Trading Lot Adjustment Post-Split and Bonus
After a stock split or bonus issue, stock exchanges adjust the trading lot size and the market lot for derivatives to reflect the new face value and share count, and odd-lot shares arising from fractional entitlements in bonus issues are handled through a special trading window.
A stock split or bonus issue changes the number of shares outstanding without changing the company's intrinsic value. The mechanical consequence for market infrastructure is that the face value (in a split) or share count (in a bonus) changes, requiring adjustments to the trading lot, price band parameters, and derivatives contracts.
For equities trading on NSE and BSE, shares are traded in the regular lot, which is one share for most liquid stocks following the transition to compulsory dematerialisation. However, the tick size, daily price bands, and the minimum application lot remain relevant considerations. When a face value is split — for example from Rs 10 to Rs 2 (a five-for-one split) — the exchange adjusts all reference prices, index representations, and the base price on the ex-date to the post-split equivalent. All open orders on the exchange order book are cancelled and market participants must re-enter their orders at the adjusted price.
Derivatives adjustment is more complex. SEBI's framework for corporate action adjustments to derivatives contracts requires exchanges to adjust the strike prices and lot sizes of in-the-money options and futures contracts. For a bonus issue in the ratio of one additional share for every existing share (one-for-one bonus), the lot size doubles and the reference price halves for all existing derivative contracts, so that the economic value of open positions is preserved. Market participants with open futures or options positions do not need to take any action — the exchange adjusts contract specifications automatically effective from the ex-date.
Odd lots arise specifically in bonus issues when the ratio is not a whole number multiple. For a three-for-two bonus (three new shares for every two held), a shareholder with an odd number of shares — say, one hundred and one shares — would be entitled to one hundred and fifty-one shares (one hundred and one plus fifty and a half new shares). Since fractional shares cannot be issued in India, the half share is typically handled by the company rounding down the fraction and disposing of the aggregate fractional entitlements through a special call auction or through a separate odd-lot window on the exchange, with the proceeds distributed to affected shareholders.
For investors, understanding the trading lot adjustment is important because post-adjustment prices can confuse unsophisticated market participants who are unaware of the corporate action. Price data services apply retroactive adjustments to historical price charts to account for splits and bonuses, ensuring continuity in time-series analysis. Analysts using historical price data for backtesting or technical analysis must confirm that the data source is adjusted for corporate actions to avoid distortions in return calculations.