Impact Cost
The hidden slippage a large order pays as it walks the order book — a real measure of what liquidity actually costs you.
Definition
Impact cost is the percentage difference between the actual average execution price of a large order and the theoretical ideal price — defined as the mid-point of the best bid and ask at the moment the order is placed. Unlike the bid-ask spread, which measures the cost for a single-lot trade, impact cost captures how much a bigger order moves the market against the trader as it consumes successive levels of the order book. NSE uses it as its official liquidity metric and publishes monthly impact cost figures for all actively traded securities.
Why it matters
For retail traders buying or selling a handful of Nifty or Bank Nifty option lots, impact cost is usually negligible — the order is too small to dent the top-of-book. The picture changes completely for institutional desks, proprietary trading firms, or any trader executing dozens of lots simultaneously. A strategy that looks profitable on paper can be eroded or even inverted once the true fill price is factored in.
Impact cost also matters indirectly for all traders because it governs index composition. NSE mandates that a stock must have an average impact cost of 0.5% or below for at least 90% of intraday observations over a six-month review period to qualify for the Nifty 50. Stocks failing this threshold get dropped from the index regardless of market cap. This directly affects which stocks have liquid futures and options contracts available.
During periods of high volatility — RBI policy announcements, Budget day, or heavy FII sell-offs — order books thin out and impact cost spikes even on normally liquid contracts. Options on mid-cap stocks or far-expiry monthly contracts can see impact cost jump to levels that make quick in-and-out trades deeply unprofitable after accounting for actual fills.
Formula
Impact Cost (%) = ((Actual Average Fill Price − Ideal Price) / Ideal Price) × 100, where Ideal Price = (Best Bid + Best Ask) / 2. For a buy order the actual fill will be above the ideal price; for a sell order it will be below. NSE calculates this for a standard order value (e.g., ₹1 lakh or ₹5 lakh depending on the security) to make comparisons consistent across stocks.
Example
Suppose the best bid for a Nifty 50 stock's futures contract is ₹500.00 and the best ask is ₹500.20, making the ideal mid-price ₹500.10. A retail trader buys one lot and gets filled at ₹500.20 — a cost of ₹0.10, or 0.02% above mid. Now suppose an institution needs to buy 200 lots. After the top ask at ₹500.20 is absorbed, the next layer sits at ₹500.50, then ₹500.90. The average fill across all 200 lots might come to ₹500.60 — an impact cost of (500.60 − 500.10) / 500.10 × 100 = 0.10%. On a notional position of ₹2 crore, that 0.10% is ₹20,000 in pure execution friction before any brokerage or STT.
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