The one-sentence version
A cluster buy is three or more insiders at the same company buying stock in a short window — usually a week or two. That's it. The reason it matters is why they're all buying at the same time.
Why any single insider buy is noise
One executive buying their own stock can mean anything. Maybe they want to look confident. Maybe they're diversifying a salary bonus. Maybe they inherited money and put it somewhere familiar. Any one buy is easy to explain away.
But when four executives independently decide to buy the same stock in the same week? That requires a coincidence, and coincidences are rare. What's much more likely is thatsomething specific is happening inside the company that's making them all confident at once.
Why the signal is strong
Insiders see the business first
Executives read internal sales reports, sit in strategy meetings, and talk to major customers weeks or months before that information becomes public. When they act on what they see, they're trading with information the market hasn't priced in yet.
Legal risk keeps it honest
Insider trading laws are strict. Every one of these executives knows their buy will show up on a public SEC filing within two days. They're putting a signal on the tape and betting they're right — that's not a decision you make lightly.
They're risking their own money
An analyst upgrade costs the analyst nothing. A CEO buying $500K of their own stock costs the CEO $500K. Skin in the game beats speculation every time.
Insiders don't coordinate their personal trades. When they appear to, something at the company is coordinating them.
What counts as a “cluster”
There's no official definition, but most serious analysts use some version of this:
- 2 or 3+ distinct insiders, not just one person splitting a trade across days
- Buys only — sells happen for a hundred non-signal reasons (tax planning, home purchases, pre-scheduled 10b5-1 plans)
- Within a rolling window, usually 10 to 30 days. Tighter is stronger.
- Meaningful dollar amounts — a $5K buy from a CEO worth $50M is symbolic; a $500K buy is a real bet
How to actually track them
Manually, you'd watch SEC Form 4 filings (public within two business days of every trade) and cross-reference by ticker and date. This is tedious — hundreds of Form 4 filings hit daily. You'd miss most clusters just because you weren't looking at the right rows.
Kestrel Terminal flags cluster buys automatically as they form, so you can set an alert like “email me when 3+ insiders buy in a 10-day window with total value over $500K.” You can also backtest cluster-buy strategies against historical data — see how they would've performed before you commit.