Thinkorswim Gap Scanner Settings

What a pre-market gap scanner does

A pre-market gap scanner finds the stocks trading meaningfully above or below their prior close before the 9:30 AM ET open — usually on a fresh catalyst like earnings, FDA news, an analyst upgrade, or a buyout. Its job is to reduce the whole market to a short list of names worth a closer look before the bell, ranked by gap size, with the volume and float context that tells you whether a gap has follow-through potential or is a thin, low-volume head-fake.

Why Thinkorswim is the popular free choice: thinkorswim (often abbreviated TOS) is a full-featured trading platform that’s included at no separate fee for eligible Schwab and TD Ameritrade brokerage accounts — TD Ameritrade clients migrated to Schwab, and thinkorswim is now Schwab’s flagship active-trader platform. There’s no monthly subscription for the platform itself, and that includes its built-in scanner, Stock Hacker. That makes it the default first stop for a lot of traders who want to scan pre-market gappers without paying for a third-party scanner.

How to build a pre-market gap scan in Stock Hacker

thinkorswim’s scanner is called Stock Hacker, and it’s where you assemble a filtered scan of the market. One honest note up front: exact menu labels and filter locations shift between thinkorswim versions and between the desktop, web and mobile apps, so the steps below are described at the level that stays true across builds rather than as a click-by-click walkthrough. The filters themselves, though, are the same set gap traders use everywhere:

• Gap % (net change vs. prior close) — the core filter. Start with ±3–5%. A stock trading 5% or more above its prior close before the open is a “gap up”; 5% or more below is a “gap down.” thinkorswim’s built-in % change / net % change filter (measured against the prior session’s close) is usually the simplest way to express this without custom code.
• Pre-market volume / relative volume — a gap with no volume tends to fade. Look for unusual pre-market volume, or relative volume (current volume vs. its normal average) of roughly 3–5× or more. This is the filter that separates a real move from a thin one.
• Float — the number of shares available to trade. A lower float (often under ~20 million shares) is what turns an ordinary gap into a squeeze candidate. A clean float filter isn’t available in every thinkorswim build, so some traders pull float from elsewhere and cross-reference.
• Price range — band the universe to names you’d actually trade. A common starting point is $1–$20 for low-float runners, or higher for large caps.

If you want to compute the gap precisely yourself, thinkorswim’s thinkScript language can express it as gap % = (current price − prior close) ÷ prior close × 100. It’s powerful, but it’s also the point where a quick “show me today’s gappers” job starts turning into a scripting task — and where the time cost creeps in. (A small thinkScript snippet can do this, but syntax varies by build, so treat any snippet you find as a starting point rather than a copy-paste guarantee.)

Thinkorswim’s honest limits for this job

• It’s build-it-yourself, not turnkey. There’s no “show me today’s top gappers” button out of the box — you configure the filters, save the scan, and re-run it each morning.
• Clunky for a fast pre-market look. Between opening the platform, navigating to the scan, tweaking filters and waiting for results to populate, it’s a slower loop than a purpose-built scanner that shows ranked gappers the moment it loads.
• Pre-market data caveats. Real-time pre-market and after-hours data can depend on your account’s data entitlements and whether the platform is open and connected; some extended-hours quotes may be delayed or incomplete, and real-time depth (Level II) can require an additional subscription. What’s enabled varies by account, so it’s worth confirming yours before the open.
• No catalyst column by default. thinkorswim will show you the numbers, but “why is this stock gapping?” still means checking the news yourself. A gap without a catalyst is a very different trade than one with a hard catalyst.

Why a purpose-built scanner is faster

None of this is a knock on thinkorswim — it’s an excellent general trading platform, and a lot of traders do their charting and order execution there. It’s simply that a pre-market gap scanner isn’t its focus. If the job is “show me today’s pre-market gappers right now,” a purpose-built scanner is faster because it’s built for exactly that:

• Real-time top gappers out of the box — no setup, no saved scans, no thinkScript.
• Gap %, relative volume, float and short interest on every row, plus a catalyst so you know why it’s moving.
• A Gap Score that ranks setup quality at a glance, instead of leaving you to piece together the same read manually.

Day Trade Scanner (MarketBeacon) is built for the pre-market specifically. The free tier shows the day’s top 5 gappers with live relative volume and a catalyst — no signup required to look. Pro ($1.99/wk) unlocks the full scanner, advanced filters (price, float, RVOL, short %), Gap Score, real-time alerts, and premarket charts. You can keep using thinkorswim for the parts it’s great at, and use a purpose-built scanner for the part it isn’t.

Frequently asked questions