Gap Analysis Calculator

This gap analysis calculator measures the difference between Today’s Open and Previous Close, expresses it as a percentage, and uses the previous close as the fill reference. All prices are entered manually. Its Gap Fill Probability, Trading Bias and Entry Strategy labels come from fixed rules without a measured historical success rate. Its Fill Status compares only the open and current price, so check the session chart before treating a gap as unfilled.

Opening gap versus a gap on the full chart

An opening gap compares an opening price with the preceding close. A positive difference means an opening above that close; a negative difference means an opening below it. That comparison can exist even when the new price remains inside the previous session’s high-low range.

A gap between complete price bars is a separate chart observation: the trading ranges leave a price area between them. Fidelity’s chart-pattern material describes gaps as areas without trading at specific prices. Do not assume that an open-to-close difference establishes a completely empty chart zone.

For chart examples, see gap-up and gap-down basics. Keep the definition you use beside any recorded gap study so that separate sessions remain comparable.

Gap up and gap down percentage formula

A gap is the distance between where a market opened and where it closed last time. The calculator does this subtraction for you, and you can check it by hand.

• Gap in points = today's open − previous close. A positive answer is a gap up and a negative answer is a gap down.
• Gap percentage = gap in points ÷ previous close × 100.

Gap-up example, using the numbers in the worked example below: open 20,200, previous close 20,000. The gap is +200 points, and 200 ÷ 20,000 × 100 = +1%. Gap-down example, with made-up numbers: previous close 24,800 and open 24,632. The gap is 24,632 − 24,800 = −168 points, and −168 ÷ 24,800 × 100 = −0.68% after rounding. The negative sign tells you the direction, and the distance back to the previous close is the same size.

Full gap vs partial gap

Traders use "full gap" and "partial gap" for how far the open lands from yesterday's range. The calculator has Previous High and Previous Low boxes, so you can check this yourself.

In the worked example below, the open of 20,200 is above the previous high of 20,150, so it is a full gap up. If the open had been 20,100, it would be a partial gap up because it stays inside the previous range. A full gap leaves a visible empty space between the two sessions on the chart. A partial gap does not.

Enter the same instrument across every field

• Previous Close: the closing reference from the prior completed session.
• Today’s Open: the opening price for the session being examined.
• Previous High and Previous Low: the prior session’s range boundaries.
• Current Price: the latest price you supply yourself; the page does not refresh it from an exchange.

Use one symbol and one session convention throughout. Mixing the cash index’s close with a futures opening produces a different comparison from a same-instrument overnight gap. An underlying-index move also does not equal an option-premium move.

The form has no volume, news, session high, session low or price-history input. It cannot independently distinguish a breakaway gap from an exhaustion gap, or identify every price reached earlier in the session.

Worked example: gap size and a partial retreat

Use this made-up example: Previous Close 20,000, Today’s Open 20,200, Previous High 20,150, Previous Low 19,850 and Current Price 20,100. These are illustrative prices, not a record of an actual session.

• Opening gap = 20,200 − 20,000 = +200 points.
• Gap percentage = 200 / 20,000 = +1%.
• Previous range = 20,150 − 19,850 = 300 points.
• Retreat from the open to the entered current price = 20,200 − 20,100 = 100 points.
• Fill Level = the entered Previous Close, 20,000.

In this example, the current price remains above the fill reference. The tool prints Gap Unfilled because the open-to-current comparison has not reached the previous close. It does not know whether the market touched that close earlier, then rebounded.

For a separate downside illustration, keep Previous Close at 20,000 and change Today’s Open to 19,800. The opening is 200 points below the close. The subtraction gives a negative signed gap, while the distance back to the close is positive.

What the result labels can and cannot establish

Gap (Points) is rounded to a whole point by the tool. Gap % describes the opening difference relative to Previous Close. Fill Level is simply that previous close; it is not a predicted price target.

Gap Type uses fixed percentage thresholds. A small positive or negative opening can therefore be labelled Flat Open. Read the signed numerical gap as well as the category.

Gap Size uses the prior high-low range as a substitute for a volatility measure. The form does not calculate average true range, an average of true ranges across successive bars, from a price history. ATR means average true range. The displayed ATR comparison uses only the previous session’s range, so it does not measure that average.

Gap Fill Probability is selected from the gap’s size alone. There is no historical sample, instrument-specific study, look-ahead period or measured success rate behind it. Entry Strategy and Trading Bias likewise cannot account for your trade plan, fees, execution price or risk limit.

Four types of gaps and what they suggest

Chart guides usually split gaps into four types. The labels depend on where the gap sits in a trend, so they are usually decided after the fact. Treat them as a way to describe a gap, not to predict it.

The calculator does not know whether the market is in a trend or what volume was. You have to look at the chart. Read how to trade gap up and gap down for how traders approach each situation.

Do gaps always fill? Half-gap vs full-gap targets

A gap is "filled" when price returns to the previous close. Some gaps fill within hours, some take weeks and some never do. No fill rate is published on this page. If you want one, you have to define the fill level and the time window and count the results.

Your choice of target changes how often it is reached. The previous close is the full-fill level, which the calculator shows. A half-gap target is the midpoint between the previous close and the open. In the worked example the midpoint is (20,000 + 20,200) ÷ 2 = 20,100. A price that retreats to 20,100 has half-filled the gap but not completely filled it, so it counts as a half fill. It is a closer level, so it is reached more often than the full fill, but it also leaves less room.

The fill level in the calculator is just the previous close. It is not a prediction. A gap that does not fill can still be a good or bad trade depending on your entry, stop and costs.

Why Nifty gaps at the open

The Nifty cash market is closed overnight, but the world is not. Several things can make the next open differ from the last close.

• Global markets: US and Asian markets trade while India is closed, and big moves there often spill over.
• GIFT Nifty: a Nifty contract that trades on an exchange at GIFT City outside Indian cash hours. Many traders watch it as a clue to the opening level, though it does not guarantee the open.
• Overnight news: results, policy statements, currency or oil moves and company announcements arrive after the close.
• The pre-open session: before the market opens, orders are collected and a price is set. The open can differ from the previous close because of the orders gathered.

Gap size on Nifty is the open minus the previous close of the same index. Futures and options trade at different prices from the index, so do not mix an index close with a futures open. Keep each instrument separate.

Gap trading rules traders write down

Gap trades are fast and hard to time. These rules are common ideas, not advice, and the calculator does not apply any of them.

• Wait for the opening range to form before acting, such as the first several minutes, so that you are not reacting to the very first print.
• Decide the stop before the entry. A stop-market order can fill at a worse price in a fast gap.
• Check the gap type on the chart. A breakaway gap and a common gap call for different expectations.
• Check the news. A gap caused by a company result behaves differently from a gap caused by a global move.
• Size the position by the loss you accept if the stop is hit, not by the size of the gap. The stop-loss calculator can do the arithmetic.

Building your own gap-fill record

The most useful gap statistics are your own. For each gap trade, write down the date, instrument, gap in points and percentage, gap type, your entry, stop and target, whether it filled, and how long it took. After enough trades, sort them by gap size and direction.

OneTradeJournal logs trades by hand, so these notes are written by you at the time. Adding the gap percentage to the notes lets you compare later how small and large gaps worked for you. The tool does not supply a fill rate, and the Gap Fill Probability label in the calculator above is not one either.

An earlier fill can disappear from this snapshot

Consider a session that opens above the prior close, trades down through that close and then returns above it. The gap has reached the close during that session, but entering only the later current price can make this form report Gap Unfilled. A complete fill check needs the session path or its relevant extremes.

A return to the previous close also says nothing about whether a particular gap trade was profitable. A trade could hit its stop before the price later fills the gap. The sequence of events matters.

Read the gap-fill strategy explanation with that sequence in mind. For a separate planned-loss calculation, the stop-loss calculator uses your chosen prices, while position sizing explains how quantity changes exposure.

Gap trading questions

Sources

• Fidelity: Identifying Chart Patterns (checked 15 Oct 2026).
• Charles Schwab: Market, limit and stop orders (checked 15 Oct 2026).