Impact cost
Also written Market impact cost
The percentage by which a market order's actual execution price degrades against the ideal price — the mid-point of the best bid and the best offer — and so the real cost of trading in size.
In plain language
Everybody quotes "the price" of a stock as a single number. There is no single number. There is a price at which somebody is willing to buy from you and a slightly higher price at which somebody is willing to sell to you, and behind each of those sits only a limited quantity.
The ideal price is the mid-point of the best buy and the best sell order — the fiction of a single price. Impact cost measures how far your actual fill lands from that fiction, as a percentage.
Buy a small quantity and you pay close to the ideal price. Buy a large one and you exhaust the best offer, then the next, then the one after, each worse than the last. Your average fill drifts away from the ideal price, and the percentage drift is the impact cost. Nobody invoices you for it. It is a cost all the same.
How it works
Three properties matter, and all three are examined.
Impact cost varies with transaction size. It is not a property of the stock alone; a stock can have a trivial impact cost for 500 shares and a punishing one for 50,000.
It is different on the buy side and the sell side, because the buy and sell halves of the order book are not mirror images of each other.
It is a measure of liquidity, and it moves opposite to it. A liquid market is one where large orders execute without moving the price, so high liquidity means low impact cost. It is also the reason a wide bid-ask spread and a high impact cost travel together: the larger the spread, the larger the impact cost.
The bid-ask spread is the special case of impact cost for the smallest trade — the one that fits entirely inside the best quote.
The formula
Ideal price = (Best bid price + Best ask price) ÷ 2
Actual price = Quantity-weighted average price of the orders your trade consumes
Impact cost = (Actual price − Ideal price) ÷ Ideal price × 100
For a sell order the numerator becomes Ideal price − Actual price; the degradation is always measured as a positive cost.
A worked example
The order book for a mid-cap stock at a moment in time:
| Buy qty | Bid (Rs) | Ask (Rs) | Sell qty |
|---|---|---|---|
| 800 | 482.50 | 483.50 | 600 |
| 1,200 | 481.90 | 484.20 | 1,400 |
| 2,000 | 481.20 | 485.00 | 3,000 |
Ideal price = (482.50 + 483.50) ÷ 2 = Rs 483.00
Buying 2,000 shares at market. The order takes all 600 at 483.50, then 1,400 at 484.20:
Actual price = (600 × 483.50 + 1,400 × 484.20) ÷ 2,000
= (2,90,100 + 6,77,880) ÷ 2,000 = Rs 483.99
Impact cost = (483.99 − 483.00) ÷ 483.00 × 100 = 0.20%
In rupees = (483.99 − 483.00) × 2,000 = Rs 1,980
Now buy 5,000 shares instead. The order runs three levels deep:
Actual price = (600 × 483.50 + 1,400 × 484.20 + 3,000 × 485.00) ÷ 5,000
= Rs 484.60
Impact cost = (484.60 − 483.00) ÷ 483.00 × 100 = 0.33%
In rupees = (484.60 − 483.00) × 5,000 = Rs 7,980
Two and a half times the order size, four times the rupee cost. The brokerage on both trades is the same percentage; the impact cost is not, and on the larger order it dwarfs it.
The workbook's own illustration uses the same arithmetic on a cheaper stock: to buy 1,500 shares with a best bid of 9.80 and a best ask of 9.90, the ideal price is Rs 9.85, the actual price is (1,000 × 9.90 + 500 × 10.00) ÷ 1,500 = Rs 9.9333, and the impact cost is 0.84%.
Why NISM asks about it
Chapter 2 (Understanding Index), section 2.4 on the attributes of an index, introduces impact cost — because an index is only investable if its constituents can be traded in size, so low impact cost is a selection criterion for index stocks. Chapter 6, section 6.5, returns to it as one of the three classes of trading cost, alongside user charges and statutory charges. The two questions that recur are the computation above and the true/false statement "impact cost is low when liquidity in the system is high" — which is true.
Common exam traps
- Impact cost and the bid-ask spread are not paid to anyone. They are not charges. The workbook groups them separately from user charges and statutory charges for exactly this reason, and asks which cost "is not actually paid by market participants".
- It is measured against the ideal price, not against the best bid or the best ask. Using the best ask as the benchmark for a buy order gives a smaller, wrong answer.
- High liquidity means low impact cost. The relationship is inverse, and the sentence reads naturally either way, which is why it is asked.
- The buy-side and sell-side figures differ. Do not compute one and assume the other.
- A limit order avoids impact cost but risks non-execution. Impact cost is the price of demanding immediacy with a market order.
- Impact cost rises with order size, so quoting "the impact cost of stock X" without a transaction size is meaningless.
Check yourself
1.In an order book with a best buy at ₹4.00 and a best sell at ₹4.50, a person buys 100 shares and immediately sells them. What has he lost, and what does that loss represent?
- a)₹50, representing the bid-ask spread — the transaction cost the market charges for a small trade
- b)₹50, representing the impact cost of the trade
- c)₹25, representing the degradation against the ideal price of ₹4.25
- d)Nothing, since the buy and the sell offset each other
Show the answer
Answer: (a) ₹50, representing the bid-ask spread — the transaction cost the market charges for a small trade
A market buy order for 100 shares is matched against the best available sell order at ₹4.50. A market sell order for 100 shares is matched against the best available buy order at ₹4.00. So he buys at 4.50 and sells at 4.00, losing 0.50 × 100 = ₹50.
The difference between the best buy and the best sell orders is 0.50 — called the bid-ask spread, and this spread is regarded as the transaction cost which the market charges for the privilege of trading, for a transaction size of 100 shares.
Option B misapplies the term. Impact cost is what you need when an order is large enough to move through several price levels, and it is measured as a percentage degradation against the ideal price. For 100 shares here, the whole order fills at the single best price on each side — the spread covers it.
Option C mixes in the ideal price of ₹4.25, which is correct as a number but belongs to the impact cost calculation, not this one.
Option D ignores that you buy on one side of the book and sell on the other. The key line: the bid-ask spread conveys the transaction cost for a SMALL trade — for larger orders the cost rises, which is exactly why impact cost exists.
2.In an option order book the best bid is ₹16.60 and the best ask is ₹16.80. An investor buys at the market and immediately squares off at the market. What has happened, and what is this cost called?
- a)He has paid a ₹0.20 exchange levy known as the transaction charge
- b)He has lost ₹0.20 per lot to the bid-ask spread — a cost not actually paid to anyone, which arises from market imperfections or lack of liquidity
- c)He has broken even, since the bid and ask converge at settlement
- d)He has lost ₹0.20 per lot, which is refunded as impact cost compensation by the exchange
Show the answer
Answer: (b) He has lost ₹0.20 per lot to the bid-ask spread — a cost not actually paid to anyone, which arises from market imperfections or lack of liquidity
An investor in a hurry to buy pays the ask of ₹16.80; one who wants to sell immediately receives the bid of ₹16.60. Doing both incurs a loss of ₹0.20 per lot, and this gap between the bid and ask prices is the bid-ask spread.
The crucial characterisation: bid-ask spread and impact cost are not actually paid by market participants — they arise because of market imperfections or lack of liquidity. There is no levy, no invoice and no counterparty collecting the ₹0.20 as a fee.
Option A misclassifies it as a transaction charge, which is a real exchange fee levied on both buy and sell sides and, for options, charged on the premium — an entirely separate item on the cost sheet.
Option C is wrong: the loss is realised the moment both trades are done, and nothing converges afterwards.
Option D invents a refund mechanism. Nothing compensates this cost — the only defence is the one the workbook names: place limit orders rather than market orders, since it is precisely buyers who place orders at the market price instead of limit orders who end up paying higher prices.
And remember the driver: the bid-ask spread tends to be larger for illiquid stocks which have lesser participation by traders, and the larger the bid-ask spread, the larger the impact cost.
Where this is taught
- Series XIX-C · Chapter 10: Introduction to Indices and Benchmarkingintroduced here
- Series XVI · Chapter 6: Trading Mechanismintroduced here
- Series VIII · Chapter 2: Understanding Indexintroduced here
- Series IV · Chapter 3: Exchange Traded Interest Rate Futuresintroduced here
- Series V-D · Chapter 14: Understanding Indexintroduced here
- Series VII · Chapter 4: Risk Managementintroduced here
- Series XII · Chapter 4: Secondary Marketsintroduced here
- Series X-A · Chapter 8: Investing in Stocksintroduced here
- Series VIII · Chapter 6: Trading Mechanism
- Series IV · Chapter 5: Strategies using Interest Rate Derivatives
Related terms
- Liquidity riskThe risk of being unable to get out of a position at or near the quoted price — because the contract is bilateral, because the order book is thin, or because volumes dry up near expiry.
- LiquidityThe degree of ease with which you can turn an investment back into cash at a fair value — one of the three pillars of investing, alongside safety and return.
- Bid-ask spreadThe difference between the best buy order and the best sell order.
- Ideal priceThe average of the best bid and the best offer price — the price at which a large order would execute in an infinitely liquid market.
- IndexA statistical indicator that measures changes in the economy in general or in specific areas.
- Market orderAn order to buy or sell at the best bid or offer currently available.
- Tick sizeThe smallest price change a contract may be quoted in — prices move only in whole multiples of it, and it differs from one commodity to another.
- Securities Transaction TaxA central government tax collected by the exchange on the sell side of every futures and option trade — 0.05% of futures traded value, 0.15% of option premium, and 0.15% of settlement price on exercise.