how consistently you cut the times you're wrong before they compound into something that erases many good decisions at once.

This is also, in my experience, the hardest rule to actually follow — not because it's complicated, but because it asks you to act against a very natural instinct. The instinct when a position is down is to wait for it to come back, especially when the original thesis still feels intact. O'Neil's rule exists specifically to override that instinct with a mechanical line, precisely because the instinct is unreliable exactly when it matters most.

How I think about this outside of a single stock position

I don't run a stock-trading operation at Glencore Associates in the sense O'Neil was writing for — the structure holds equities alongside gold, real estate, and vehicles, on longer horizons than an active trader typically works with. But the underlying discipline generalizes past the specific 7-8% number: predefine, before committing capital, what evidence would tell you the original thesis was wrong, and act on that evidence promptly rather than waiting for it to resolve itself. Applied to real estate, that might be a property whose operating numbers don't recover after a reasonable stabilization period. Applied to a vehicle position, it might be a maintenance or depreciation profile that no longer matches the original case for holding it. The number changes by asset class; the discipline of defining the exit before you need it doesn't.

The part most people skip

O'Neil's rule works because it's simple enough to actually follow in the moment, when following it is hardest. A more sophisticated-sounding rule that requires judgment calls under stress tends to get rationalized away exactly when discipline matters most. That, more than the specific 7-8%, is the actual lesson: a good risk rule is one plain enough that you follow it even when you don't want to.

This article is a summary of a publicly published investing principle by William O'Neil, offered for informational and educational purposes only. It is not investment advice, a recommendation of any specific security or strategy, or an endorsement by William O'Neil or Investor's Business Daily of Glencore Associates or any approach described here. Glencore Associates does not manage capital on behalf of outside investors and is not currently open to outside investment.

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About David Bellairian

David Bellairian is the founder of Glencore Associates, where he personally allocates capital across public equities, gold reserves, income-producing real estate, and a commercial vehicle portfolio. He is also the founder and CEO of AIDiscover, an AI-visibility and brand-discovery agency in Los Angeles. Connect with David on LinkedIn, X, or Instagram.

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Sources referenced in this article:

• William O'Neil, How to Make Money in Stocks (source material for the 7-8% sell rule)

• TraderLion, "William O'Neil's Sell Rules and Risk Management"

• Investor's Business Daily, background on William O'Neil