At 8:20 on Tuesday morning, Sam opened his charts and realized he had nothing to look at. XYZ was flat. ABCD was off his list for good after Lesson 34: The Bid-Ask Spread showed him what its 12-cent spread would cost.

Somewhere among the thousands of US-listed stocks, a few were moving on real news with real VolumeVolume is the total number of shares (or contracts) traded during a given time period — it's the primary confirmation tool for all price action, because a price move accompanied by high volume indicates genuine market participation, while tRead definition →. He had no way to find them except clicking through tickers one at a time, and the open was 70 minutes away.

That's the job a stock scanner does. Module 4 is about finding stocks worth watching, and it starts here.

In 30 seconds

  • A stock scanner is software that checks thousands of stocks against conditions you set, such as price, percentage move and volume, and lists only the ones that match.
  • A screener runs your conditions once, like a search; a scanner keeps checking and updates the list as prices change.
  • A scanner tells you what to look at, not what to trade. Its list is a set of candidates you still have to judge on CatalystA catalyst is the specific event or news item that triggers unusual price movement in a stock — earnings reports, FDA decisions, analyst upgrades, acquisition announcements, product launches, and short squeeze setups are all catalysts. WithRead definition →, chart, LiquidityLiquidity in trading refers to how easily you can enter and exit a position at a fair price — a liquid stock has high volume and a tight bid-ask spread, allowing large orders to be filled quickly without moving the price, while an illiquid Read definition → and risk.
  • Scanners surface attention: big moves, heavy volume and news. Research shows individual investors tend to buy attention-grabbing stocks, so treat a scan result as a reason to check a stock and nothing more.

In this lesson you'll learn what a scanner is, how it differs from a screener, how a scan narrows the market, when traders run one, and what a beginner actually needs. If you're new, start with Lesson 25: Liquidity and Volume, because liquidity is the first thing you'll check on every name a scan returns.

What is a stock scanner?

A stock scanner is a software tool that checks a large list of stocks against conditions you choose and shows you only the stocks that meet all of them.

Think of a recruiter with 5,000 applications and one afternoon. She doesn't read every one. She runs a keyword search for the skills the job needs and gets a shortlist of 30. That search hasn't hired anyone. It has told her whose applications to read properly.

A scanner works the same way for stocks. You might tell it: "Show me stocks priced between $5 and $100 that are up at least 4% from yesterday's close on at least 50,000 shares traded." Instead of thousands of tickers, you get a list short enough to read.

The list is where your thinking starts. Every name on it still needs the checks you learned in Module 3: is it liquid, what's the spread, what does the daily chart say, and where would you be wrong?

A scanner narrows the field; you still review catalyst, chart, liquidity and risk before anything makes your watchlist.

What's the difference between a scanner and a screener?

A stock screener filters stocks once against your conditions and gives you a fixed list; a stock scanner keeps re-checking those conditions during the session and adds or drops stocks as prices and volume change.

Screener
How it runs
Scanner
Once, when you press search
Screener
Typical data
Scanner
Recent or end-of-day values
Screener
Best used for
Scanner
Preparing a list the night before or early pre-market
Screener
What it gives you
Scanner
A fixed shortlist

In practice the line has blurred. Many tools, including TradingView's screener, can refresh their results, and many scanners also let you run a one-off search. What matters for you is the question you're asking: "What's set up for tomorrow?" is a screener question, and "What's moving right now?" is a scanner question.

A screener answers your question once; a scanner keeps checking. Neither one makes the trade for you.

How does a scan narrow thousands of stocks to a short list?

A scan narrows the market by applying each condition to every stock in its universe and keeping only the stocks that pass all of them, so each condition you add removes more names.

Every scan has three parts:

  1. The universe. The pool being searched, usually US-listed stocks, sometimes with ETFs or over-the-counter stocks added or removed.
  2. The conditions. Minimum and maximum price, percentage change, volume, Relative VolumeRelative volume (RVOL) compares a stock's current trading volume to its average volume for the same time of day, expressed as a multiple — a stock with 5x RVOL is trading five times more actively than normal, signaling unusual interest and Read definition →, FloatFloat is the number of shares of a company that are actually available for public trading — it excludes shares held by insiders, major institutions with lock-ups, and the company itself.Read definition → and many more.
  3. The sort. How the results are ranked: by percentage gain, by volume, by relative volume.

Because every condition must be met, stacking them shrinks the list fast. Add too many, or set them too strictly, and you get nothing. Too many or too-strict conditions are the most common reason a beginner's scan comes back empty.

Which conditions to use, and what values to start with, is the subject of Lesson 39: Screener Filters. For now, three simple ones are enough: price limits, a minimum percentage move and a minimum volume.

When do day traders run scans?

Most day traders run their main scan before the market opens, then keep a live scan running through the first part of the session to catch stocks that start moving after 9:30 a.m. ET.

  • Pre-market. A gap scan finds stocks trading well above or below yesterday's close, usually because of overnight news. This is where most morning lists begin. Lesson 42: Pre-Market Gappers shows how to grade them.
  • The open. A live scan catches stocks that were quiet before the bell but suddenly trade heavily, often on news released during the session.
  • After the open. Many traders scan less, or switch to a narrower scan, once the first hour's activity fades.

One warning about the pre-market list. The SEC notes that extended-hours sessions can have less trading activity, wider spreads and bigger price swings than regular hours (Investor.gov, extended-hours trading). A stock that's up 30% at 8:00 a.m. on a few thousand shares may look very different at 9:31. Lesson 11: Stock Market Hours covers the sessions themselves.

Why doesn't a scanner tell you what to buy?

A scanner can't tell you what to buy because it only measures what you asked it to measure, usually price movement and volume. Those show where attention is, which is a different thing from a good trade.

That difference matters more than it sounds. In a well-known study of brokerage records, Barber and Odean found that individual investors are net buyers of attention-grabbing stocks: stocks in the news, stocks with unusually high trading volume and stocks with extreme one-day returns. Their explanation is simple. With thousands of stocks to choose from, people mostly consider the ones that catch their eye first (Barber and Odean, 2008).

A later study of Robinhood users found the same pull and put a cost on it: the stocks those users bought most heavily on a given day averaged abnormal returns of about -4.7% over the following 20 days (Barber, Huang, Odean and Schwarz, 2022).

A scanner is an attention machine. It puts the loudest stocks of the day in front of you, which is exactly what you want for finding candidates and exactly what makes it dangerous if you treat the list as a shopping list.

Some of that attention is manufactured. The SEC warns that a jump in a stock's price or trading volume linked to promotional activity is a red flag, and that microcap and penny stocks are more open to manipulation (Investor.gov, fraudulent stock promotions). A tiny stock at the top of a gainers scan, pushed by social media posts, is a classic version of that pattern.

So read every result with a question: why is this moving, and can I trade it cheaply? The first half of that question belongs to Lesson 37: Catalysts. The second is the liquidity and spread check you already know.

Do you need a paid scanner as a beginner?

No. A free screener is enough to learn how scans work and to practice building a morning list; a paid real-time scanner starts to matter when you trade live and delayed data would leave you minutes behind.

Free tools, such as the screener in a free TradingView account or the one built into many brokers, let you set conditions, sort results and save a scan. Their usual limits are delayed data, fewer conditions and no live alerts. For learning and paper practice, those limits are fine.

Disclosure: This lesson contains affiliate links. If you buy through them, DayTradingToolkit may earn a commission at no extra cost to you.

When you're ready to scan live, this guide's top pick for an all-in-one active trading platform is Trade Ideas. It combines real-time scanning with hundreds of filters and ready-made scans, charting, simulated (paper) trading and, on its top plan, the Holly AI trade-idea engine, whose results vary with market conditions. The lowest-commitment way in is its free plan, which runs on delayed data and is best for learning the layout. A few times a year Trade Ideas runs a low-cost Test Drive of the full platform; our Test Drive guide explains what it includes. Our Trade Ideas review covers who it suits and who should skip it.

A paid scanner doesn't make a stock worth trading. The thinking you do after the scan is the part no tool does for you.

Sam runs his first pre-market scan (hypothetical)

Sam opens a scanner (this example uses Trade Ideas) and builds the simplest gap scan he can, with three conditions.

Hypothetical: Sam's first scan, Tuesday pre-market

Illustrative example
  1. Three conditions

    Price between $5 and $100. Up at least 4% from yesterday's close. At least 50,000 shares traded pre-market. He leaves everything else blank.

  2. 40 names

    The scan returns 40 stocks, sorted by percentage gain. The top one is up 61%. Number 40 is up 4.1%.

  3. The top of the list

    He opens the top three. Each trades only a few thousand shares a minute, with pre-market spreads of 8 to 15 cents. By his Lesson 34 rule, any of them would cost more than 10% of his $60 risk in spread alone.

  4. Too many to watch

    He's checked six names in 25 minutes. At that pace, the other 34 would take until after the open. No trade, no orders: this was practice.

Sam writes one line in his notebook: "The scan found what's loud. It didn't find what's good." Forty candidates is a better problem than none, but it's still a problem.

Starting at the top of the gainers list

You're doing it if
You sort by biggest percentage gain and look at the first name first, every morning, because it's the most exciting.
The fix
Sort once by percentage gain to see the market's mood, then re-sort by volume or relative volume and start with names you could actually trade at your size. Check the spread before the chart.

Your turn: build and break a scan

Scanner practice (15 minutes, no orders)

0 of 6 done

Check your understanding

Check your understanding

Your scan returns zero stocks every morning. What's the most likely cause?

Check your understanding

Which question is a screener built to answer, rather than a live scanner?

Check your understanding

A tiny stock tops your gainers scan, up 70% pre-market, and social media is full of posts about it. What's the best first reaction?

Key takeaways

  • Scanners save you from searching; they don't do your judging.
  • Each condition removes stocks, so start with three simple ones and adjust one at a time.
  • Pre-market results come from thin trading, so spreads and prices can change a lot by the open.
  • The loudest stock on the list is often the riskiest. Check why it's moving and what it would cost to trade.
  • Free tools are enough to learn; real-time data matters once you trade live.

Frequently asked questions

Can a scanner find stocks that are falling?
Yes. Most scanners can sort by the biggest losers or scan for stocks gapping down, which short sellers use. The same checks apply: why is it moving, how liquid is it and what's the spread. Short selling has its own extra risks, which later lessons cover before you'd ever try it.
What's the best stock scanner for day trading?
It depends on your stage and budget. This guide's top pick for an all-in-one platform is Trade Ideas, but a free screener is the right place to learn. Our stock scanner comparison ranks the main options side by side.
Can I use TradingView's free screener for day trading?
You can use it to learn and to build a pre-market list. Check what data delay applies to your plan and the exchanges you watch. Many traders use a screener like this for preparation and a real-time scanner or broker tool once the session starts.
Why are so many scanner results cheap, tiny stocks?
Because small stocks can move much further in percentage terms on less money, so they dominate any "biggest gainers" sort. A price floor and a volume minimum remove most of them, and that's usually wise for a beginner, since thin, cheap stocks have wider spreads and are more open to manipulation.
How many stocks should a scan return?
Enough to choose from and few enough to check properly, often somewhere around 10 to 30 before you narrow it down. If you get hundreds, tighten a condition; if you get none, loosen one. Turning that list into the few you'll watch is the next lesson.

Up next: how does Sam get from 40 names to 3?

Sam's scan worked, and that was the problem: 40 candidates and less than an hour. He needs a quick way to throw most of them out and keep the two or three worth watching closely. Lesson 36: Building a Watchlist gives him that routine.

Disclaimer

This lesson is for educational purposes only and is not financial advice. Scanner results are lists of stocks that met conditions and are never recommendations. Pre-market prices can change sharply by the open. Free tools often use delayed data that is unsuitable for live orders, and heavily promoted small stocks carry extra risk of manipulation. All Sam examples and numbers are hypothetical. Trading involves the risk of loss, and nothing here guarantees any result. For our complete disclaimer, visit https://daytradingtoolkit.com/disclaimer.

Article Sources

This lesson relies on SEC investor guidance and peer-reviewed research.