A stock scanner watches every trade and quote in the market, fires an event the instant a stock does something specific (new high of day, volume spike, halt), and shows you only the stocks that also pass your numeric filters. That’s the whole machine: events plus filters, running against live data, all day.
Understanding that machine is the difference between a scanner that hands you two or three stocks in play each morning and one that buries you in noise. This guide explains the mechanics, then walks through building a scan step by step.
Who this is for: traders who own a scanner and aren’t getting clean results from it, and traders deciding whether they need one at all. Prerequisite: none. The menu names and engine details below come from the Trade Ideas documentation, verified in June 2026, because its user guide is public and unusually specific about how the engine works. The concepts transfer to any real-time scanner. If you want to follow along live, a free Trade Ideas account includes browser dashboards on 15-minute delayed data per the official help center; delayed data is fine for learning the mechanics, useless for trading the open.
The engine: events, lists, and how they combine
Real-time scanners present results in two fundamentally different ways, and knowing which one you’re looking at changes how you read it.
Alert windows stream events. Per the Trade Ideas user guide, an alert window monitors and streams events in real time, with each new event appearing as a time-stamped line item at the top. An “event” is a thing that just happened: a stock crossed above VWAP, printed a new high, got halted, spiked on volume. The platform’s published index of alert types runs from new highs and premarket highs through halts, gap reversals, opening range breakouts on 1–60 minute timeframes, moving average crosses, and candlestick patterns like dojis and topping tails. You’re watching a tape of triggers, newest first.
Top lists rank snapshots. A top list window, per the same documentation, refreshes every 30 seconds and behaves like a self-updating spreadsheet: it ranks every stock by one sort criterion (say, relative volume) and shows the top 100 by default, extendable to 1,000. No event needs to happen. A premarket gappers list is a top list; a high-of-day momentum stream is an alert window.
The combination logic matters more than anything else on this page. In the Trade Ideas engine, alerts work in an OR fashion: each alert triggers independently when its event fires. Filters work in an AND fashion: a stock must pass every filter to appear at all. So a scan with three alerts and four filters reads as “show me any of these three events, but only on stocks that clear all four hurdles.” Every filter you add cuts the result count; every alert you add grows it. When your scanner is too noisy, the fix is almost always another filter, not fewer alerts.
Scanner vs screener
The terms get used interchangeably, but the working distinction is time. A screener queries a static database, usually end-of-day data, and answers “which stocks have a P/E under 10 and a float under 50 million?” A scanner runs continuously against live intraday data and answers “which of those stocks is breaking out right now?” Screeners build watchlists the night before; scanners find the trade while it’s happening. Day traders need the second one, because the move that matters is measured in minutes.
How to build a scan that finds your setup
The steps below use Trade Ideas menu paths where a path is needed. The logic is universal.
Step 1: define the setup in one sentence
A scanner can’t find “good opportunities.” It can only find quantifiable conditions. So start with a sentence a machine could parse: “stocks under $20 that gapped up premarket on news, trading at least five times their normal volume, breaking the premarket high after the open.” If you can’t write that sentence, the scanner isn’t your problem; the missing trading plan is. Pick one setup, ideally from a strategy you already understand, like the gap and go (the gap scanning guide covers that translation in full).
Step 2: translate it into numbers
Turn each clause of the sentence into a measurable criterion:
- “Under $20” becomes a price filter: minimum $2 (to skip the sub-dollar churn), maximum $20.
- “Five times normal volume” becomes relative volume of 5 or higher. RVOL is the single best noise cutter in scanning, because it normalizes for each stock’s own baseline.
- “Gapped up” becomes gap percentage of 4% or more from yesterday’s close.
- “Capable of a real move” usually means a float cap, commonly under 20 million shares for momentum names, since low float plus high RVOL is what produces outsized intraday ranges.
Treat these as starting values to test, not magic numbers. The point is that every word in the setup sentence now has a number attached.
Step 3: start from a prebuilt scan, then modify
Don’t build from a blank page on day one. The Trade Ideas Channel Bar ships more than 40 preconfigured, customizable scans covering momentum, short squeezes, premarket and after-hours activity, gaps, and sector plays, per the Channel Bar documentation; open it via Toolbar, then New Tab, then Channel Bar. To see what’s inside any scan, right-click the alert window, select Configure, and open the Strategies Tab, which lists the prebuilt bullish, bearish, and neutral strategies along with each one’s alerts and filters. Load the closest match with Load Settings. The Flip option converts a bullish configuration to its bearish mirror in one click. Other platforms ship preset scans under different names; the principle of “modify a working template before building from scratch” holds everywhere.
Step 4: tighten with filters
In the Configure menu, the Window Specific Filters Tab is where your Step 2 numbers go. Each filter takes a minimum and a maximum value. When you’re unsure where to set a threshold, the Show Me feature (the magnifying glass icon between the min and max fields) displays how many stocks pass at different values, as a histogram and worked examples, so you can see whether RVOL ≥ 5 leaves you 12 candidates or 200 before you commit. One detail worth stealing for any platform: a filter left blank but displayed as a column (Show Column) reports the value at alert time without constraining results. That’s how you audit a criterion before you enforce it.
Step 5: control the output
A scan that fires 400 times a day is a slot machine, not a tool. Two controls keep it honest. First, columns: display the fields you actually decide on (price, RVOL, float, gap %) so each line answers “do I care?” in one glance. Second, sound: reserve audio alerts for the one or two scans whose signals you’d act on immediately, and leave everything else silent. The logic is mechanical. If every window dings, no ding carries information.
Step 6: test, count false positives, refine
Run the scan for a week without trading it. For every alert, note whether the stock followed through on your setup or stalled, and which criterion the failures shared. Then adjust one value at a time: raise the RVOL floor, add a spread filter, narrow the price band. Loosening filters to catch moves earlier always buys speed with false positives; tightening does the reverse. There is no setting that gives you both, only a trade-off you choose deliberately. A trading journal is the natural place to log which scan surfaced each trade, because over a month it tells you which scans earn their screen space.
Recommended starting values for a momentum scan
For a first momentum scan on US stocks, these values produce a watchable, not overwhelming, stream on a typical session: price $2–$20, relative volume at least 5, float under 20 million shares, up at least 4% on the day, with new-high and high-relative-volume alerts active. On a quiet tape expect a handful of names; when the market’s hot, expect dozens, and tighten the RVOL floor rather than trying to watch them all. A stock that keeps re-hitting the scan, printing alert after alert, is telling you more than a stock that fired once and went quiet. Repetition is the momentum signal.
Where scanners live, and what each location costs you
Broker platform scanners come bundled with your trading platform. Zero extra cost, and adequate for end-of-day screening and basic intraday lists. Dedicated scanning software is a separate subscription whose entire engineering budget goes into scan speed, alert breadth, and filter depth; the alert index linked above is what that specialization looks like. Free and delayed tiers exist in between: the free Trade Ideas account noted earlier runs browser dashboards on 15-minute delayed data, and the same help article states plainly that no free fully functional trial exists because exchanges set requirements around live real-time data. That constraint applies industry-wide: real-time data costs real money, so any “free real-time scanner” is worth a skeptical second look at what data it’s actually feeding you.
Which option you need depends on how you trade. If you trade the open every day on momentum, scan speed is your edge and a dedicated tool earns its subscription. If you check the market twice a week, it almost certainly doesn’t. The stock scanner rankings compare the current options by use case.
One reality check before you subscribe to anything: a scanner finds candidates, not profits. Most day traders lose money, and a faster pipeline of stocks to trade does nothing for a trader without an edge to apply to them. The scanner amplifies whatever process you already have, including a bad one.
Troubleshooting
Too many alerts. Add filters, don’t remove alerts. RVOL and spread filters cut the most noise per click. Filters combine AND-fashion, so each one strictly shrinks the stream.
Too few alerts. Check the restrictive filter first; one over-tight maximum (a $10 price cap, a 5 million float cap) can zero out a scan on its own. Use a value-testing feature like Show Me, where available, to see which criterion is doing the strangling.
Alerts arrive after the move. Some lag is structural in any alert chain, but most “late” alerts are really early-confirmation settings: confirmed-breakout alert types wait for follow-through by design. Trading the faster unconfirmed variant means accepting more false signals. Same trade-off as Step 6, surfaced in a different menu.
Great alerts, bad trades. That’s not a scanner problem. Log the trades in a journal and look at entries and exits, not the scan.
Next steps
Once the mechanics make sense, the highest-leverage move is tuning real configurations: the Trade Ideas scan settings guide walks through tested setups for momentum and gap trading, and the full walkthrough covers the platform end to end. If you don’t own a scanner yet and the alert-and-filter engine described here fits how you trade, start with the Trade Ideas review to see whether the price is justified for your trading frequency, and what the cheaper alternatives give up.
FAQ
What’s the difference between a stock scanner and a stock screener?
Data recency and purpose. A screener filters a mostly static, often end-of-day database to build watchlists and research lists. A scanner runs the same kind of criteria continuously against live intraday data and alerts you the moment a stock qualifies. Day traders rely on scanners; longer-term investors are usually fine with screeners.
Do stock scanners work in the premarket?
Yes, on platforms that scan extended hours. Trade Ideas, for example, documents premarket-high and premarket-low alert types and ships a dedicated premarket channel in its Channel Bar. Premarket scanning is where most momentum traders build the day’s watchlist, since the stocks gapping on news before 9:30 are usually the stocks in play after the open.
Are free stock scanners good enough for day trading?
Good enough to learn on, rarely good enough to trade the open with. Free tiers typically run on delayed data: the free Trade Ideas account, for instance, provides browser dashboards on a 15-minute delay. A 15-minute-old alert on a momentum stock describes a move that already happened. For end-of-day screening and watchlist building, free tools are genuinely fine.
Does a scanner tell you what to buy?
No. It tells you which stocks currently match conditions you defined. Whether any of them is a trade depends on your strategy, your risk plan, and the chart in front of you. Treat every alert as a candidate for analysis, not a signal to click buy.
How many scans should you run at once?
As few as cover your actual setups. One or two primary scans with audio, plus a gappers list for the morning, beats ten windows competing for attention. Every scan you add raises the odds you’re watching alerts instead of managing the trade you’re in.