Risk & volatility

Gap risk

Gap risk is the possibility that a stock’s next available trading price jumps above or below its previous price, causing a fill or loss beyond the level expected.

What it means

A stock can move sharply between one available trading price and the next, with no opportunity to trade at the prices in between. This often happens between the regular-session close and the next open, but gaps can also follow a trading halt or occur while trading is thin. News, earnings, and broader market events can contribute.

For a pending order, the next available price may differ substantially from the price used to plan the trade. For an existing position, a gap can create a loss larger than the distance to a planned stop level. Gap risk describes this uncertainty; it does not mean every gap causes a loss. The direction of the move and whether the investor holds a position or has an unfilled order both matter.

How it is calculated

There is no single formula that measures the probability or full financial consequence of a future gap. To describe a gap after it occurs, use:

Gap (%) = (first available price − reference price) / reference price × 100

  • First available price: The price at the next relevant trade, such as the opening price after an overnight close, in US$ per share.
  • Reference price: The earlier price used for comparison, commonly the previous regular-session closing price, in US$ per share.
  • Gap (%): The signed percentage change. A positive result is a gap up; a negative result is a gap down.

The absolute value of the result gives the gap’s size without its direction. An opening gap is not necessarily an investor’s execution price: the actual fill depends on the order and available liquidity. A percentage gap also does not, by itself, measure the probability of a future gap.

Worked example

Assume a hypothetical stock closes at US$100 per share. An investor holds 100 shares, valued at US$10,000 at that close, and has a sell stop order with a stop price of US$95.

  1. Before the next regular session, news arrives. The first available trading price is US$90. The opening gap is (US$90 − US$100) / US$100 × 100 = −10%.
  2. At US$90, the position’s marked value is US$9,000, or US$1,000 below its closing value. This is an illustrative change in value, not necessarily a realized loss.
  3. A conventional sell stop order becomes a market order when its stop condition is triggered. If it fills at US$90, the proceeds are US$9,000 before fees. That is US$500 less than the US$9,500 implied by selling 100 shares at the US$95 stop price.

The US$90 fill is an assumption for illustration, not a guaranteed opening fill. Depending on the broker’s stop-trigger rules and market conditions, the order could fill at another price or remain unfilled. A stop-limit order can restrict the acceptable price but may not execute at all.

Why it matters before you trade

A pre-trade check can distinguish a planned order price from a price that is actually available after a gap. It can also identify whether an order is exposed to an overnight period, a scheduled announcement, or a possible trading interruption. These checks do not establish where the stock will open.

Position size affects the dollar exposure: the same per-share gap produces a larger dollar change across more shares. Spread, market depth, and trading volume matter too, because a quoted opening price may not be available for an entire order. Historical daily volatility and one-day value at risk can provide context, but neither guarantees a limit on gap losses, especially during unusual events.

How PreTrAIde uses it

PreTrAIde’s stated quantitative methods do not include a separate gap-risk probability or gap-risk score. Its pre-trade analysis evaluates a proposed stock basket before an order is placed using measures including daily volatility, estimated spread, market impact, liquidity, and one-day 95% value at risk. Recent news is among the inputs to its AI layer.

The basket and each line receive a GO, NO-GO, or MODIFY verdict with reasoning and a suggested execution strategy. Those outputs are decision support, not a prediction of a gap or an assurance that an order will fill at a chosen price. PreTrAIde does not place orders; users trade through their own brokers.

Common mistakes

  • Treating a stop price as a guaranteed sale price: A stop-market order can fill beyond its stop price after a gap.
  • Equating the opening price with every fill: Available volume, order type, and timing affect execution.
  • Assuming a limit order removes gap risk: It limits the permitted execution price, but the order may receive no fill.
  • Using ordinary volatility as a maximum-loss figure: An exceptional gap can exceed a routine volatility estimate.
  • Counting every gap as a loss: A gap’s effect depends on its direction and the position or order involved.

Frequently asked questions

Can a stop-loss order protect against a price gap?

A conventional stop order does not guarantee execution at its stop price. Once triggered, a stop-market order may fill at the next available price, which can be substantially worse after a gap.

How do you calculate an overnight gap percentage?

Subtract the previous regular-session closing price from the next opening price, divide by the closing price, and multiply by 100. A negative result indicates a gap down.

Is gap risk the same as daily volatility?

No. Daily volatility summarizes price variation using a particular method and observation period; gap risk concerns a discontinuous move between available prices. A volatility estimate does not set a maximum gap size.

Related terms

Overnight risk Overnight risk is the possibility that a stock’s value changes while its main exchange is closed, leaving an investor unable to trade at the previous closing price. Event risk Event risk is the possibility that a specific announcement or occurrence causes an abrupt change in a stock’s price, liquidity or trading conditions. Stop order Stop order is an instruction that becomes a market order when a specified stop price is reached, so its execution price can differ from the stop price. Stop-limit order Stop-limit order is an instruction that, once a specified stop price is reached, becomes a limit order that can execute only at its limit price or better. Slippage Slippage is the difference between a trade’s expected price and its actual fill price, expressed per share, as a percentage or in total dollars. Trading halt Trading halt is a temporary pause in trading a security, imposed by an exchange or regulator, during which orders generally cannot execute on the affected market.

Risk & volatility

Volatility Historical volatility Implied volatility Annualized volatility Daily volatility Parkinson volatility estimator Yang-Zhang volatility estimator GARCH model All glossary terms →

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Educational content, not investment advice. PreTrAIde is analysis software, not a broker: it never executes orders, and its verdicts are decision support, not recommendations to buy or sell.