Price Gaps: Why a Gap Fill Is a Reference, Not a Promise

Price Gaps: Why a Gap Fill Is a Reference, Not a Promise

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Learning objective: define a price gap consistently, distinguish an opening gap from a full session gap, and test a gap-fill idea without assuming that every gap must close.

A gap marks an area between prices where little or no trading occurred in the chosen session data. Traders often describe that empty area as a magnet and say the market “must fill the gap”. No market mechanism guarantees that outcome. New information can move the auction to a different value range, and price may continue away from the old session for days, years or permanently.

The gap can still be useful. It identifies a discontinuity, a reference area and a testable question: does this type of gap, in this instrument and context, tend to retrace enough to support a strategy after costs?

This tutorial uses hypothetical daily share data in British pounds. All prices and probabilities are examples rather than trading instructions.

What exactly is a price gap?

CME Group's glossary defines a gap as a price area where the market did not trade from one day to the next. That simple definition needs a consistent session and data source.

Two measures are commonly confused:

Opening gap

The difference between the new session's opening price and the previous session's close.

Opening gap % = (current open − previous close) ÷ previous close × 100

Full price gap

For a gap up, the current session's low remains above the previous session's high. The untraded area lies between those two points. For a gap down, the current high remains below the previous low.

A market can open far above the previous close, then trade back through the previous high during the day. That still had an opening gap, but it no longer has a full daily gap on the completed candle.

Step 1: fix the session before measuring

Stocks may have regular and extended-hours sessions. Futures trade for much of the day but still have maintenance breaks and settlement conventions. Spot Bitcoin trades continuously, while Bitcoin futures can follow a venue-specific calendar.

Choose and document:

• exchange and instrument;
• timezone;
• regular session or extended-hours data;
• official open and close definition;
• whether adjusted prices are used for shares;
• treatment of dividends, splits and contract rolls.

Without a fixed session, two charts can show different gaps for the same instrument. A futures roll can also create an apparent discontinuity that reflects contract construction rather than an executable market move.

Step 2: define what “filled” means

Suppose the prior session has:

• high: £105;
• close: £104.

The next session opens at £110, trades no lower than £108 and closes at £112.

Measurement	Level	Distance from £110 open
First move into the full gap	£108	£2
Full-gap boundary	£105 prior high	£5
Prior-close reference	£104	£6

The opening gap is £6, or 5.77%, from £104 to £110. The completed candles also leave a £3 full gap between the prior high at £105 and the new low at £108.

Different traders might call the gap “filled” when price:

1. first enters the empty area;
2. crosses the entire full gap to £105;
3. reaches the prior close at £104;
4. closes beyond one of those levels.

Those are different outcomes. Select one definition before the test; changing it after seeing the chart produces an unreliable result.

Step 3: identify why the discontinuity occurred

Common sources include:

• earnings, guidance or company announcements outside regular hours;
• macroeconomic data or central-bank decisions;
• geopolitical news while a venue is closed;
• an opening auction with a large order imbalance;
• a thin market where little depth separates prices;
• a dividend, split, symbol change or data adjustment;
• a futures contract roll.

A data or corporate-action gap should not be analysed as if it were a fresh supply-demand shock. Verify the chart before creating a trade thesis.

CME Group's event-risk research explains that abrupt price gaps can arise when new information shifts the expected market value. A gap is evidence that the new auction began elsewhere; it is not evidence that the old price remains fair.

Step 4: treat gap labels as hypotheses

Technical literature often uses labels such as:

• common gap: a discontinuity inside a wider range;
• breakaway gap: a move out of a base or established range;
• continuation or runaway gap: a gap during an existing trend;
• exhaustion gap: a late move that is followed by reversal.

These descriptions can organise research, but some are only obvious afterwards. A strong breakaway gap and an exhaustion gap may look similar at the open. Future price action determines which label appears to fit.

Use measurable context instead:

• gap size relative to recent volatility;
• location relative to a tested range or trend;
• opening volume and auction imbalance;
• breadth and movement in related assets;
• whether the catalyst changes estimated cash flow, rates or supply;
• acceptance above or below the gap after the first trading window.

Step 5: build a conditional plan, not a certainty

For a hypothetical gap-up fade, a plan might require:

1. price fails to hold above the opening range;
2. the instrument re-enters the gap;
3. related market strength weakens;
4. risk is invalidated if price closes back above a defined opening structure.

A continuation plan might instead require price to hold above the gap boundary and form a higher low. The gap provides reference levels; price behaviour determines whether fading or following it is being tested.

Never short solely because “gaps always fill”. A large positive revaluation can continue, and short losses are not capped by the purchase price.

Step 6: calculate the trade, including the fill probability

Return to the hypothetical £110 open. Suppose a tested rule enters a short at £110, uses £115 as invalidation and targets the prior close at £104.

• risk per share: £115 − £110 = £5;
• potential gain per share: £110 − £104 = £6;
• gross reward-to-risk: 1.2R.

That ratio is not an edge. If 40% of comparable trades reach the target and 60% hit the stop:

Expectancy = (0.40 × 1.2R) − (0.60 × 1R) = -0.12R per trade

If a robust out-of-sample study instead finds a 55% target rate under the same rules:

Expectancy = (0.55 × 1.2R) − (0.45 × 1R) = +0.21R per trade before costs

Spread, commission, borrow fees and slippage can reduce or reverse that result. The probabilities are hypothetical and do not describe any current market.

Step 7: respect opening execution risk

The open can combine wide spreads, rapid repricing and order imbalance. A visible chart price is not necessarily available for the desired quantity.

The SEC's updated stop-order bulletin states that a stop price is a trigger rather than a guaranteed execution price. A market order can fill far from the stop, while a stop-limit order may not execute if price moves past its limit.

For a gap trade, model:

• the opening spread and available depth;
• partial fills;
• short-borrow availability where relevant;
• halts and volatility controls;
• stop slippage during another discontinuity;
• extended-hours restrictions at the broker;
• the difference between an indicative auction price and the final open.

Step 8: backtest without look-ahead bias

A credible study should:

1. use point-in-time session data with corporate actions handled correctly;
2. calculate the gap using information available at the open;
3. group gaps by size, catalyst, trend and volatility regime;
4. define entry, fill, invalidation and maximum holding time before testing;
5. resolve intrabar order when the target and stop occur in one candle;
6. include delisted stocks and failed gaps;
7. deduct realistic costs and borrow fees;
8. validate on later unseen data.

Report more than the fill rate. Include time to fill, maximum adverse excursion, drawdown, average return, tail losses and how much profit depends on a few reversals.

For supporting execution concepts, read Opening and Closing Auctions, Overnight Gap Risk and Order Book and Market Depth.

Common mistakes

Treating an open gap as a debt the market owes

Price has no obligation to revisit a level. A gap can mark a lasting change in valuation.

Mixing opening-gap and full-gap statistics

Reaching the prior high is different from reaching the prior close. Keep definitions consistent.

Ignoring corporate actions and futures rolls

Bad adjustments can create gaps that were not tradable.

Entering before liquidity forms

The first print can have wide spreads and shallow depth. Model realistic fills rather than chart midpoints.

Using a stop inside normal opening noise

An invalidation should follow the tested setup and current volatility, not the trader's preferred position size.

Calling every late gap exhaustion

The label often depends on the reversal that occurs later. Define evidence available at entry.

Waiting indefinitely for a fill

A time stop matters. Capital can remain trapped while price establishes a new range away from the old close.

Practical checklist

• Which instrument, venue, session and timezone define the gap?
• Is this an opening gap or a completed full price gap?
• What exact price or close counts as a fill?
• Have corporate actions, data errors and contract rolls been checked?
• What catalyst or order imbalance created the discontinuity?
• What evidence supports fade versus continuation?
• What price action invalidates the plan?
• Are opening spread, depth, partial fills and stop slippage modelled?
• What is the maximum holding time if the gap remains open?
• Does out-of-sample expectancy remain positive after all costs?

The Money Man lesson

A gap is a map reference, not a promise. Define the session, gap boundary and fill rule first. Then use context and new price behaviour to form a conditional plan. The strategy must earn its place through observed probabilities, realistic execution and controlled losses—not through the belief that every empty area on a chart must eventually disappear.

This tutorial was prepared with AI-assisted research and writing under The Money Man editorial workflow. All prices, probabilities and trade outcomes are hypothetical and used only to explain the method. It is educational information, not personalised investment advice or a promise of performance. Gaps can continue, stops can slip and losses can exceed the planned amount.