Stock Scanners and Screeners
A stock scanner tests a whole market against conditions such as price, volume, relative volume or gap size, and returns the stocks that match. The result is a shortlist to study on a chart.
MadStockAlerts Research · Updated October 2, 2026
What to Take Away
- A scanner applies a list of conditions to every stock in a universe and returns the ones that pass.
- A screener usually runs on slower data and answers a slower question; a real-time scanner reruns as the session trades.
- Liquidity filters such as price and volume come first, because a match nobody can trade is not useful.
- Delayed data, thin small caps and unadjusted relative volume all produce results that mislead.
- Every row in a scan result is a candidate to study, and the conditions decide what the list means.
MAD Academy Training Video · 0:51
Thousands In, a Handful Out
Two scans over the same market at the same moment return two different lists, because the conditions are the whole definition of the result.
This lesson is part of a MAD Academy plan.
What a scanner is
A stock scanner is software that tests every stock in a universe against a list of conditions and returns the ones that pass. Traders use one to narrow thousands of listed securities to a short list worth opening on a chart. The conditions decide what the list means, and every name on it is a candidate to study.
The two words are often used interchangeably, but they describe tools built for different speeds of question.
| Aspect | Screener | Real-time scanner |
|---|---|---|
| Data | End-of-day prices and reported fundamentals | The live session, trade by trade |
| Refresh | Once a day, or on demand | Continuously while the market is open |
| Typical conditions | Valuation, growth, dividend, sector | Percent change, volume, relative volume, new highs |
| The question it answers | Which companies have these characteristics? | Which stocks are doing this right now? |
The common filters
| Filter | What it measures | Why it is used |
|---|---|---|
| Price | The last trade | Sets a floor below which spreads and volatility behave differently |
| Volume | Shares traded in the session | A floor on liquidity, so a match can actually be traded |
| Relative volume | Today's volume against the stock's own average | Finds unusual participation, which is comparable across stocks |
| Gap percent | The open against the prior close | Finds stocks repriced overnight, usually by news |
| Float | Shares available to trade | A small float can move further on the same demand |
| Market capitalization | Total value of the shares outstanding | Separates very different kinds of company |
| Technical condition | Price against a moving average, a 52-week high, an RSI reading | Selects a chart state, such as a trend or a stretched reading |
Each filter removes stocks; none adds any. A scan with seven conditions returns the stocks that pass all seven at once, which is why adding a condition narrows the list faster than intuition suggests. A combination of a price floor, a volume floor and one condition describing the move is the shape most scans share.
From scan to shortlist
- 1UniverseEvery listed stock the scanner covers
- 2Liquidity floorPrice and volume minimums
- 3The conditionA gap, a volume surge, a new high, a technical state
- 4ShortlistThe handful of names that passed everything
- 5Chart reviewStructure, levels, news and the filing behind the move
A preset is a saved list of conditions with a name: top gainers, gap ups, unusual volume, new 52-week highs. The name is a summary, and the conditions underneath it are the definition. Two presets with similar names can apply quite different thresholds, so reading the conditions is how the result is read.
Where scans mislead
| Pitfall | What happens |
|---|---|
| Delayed data | The list describes the market as it was several minutes ago, and the fastest movers have moved again |
| Thin small caps | One trade can move the percentage change, and the last price may be minutes old |
| Unadjusted relative volume | Early in the session the figure is distorted unless it is adjusted for time of day |
| Unadjusted corporate actions | A split can appear as an enormous gap that never traded |
| Survivorship in the universe | A universe of today's listings omits everything delisted, which matters for any historical scan |
A match describes the past
Every condition in a scan is measured on prices that have already printed. A stock that passed a gap scan has gapped; whether it does anything further is a separate question, and the scan contains no information about it. Treating the list as a set of signals skips the step where the chart, the levels and the news are read.
How MadStockAlerts uses this
The free MadStockAlerts scanner runs the whole US market through preset scans for gainers, losers, momentum, gaps, unusual volume and 52-week extremes, with filters on top. Its prices are not real-time; members scan in real time. Our premium alerts start from a market scan as well, and each one is reviewed by an analyst before it sends.
Run a preset, then open its conditions to see exactly what the list was selected for.
Open the free scannerPrimary Sources
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