Corporate Action Adjustment
A revision to futures and options contract parameters — strike prices, lot sizes, and settlement prices — made by NSE to preserve the economic value of open positions when a corporate action mechanically alters the underlying share price.
Definition
A corporate action adjustment is a modification applied by the National Stock Exchange (NSE) to the contract specifications of listed futures and options when a corporate event causes a predictable, non-discretionary change in the price or quantity of the underlying shares. Events that trigger adjustments include stock splits, bonus issues, rights issues, and large dividends (those exceeding 10% of the market price). Without such adjustments, an open option or futures position would appear to gain or lose value purely because of the mechanical price change — not because of any change in market sentiment. The adjustment ensures that the economic value of open positions is preserved across the ex-date, making the corporate event neutral to derivative holders.
Why it matters
For options traders on NSE, failing to understand corporate action adjustments can lead to significant confusion and unintended risk. When a stock undergoes a 1:1 bonus, for instance, its price halves on the ex-date — any unadjusted call option that was in-the-money could suddenly appear deep out-of-the-money the next morning. NSE's adjustment prevents this by halving all strike prices and doubling the lot size simultaneously, so that the option holder's P&L is unchanged. Similarly, a large special dividend reduction in the stock price is neutralised by reducing all open strikes by the dividend amount. Traders also need to monitor NSE circulars closely because the adjusted contracts may show different ticker representations (e.g., a suffix indicating adjustment), and the adjusted strikes may not align with standard strike intervals — which can affect liquidity and the bid-ask spread on those strikes.
How it works
NSE publishes an adjustment circular before the ex-date, specifying: (1) the adjustment factor or method; (2) the revised strike prices for all open series; (3) the revised lot size if applicable; and (4) the effective date (the ex-date of the corporate action). The adjustment factor for a bonus issue of 1:1 is 0.5 — all strikes are multiplied by 0.5 and the lot size is multiplied by 2. For a 2:1 stock split, the factor is also 0.5. For a dividend adjustment, each strike is reduced by the per-share dividend amount. The daily settlement price of futures contracts on the ex-date is also adjusted by the exchange so that mark-to-market gains and losses are computed on a level playing field.
Example
Suppose ABC Pharma Ltd trades at ₹2,000 and announces a 1:1 bonus issue. A trader holds a call option with a strike of ₹2,100 covering a lot of 250 shares. On the ex-date, the stock price halves to approximately ₹1,000. NSE applies an adjustment factor of 0.5: the strike is revised to ₹1,050, and the lot size doubles to 500 shares. The notional value of the position (500 × ₹1,050 = ₹5,25,000) is unchanged compared to the pre-adjustment position (250 × ₹2,100 = ₹5,25,000). The trader's position remains exactly as it was in economic terms — the adjustment is transparent and automatic.
Monitor open interest before corporate ex-dates
TradePulse's live option chain highlights unusual open interest concentration ahead of corporate actions so you can plan your adjustments in advance.