Gap Fill Strategy for Nifty and Bank Nifty: Real Odds and a Worked Example

What a Gap and a Gap Fill Actually Are

A gap is empty space on the candlestick chart where no trading occurred. It appears when today's open is meaningfully away from yesterday's close. In Indian markets this almost always forms overnight, because the cash market is closed from 15:30 to 09:15 the next day. During those 17 hours the US market closes, Asian markets move, crude and the rupee shift, and global risk sentiment changes. All of that gets priced into the first print at 09:15 in one jump rather than smoothly. The GIFT Nifty (the offshore Nifty futures that now trade nearly around the clock from Gujarat) gives a live preview of where the index is likely to open.

A gap fill means price travels back to the prior session's closing level and touches it. If Nifty closed at 24,500 and opened at 24,400, the gap is filled the moment price trades back up to 24,500. Traders who play the fill are betting that the overnight move was an overreaction and that the cash market will drag price back to where the bulk of yesterday's volume sat. This is a mean reversion idea, not a trend idea, and that distinction decides which gaps are worth trading.

The key skill is telling apart the four classic gap types, because they behave very differently. A common gap is small and noise driven and usually sits inside the prior day's range. A breakaway gap launches out of a base on heavy volume and real news. A runaway or continuation gap appears mid trend and signals acceleration. An exhaustion gap appears at the end of an extended move and often reverses. Only common gaps and exhaustion gaps are natural fill candidates. Breakaway and runaway gaps are the ones that hurt fill traders.

Real Gap Fill Probability for Nifty and Bank Nifty

Generic articles claim gaps almost always fill. That is misleading. The fill rate depends heavily on gap size and gap direction. Backtests of NSE index data across several years point to a clear pattern: the smaller the gap, the higher the chance it fills the same day, and the bigger the gap, the lower that chance. The figures below are illustrative ranges drawn from historical Nifty and Bank Nifty behaviour and should be treated as rough guidance, not a fixed law. Markets change, and any single year can deviate.

Two more patterns matter for India. First, gap down opens fill slightly more often than gap up opens in an index that has a long term upward drift, because buyers tend to step in on weakness. Second, Bank Nifty gaps are larger and noisier than Nifty gaps because Bank Nifty is more volatile and concentrated in a dozen heavyweight banks. A 0.5 percent gap on Nifty is routine, but the same percentage on Bank Nifty can be 250 to 300 points, so the rupee risk per lot is bigger even though the percentage is identical.

Why Indian Index Gaps Form Overnight

Understanding the cause helps you judge whether a gap is fade-able. NSE runs a pre-open session from 09:00 to 09:08 for order collection, with the equilibrium price determined by 09:08 and continuous trading from 09:15. There is no overnight trading in the cash index, so every overnight development compresses into that first equilibrium price. The largest single driver for Nifty is the GIFT Nifty level, which reflects the offshore bet on where the index opens. When GIFT Nifty is up 120 points at 08:45, the cash Nifty usually opens with a similar gap up.

• US markets: the S&P 500 and Nasdaq close at 02:00 to 02:30 IST, setting global risk mood for the next Indian open.
• GIFT Nifty: live offshore Nifty futures that price the expected open before 09:15.
• Crude oil and the rupee: a sharp move in Brent or USD/INR overnight feeds straight into the open, especially for OMCs, paints, and importers.
• Stock-specific news: quarterly results declared after 15:30, block deals, regulatory action, or rating changes create single-stock gaps that can be much larger than index gaps.
• Event risk: RBI policy, the Union Budget, US Fed decisions, and election results regularly produce 1 to 3 percent index gaps that are usually breakaway gaps, not fade candidates.

Exact Entry Rules for a Gap Fill Trade

A good gap fill entry is not just buying the open. You want confirmation that the overnight move is fading. The cleanest setup is the first 15-minute reversal. After a gap down, wait for the 09:15 to 09:30 candle to complete. If price holds above the low of that first candle and the second candle prints a higher high, that is early evidence buyers are absorbing the gap. Enter on the break of the first candle's high, place the stop below the first candle's low, and target the prior close.

• Measure the gap at 09:15. Reject anything larger than about 0.8 percent on Nifty unless you have a strong reversal reason.
• Identify the gap type. If it is news driven and one directional, stand aside. Fade only common gaps with no fresh catalyst.
• Wait for the first 15-minute candle to finish. Do not enter blind at 09:15.
• Enter on a reversal trigger: a break of the first candle in the direction of the fill, ideally with rising volume.
• Set a hard stop beyond the extreme of the first 15 minutes, sized to the ATR so you are not stopped by normal noise.
• Target the prior day's close. Book a portion there and trail the rest only if momentum continues.

Dated Worked Example: Nifty Gap Down, 14 May 2025

The numbers below are illustrative and rounded to teach the mechanics, not an exact tick record. Suppose on the prior session Nifty closed at 24,600. Overnight, weak US markets and a soft GIFT Nifty point lower, and on the morning of 14 May 2025 Nifty opens at 24,470, a gap down of 130 points, about 0.53 percent. That sits in the small-gap band where the historical same-day fill rate is roughly 65 to 75 percent, so this is a reasonable fade candidate. There is no fresh negative India-specific news, which supports treating it as a common gap.

The first 15-minute candle (09:15 to 09:30) drops to a low of 24,440 and then closes back at 24,480, a bullish reversal candle. At 09:30 price breaks above the first candle's high of 24,490. You take the long fill trade in the nearest weekly expiry. Rather than buy the index future, you buy 1 lot of the 24,500 weekly call, lot size 65, at a premium of 90 rupees. Your view: Nifty fills the gap back to 24,600, lifting the call.

Say the gap fills by 11:00 and Nifty trades up to 24,600, a 110-point move from your 24,490 trigger. The 24,500 call, now 100 points in the money with some time value left, rises to about 150 rupees. You exit. Gross gain is (150 minus 90) x 65 = 3,900 rupees on the 5,850 rupees you risked, before costs.

Now the costs, because they are real on options. On NSE, Securities Transaction Tax on options is charged at 0.1 percent of the premium on the sell side only, so STT here is roughly 0.001 x 150 x 75, about 11 rupees. A typical discount broker charges 20 rupees per executed order, so 40 rupees for buy plus sell. Add exchange transaction charges, SEBI fees, stamp duty on the buy side, and 18 percent GST on (brokerage plus transaction charges). Together these come to roughly 90 to 110 rupees for this round trip. Your net profit is therefore about 4,500 minus 110, around 4,390 rupees, illustrative.

Exit Rules, Stops, and Position Sizing

Your primary target is always the prior day's close, because that is the literal definition of the fill. Many traders book the full position at the close level and walk away, since beyond the fill there is no statistical edge left. If you want to let part of it run, scale out: take two thirds at the fill and trail the last third with the prior swing low. The danger is giving back a clean gap-fill profit by hoping for a trend that the data does not promise.

Stops belong beyond the structure, not at a round number. For an index gap fill, place the stop just past the high or low of the first 15-minute candle, then size the position so that this stop distance equals a fixed fraction of capital, ideally 1 percent or less per trade. Use the Average True Range to gauge whether your stop is wide enough to survive normal opening noise. If the ATR-implied noise is bigger than your intended stop, the trade is too tight and you should skip it or trade a smaller instrument.

• Cash or futures sizing: with Nifty at 24,490 and a stop at 24,440, the per-lot risk is 50 points, which is 50 x 65 = 3,250 rupees per futures lot. Size lots so total risk stays within 1 percent of capital.
• Options sizing: risk is capped at the premium paid, but theta decay punishes you if the fill is slow, so use options only for fast same-session fills.
• Never average down on a gap that is running against you. A failed fill often becomes a breakaway gap, the single worst place to add.

Common Gaps Versus Breakaway Gaps: The Trade-Killer

The most expensive mistake in gap trading is fading a breakaway gap, thinking it is a common gap. A breakaway gap appears on heavy volume, often after a result, an order win, an RBI action, or a budget announcement, and it does not want to come back. If you short a stock that gapped up 6 percent on a blowout result expecting a fill, you can be run over for the rest of the day and the week. The volume is the tell: common gaps form on ordinary or light volume, breakaway gaps form on a clear volume surge.

Taxes and SEBI Rules You Cannot Ignore

Gap fill trading is short term and intraday, so the tax treatment is not the friendly capital gains regime. Intraday equity trades are speculative business income and F&O trades are non-speculative business income. Both are taxed at your normal income slab, not at the 20 percent STCG or 12.5 percent LTCG rates. Those capital gains rates only apply if you actually take delivery and hold: STCG is 20 percent for holdings up to one year, and LTCG is 12.5 percent on gains above 1.25 lakh rupees a year for holdings beyond one year. Because gap fills are closed the same day, expect slab-rate business taxation and the ability to set off losses and expenses, subject to the rules in force.

On the SEBI and exchange side, weekly index options now have one weekly expiry per exchange, with Nifty weekly options expiring on Tuesday and the bigger monthly contracts on the last Tuesday of the month. SEBI has tightened position limits, raised the minimum contract value, and increased margins for index derivatives, which affects how many lots you can carry. Always confirm the current expiry day, lot size, and margin on the NSE site before you trade, because these specifications have changed more than once and are illustrative here.

• Intraday and F&O profits: taxed as business income at your slab, not as capital gains.
• STT on options: 0.1 percent of premium on the sell side. STT on futures: 0.02 percent on the sell side. Both are real costs to model.
• Delivery STCG: 20 percent. Delivery LTCG: 12.5 percent above 1.25 lakh rupees a year. These apply only if you hold, not to same-day gap trades.
• Lot sizes to remember: Nifty 75, Bank Nifty 15, FinNifty 25, Sensex 10. Confirm before trading because the exchange revises them.

Combining Confirmation Tools Without Overfitting

Indicators should filter trades, not generate them. The fill direction is already defined by the gap, so use tools only to decide whether to take the trade. A simple, robust filter set works better than a cluttered screen. Watch the opening five-minute volume, because a fade on weak volume is a healthier common gap than a fade on heavy volume. Use the Relative Strength Index on a short timeframe to avoid buying a gap-down fill when momentum is still violently down, or shorting a gap-up fill when buyers clearly dominate.

• VWAP: if price reclaims the opening VWAP after a gap down, the fill has real support behind it.
• First 15-minute range: a break back through it is your trigger and your invalidation in one level.
• Prior day close line: mark it on the chart, it is both your target and the zone where momentum often stalls.

Resist the urge to stack ten indicators. Each one you add increases the chance you curve fit a setup that worked on old data and fails live. The gap, its size, its cause, and the first-candle reversal are the core. Everything else is secondary confirmation. A clean rule set you can follow under pressure beats a complicated one you abandon the moment a fast tape rattles you.

Frequently Asked Questions

Sources and Further Reading

For authoritative data and further reading on this topic, refer to NSE India, Zerodha Varsity and Investopedia. Always confirm current rules, rates and contract specifications on the official source before you trade.