Impact Cost

Impact cost refers to the cost incurred when executing a buy or sell transaction of a security. It depends on the liquidity of the asset.

What are Impact Costs?

Impact cost is the hidden cost you pay when your trade affects the stock price. In simpler terms, impact cost is the difference between the price you see in the market and the actual price at which your trade happens.

Impact cost reflects the market liquidity and price fluctuations caused by the size of the trade. These costs are mainly influenced by market liquidity, order size, and the difference between thebid-ask price.

It is low for shares or securities that have enough buyers and sellers in the market, i.e., highly liquid stocks. Generally, impact cost is higher when a large amount of stock needs to be traded or if the stock is too illiquid.

Why are Impact Costs Important?

Impact costs are high because they influence the trading strategies of both individual retail investors and high-frequency traders. High-frequency traders, who place large numbers of orders within seconds, often face challenges due to impact costs.

Here’s why impact costs are essential:

Influence on Trading Strategies

Impact cost is essential for traders, huge players like foreign or domestic institutional investors, who buy and sell millions of shares. Instead of placing one large order, they break it into smaller parts to avoid pushing up the stock price too much.

For example, an institutional investor wants to buy 1 million shares of Tata Motors. If they buy all at once, the stock price could rise from ₹600 to ₹610. To avoid this, they break the order into smaller parts.

Effect on High-Frequency Trading

High-frequency traders make hundreds or thousands of trades in a fraction of a second. For them, even small impact costs can affect their trades. If the impact cost is high due to low liquidity, their overall strategy may become less profitable.

For instance, a high-frequency trader buys shares of a less liquid stock like SpiceJet at ₹100.01 and plans to sell them at ₹100.05. Now, if impact costs raise the buying price to ₹100.03, their profit drops from ₹0.04 per share to ₹0.02, reducing their overall profits.

Profitability for Large Trades

In this category, hedge funds face the problem of impact costs when large trades can move the market price, causing higher impact costs, especially in less liquid stocks. This can reduce the share price and overall profit.

A hedge fund like HDFC Asset Management wants to sell 500,000 shares of a mid-cap stock like Apollo Tyres, priced at ₹200. Since the stock is less liquid, the selling pressure drops the price to ₹195, cutting the hedge fund’s expected profits by ₹5 per share.

What factors contribute to the impact cost?

Impact costs are mainly influenced by liquidity, order size (the number of shares bought or sold in a transaction), and market depth. Here’s a breakdown of each and how they contribute to impact costs:

Liquidity

Liquidity refers to how easily an asset or security can be sold in the market without changing its price. When there are enough buyers and sellers for the asset, the liquidity risk is lower, and this helps to minimise impact costs. Liquidity risk is significant in illiquid stocks like small-cap or penny stocks.

Order Size

This is an essential factor affecting impact costs. If there is a large number of shares to buy, it creates more demand than the available supply in the market. This increased demand pushes the stock price higher, resulting in higher impact costs.

For example, a mutual fund wants to buy 500,000 shares of Bharat Forge at ₹800. Since only 100,000 shares are available at that price, the fund buys the rest at higher prices, pushing the stock price up and increasing impact costs.

Market Depth

Market Depth refers to the number of buy or sell orders available at different price levels in a stock. If there are fewer orders, large trades can cause sharp price movements as the order book is thin, leading to higher impact costs.

For instance, if a trader places a large sell order for 100,000 shares of a less liquid stock like Jindal Steel, but there aren’t many buy orders, the stock price can quickly drop, causing sharp price changes and higher impact costs.

How To Calculate Impact Costs?

Impact cost measures the difference between the expected price of a trade and the actual execution price. It helps traders understand how much their order size is affecting the stock price.

Where:

• Ideal Price = (Best Buy Price + Best Sell Price) ÷ 2
• Actual Price = Weighted average execution price

Example for Calculating Impact Cost through Order Book:

Let’s assume stock ‘A’ has the following order book:

Example 1: Selling 3,000 Shares

Step 1: Ideal Price

(98 + 99) ÷ 2 = ₹98.5

Step 2: Actual Sell Price

(98 × 1500 + 97.5 × 1000 + 97 × 500) ÷ 3000 = ₹97.67

Step 3: Impact Cost

((97.67 – 98.5) ÷ 98.5) × 100 ≈ -0.84%

This means the shares were sold at a lower price due to market impact.

Example 2: Buying 3,000 Shares

Step 1: Ideal Price

₹98.5

Step 2: Actual Buy Price

(99 × 1000 + 99.5 × 1500 + 100 × 500) ÷ 3000 = ₹99.42

Step 3: Impact Cost

((99.42 – 98.5) ÷ 98.5) × 100 ≈ 0.93%

This shows the buyer paid more than the expected price due to order size and liquidity.

Best Strategies to Reduce Impact Costs?

Impact costs can be managed in different ways, but this also depends on the liquidity risk present in the stock. Here is a table summarising the costs to minimise the impact.

Impact Cost: Critical Insights for Retail Traders

Impact costs significantly affect institutional investors or those trading large amounts of shares. Retail traders should also consider impact costs, especially when dealing with illiquid or volatile stocks. Unlike the more obvious bid-ask spread, impact costs can quietly reduce profits, impacting overall returns without traders even realising it.

For instance, an institutional investor might want to buy 1 million shares of less liquid stock, such as Apollo Tyres. As they buy, the stock price rises from ₹400 to ₹410 due to limited availability. This price rise quietly increases their cost, reducing profits without them noticing immediately.

Conclusion

Impact costs are often an overlooked factor in trading, but they can significantly affect institutional players more than retail traders. Impact costs arise when the size of a trade affects the stock price, resulting in higher buying costs or lower selling prices. They are influenced by factors such as market liquidity, order size, and market depth. Managing impact costs is crucial for institutional and high-frequency traders. Retail investors and traders should also be mindful of impact costs, especially when dealing with illiquid or volatile stocks. To minimise impact costs, traders can split their orders or use algorithms to control execution prices.

Frequently Asked Questions (FAQs)

What do you mean by impact cost?

Impact cost simply refers to the extra cost incurred for a trade compared to the expected cost. It reflects the difference between the price you desire and the actual price at which the trade is executed.

How do you calculate cost impact?

Impact cost is calculated by comparing the actual price of the transaction with the expected price. The result is expressed as a percentage. Here’s the formula:

Impact Cost (%) = ((Actual Price – Expected Price) / Expected Price) * 100

What is the impact of liquidity on impact cost?

Liquidity is basically how easily the asset can be converted into cash; the impact cost is lower during high liquidity and higher impact costs if there is less liquidity.