Scanner

A scanner is a software tool that continuously checks a large group of stocks (or other instruments) against a set of rules you define, and shows you only the ones that currently match. Instead of a trader manually flipping through hundreds or thousands of tickers looking for something interesting, the scanner does that watching for them.

Under the hood, a scanner is fed real-time or delayed price and volume data, and it runs your chosen filters against every symbol in its universe on a repeating cycle. Typical filters include price change over a time window (say, up 5% in the last hour), unusual volume compared to the stock's average, price crossing above or below a certain level, or a stock hitting a new high or low for the day. When a stock meets the criteria, it appears on the scanner's results list, often with a sound or visual alert.

The nuance that trips people up is that a scanner surfaces candidates, not signals. It tells you a stock matches a pattern of numbers, not that the pattern means anything tradable right now. Two stocks can pass the same "up 5% on high volume" filter for completely different reasons — one on real news, one on a thin, easily-reversed spike — and the scanner alone won't tell you which is which. Scanners also depend heavily on how tightly or loosely the filters are set: too loose and you get flooded with noise, too tight and you miss moves.

It's also worth knowing that scanners range from generic, built into most trading platforms, to specialized ones built around a specific strategy's logic, which narrow the universe using that strategy's own assumptions rather than generic price/volume rules.