Impact Cost
Impact cost is a measure of the liquidity of a security, expressed as the percentage deviation of the actual transaction price from the ideal price (the average of the best bid and best ask), for a given order size under prevailing market conditions.
Impact cost is one of the most operationally meaningful measures of market liquidity, because it directly captures the cost that a trader bears when executing an order of practical size—as opposed to theoretical measures that assume infinite liquidity at the quoted price.
The concept is straightforward. At any moment, the best bid price (highest price a buyer is willing to pay) and the best ask price (lowest price a seller is willing to accept) define the spread. The midpoint of these two prices is the 'ideal price' or 'fair value' implied by the order book. When a trader places a buy order for, say, ₹10 crore worth of a stock, the order consumes multiple levels of the ask side of the order book, pushing the average execution price above the ideal price. Impact cost measures this slippage as a percentage of the ideal price.
NSE uses impact cost as the primary liquidity criterion for Nifty 50 index inclusion. One of the key eligibility conditions for a stock to be included in the Nifty 50 is that its impact cost for a notional order of ₹10 crore must be 0.50% or less, computed on 90% or more of observations over the six months prior to review. This threshold ensures that the Nifty 50 is composed exclusively of highly liquid stocks that large institutional investors can enter and exit without significant market impact.
For mid-cap and small-cap stocks, impact costs are substantially higher—sometimes several percentage points for equivalent order sizes—reflecting the shallow order books and wide bid-ask spreads typical of less-traded securities. This higher impact cost is a hidden transactional cost that erodes returns for investors who frequently trade in illiquid stocks.
Impact cost data is published by NSE as part of its market data offerings, allowing sophisticated investors and fund managers to assess the tradability of stocks relative to their portfolio size. Portfolio managers running large AUMs must account for impact cost when computing the realistic cost of entering or exiting positions, and it influences their decision on position sizing and entry/exit timing.