Stripping the Market’s Noise Out of a Stock’s Story
A stock jumps 5% the day of an earnings announcement. Great result — except the whole market was up 4% that day. How much of the move was the announcement, and how much was just the tide lifting every boat? Cumulative abnormal return (CAR) is the finance metric that answers exactly that question.
This free CAR calculator handles the arithmetic for you: enter each day’s actual return and its expected return (from your market model, CAPM estimate, or benchmark index), and it computes the abnormal return for every day, adds them into the CAR, and gives you a plain-English interpretation of what the number means.
Event study · abnormal return · CAR
Add one row per day of your event window. Enter the actual return of the stock and the expected return (from a market model or index) for that day. All values are in percent.
Cumulative abnormal return
–
How to Use This Calculator
- Add a row for each day of your event window — five rows are pre-filled (Day -2 through Day +2); use “+ Add day” for more, or remove rows you don’t need.
- Label the days — event studies usually use -2, -1, 0, +1, +2, with day 0 being the event day.
- Enter the actual return % — what the stock really returned that day.
- Enter the expected return % — what the stock should have returned per your model (market model, CAPM, or a benchmark).
- Click “Calculate CAR” — you get a per-day table of abnormal returns, the CAR, the average AR, and an interpretation.
What Is Cumulative Abnormal Return?
An abnormal return (AR) is the difference between what a stock actually earned and what it was expected to earn on a given day: actual return − expected return. The “expected” part usually comes from the stock’s normal relationship with the market — often estimated with the market model or the Capital Asset Pricing Model (CAPM).
The cumulative abnormal return simply adds up those daily abnormal returns across the whole event window — the days before, during, and after the event you’re studying. A positive CAR suggests the market reacted favorably to the event; a negative CAR suggests the opposite. It is the standard measurement in event studies, the method finance researchers use to test how news (earnings, mergers, product launches, scandals) affects stock prices.
The CAR Formulas
Cumulative abnormal return: CAR = AR1 + AR2 + … + ART
Average AR: CAR ÷ Number of days
Worked example
Imagine a company announces a new product on day 0. Its model says the stock should return 1.2% each day of the window:
| Day | Actual % | Expected % | AR % |
|---|---|---|---|
| -2 | 2.4 | 1.2 | 1.2 |
| -1 | 1.8 | 1.2 | 0.6 |
| 0 | 6.5 | 1.2 | 5.3 |
| +1 | 1.1 | 1.2 | -0.1 |
| +2 | 0.9 | 1.2 | -0.3 |
- Abnormal returns: 2.4−1.2 = 1.2%, 1.8−1.2 = 0.6%, 6.5−1.2 = 5.3%, 1.1−1.2 = −0.1%, 0.9−1.2 = −0.3%
- CAR: 1.2 + 0.6 + 5.3 + (−0.1) + (−0.3) = 6.7%
- Average AR: 6.7% ÷ 5 days = 1.34% per day
The 6.7% CAR says the stock outperformed expectations by 6.7 percentage points over the five-day window — a strong positive market reaction to the announcement, concentrated on day 0 itself.
Reading the Pattern: Leakage, Reaction, and Drift
The day-by-day shape of abnormal returns tells a story beyond the single CAR total. A build-up before day 0 suggests information leaked or was anticipated ahead of the announcement. A sharp jump concentrated on day 0 and day +1 is the classic signature of the market reacting to the event itself. Drift that continues after the news day hints at slow absorption of the news. A flat, near-zero run before the event is what a clean study hopes for — it means the event stands out on its own.
Who Uses CAR and Why
- Finance students — event studies are a standard assignment in corporate finance and investments courses.
- Equity researchers and analysts — to measure whether a company’s announcements actually moved its stock.
- Academic researchers — CAR is the backbone metric of event-study methodology, used to test market efficiency.
- Investors — to judge how the market has historically rewarded or punished similar events.
Tips for a Meaningful Event Study
- Pick the window deliberately. A tight window (like −1 to +1) isolates the event; a wide one lets unrelated news creep in.
- Keep the expected return consistent. Use the same model or benchmark for every day — mixing sources pollutes the comparison.
- Mind the event day. If day 0’s AR dominates the CAR, the market reacted mainly to the event itself — exactly what you’re measuring.
- Remember it’s descriptive. CAR tells you what happened around the event, not why. A negative CAR after good earnings might reflect high expectations, not bad news.
- Check for confounding events. If another announcement landed inside your window, it contaminates the CAR.
Frequently Asked Questions
What does a positive CAR mean?
A positive CAR means the stock earned more than expected over the event window — the market reacted favorably to whatever event you are studying.
What does a negative CAR mean?
A negative CAR means the stock earned less than expected — an unfavorable market reaction. Note it describes the market’s reaction, which can differ from whether the underlying news was objectively “good.”
Where do expected returns come from?
Typically from a model of the stock’s normal behavior: the market model (regression of the stock against an index), the CAPM, or simply the return of a benchmark index like the S&P 500 over the same days.
What is an event window?
The period of days around the event you are analyzing — for example, two days before to two days after an earnings announcement (a 5-day window). Days are usually labeled -2, -1, 0, +1, +2, with 0 as the event day.
Can CAR be zero?
Yes. A CAR of zero (or near zero) means the stock performed essentially as expected — the event had no measurable net abnormal effect over the window.
Is this calculator financial advice?
No. This is an educational arithmetic tool for event-study calculations. It doesn’t assess investment merit, and nothing here should be used to make buy or sell decisions.
What is the difference between AR and CAR?
AR (abnormal return) is a single day’s excess over expectations. CAR (cumulative abnormal return) is the sum of ARs across the whole event window — the total abnormal effect of the event.
What is the difference between CAR and CAAR?
CAR is one stock’s cumulative abnormal return around one event. Studies of many firms average first: each event day’s ARs are averaged across firms to get the AAR, and summing the AARs over the window gives the CAAR. For example, three firms’ day-0 ARs of +2.7%, +1.5%, and +0.9% give an AAR of +1.7% for day 0.
Does a nonzero CAR prove the event caused the stock move?
No — CAR measures association within your window, not proof of cause. Confounding news, a poorly fitting expected-return model, or plain volatility can all produce a nonzero CAR. Formal studies pair CAR with significance tests before claiming an effect.
Final Thoughts
Cumulative abnormal return is a beautifully simple idea: take what a stock actually did, subtract what it should have done, and add it up. The calculator above does that arithmetic for any event window you define — so you can spend your time choosing the right event and model instead of adding columns by hand.
Working on More Financial Math?
Explore all our free calculators for finance, education, and everyday numbers.