Implementation Shortfall
Implementation shortfall is the difference between the price when you decided to trade and the result you actually achieved, including spreads, market impact, fees and any part of the order that was never filled.
Implementation shortfall measures the gap between the price when you decided to trade and the result you actually got, including the shares that never filled. It captures the full cost of acting on an idea.
- Category
- Market Structure
- Reading time
- 3 min
- Also called
- Arrival price slippage, Perold shortfall
- Updated
Implementation shortfall in plain words
Implementation shortfall measures trading cost as the gap between the price when you decided to trade and what you actually achieved — including any shares you meant to buy but never got.1
It captures the full cost of acting on an investment decision, explicit and hidden alike.2
That includes market impact, the cost of delay and the opportunity cost of unfilled orders.2
How to calculate implementation shortfall
Record the decision price
Record the market price at the moment you decide to trade.1
Record fills
Note every executed price and size.
Add fees
Include commissions and other explicit costs.
Measure delay and impact
Split the total into market impact, delay and opportunity cost.2
Count unfilled shares
Price the missed shares against the decision price.
Sum and compare
Total cost covers all explicit and implicit trading costs.2
Worked examples
Example 1 — Full fill
210Decision 50.00; buy 1,000 at avg 50.20; fees 10.
| Price cost | 200 |
| Fees | 10 |
| Total | 210 |
The cost is measured against the price at the moment of the decision.1
Example 2 — Partial fill
460Decision 20.00; intend 1,000; fill 600 at 20.10; close 21.00.
| Executed cost | 60 |
| Opportunity cost | 400 × 1.00 = 400 |
| Total | 460 |
Unfilled shares count too, so the missed part adds 400 to the shortfall.1
Example 3 — vs VWAP
IS is stricterSame trade looks cheap against VWAP.
| VWAP cost | Low |
Measured against VWAP the cost looks smaller, because VWAP often misses an order's own market impact.2
Implementation shortfall vs VWAP benchmark
| Aspect | Implementation shortfall | VWAP |
|---|---|---|
| Benchmark | Decision price | Day's average price |
| Unfilled shares | Counted | Ignored |
| Impact capture | Strong | Weak |
| Gaming | Hard | Easier |
Where IS is used
Execution quality
Beyond simple Slippage.
Benchmarks
Compared with Volume-Weighted Average Price.
Costs
Adds Trading Fees and Commissions to implicit costs.
Strategy review
Checking a Trading Strategy works after costs.
Advantages and disadvantages
Advantages
- Captures all trading costs.
- Counts missed trades.
- Hard to game.
Disadvantages
- No single accepted formula.
- Needs exact decision timestamps.
- Opportunity cost depends on a chosen end price.
- More data work than VWAP.
Going deeper
Read as far as you need. Each level builds on the one before it.
Common mistakes
Ignoring unfilled shares
Missed trades are a cost.
Wrong start price
Use the decision price.
Only using VWAP
VWAP can hide impact.
Leaving out fees
Add explicit costs.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- IS compares results with the decision price.
- It includes impact, delay and opportunity costs.
- Unfilled shares count.
- It is harder to game than VWAP.
- There is no single accepted formula.
Frequently asked questions
Is IS only for funds?
What end price for missed shares?
Can IS be negative?
Further learning
- Slippage
Simple execution cost.
- Volume-Weighted Average Price
The common alternative.
- Trading Fees and Commissions
Explicit costs.
- Bid-Ask Spread
An implicit cost.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- U.S. SEC. Request for Comments on Measures To Improve Disclosure of Mutual Fund Transaction Costs (opens in a new tab) Checked .
- CFA Institute. Trading Costs and Electronic Markets (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.