What Is CAGR? The Growth Number Consultants Trust
CAGR turns messy year-to-year growth into one honest number. Learn what it means, why simple averages lie, and how to estimate it in a case interview.
CAGR (compound annual growth rate) is the one number consultants trust to describe how fast something grew when the year-to-year path was messy. Revenue, users, market size: if it moved over several years, CAGR turns that bumpy history into a single honest figure.
This guide explains what CAGR is, why it beats a simple average, and how to estimate it in your head during a case when there is no calculator in the room.
CAGR is the steady growth rate that would have taken you from the start to the end, as if every year grew by exactly the same amount.
What CAGR Actually Tells You
Imagine a 600-mile road trip. The first hour you crawl through city traffic at 20 mph. Then you hit open highway and cruise at 80. Your speedometer never sat still, but there is one number, your average speed, that would have covered the exact same distance in the exact same time at a perfectly steady pace. CAGR is that steady speed, but for growth.
A company's revenue might jump 40% one year, slip 10% the next, then climb 25%. CAGR is the single smooth rate that connects the first year's revenue to the last year's, as if it grew by the same percentage every single year. It does not pretend the journey was smooth. It just gives you one clean number to describe the whole trip.
The formula follows the analogy. Take the ending value, divide by the beginning value, raise it to the power of one over the number of years, and subtract one. If revenue went from $100M to $200M over 4 years, that is (200/100) to the power of one-quarter, minus 1, or roughly 19% a year.
Why a Simple Average Lies
The lazy move is to average the yearly growth rates. That number is almost always wrong, and interviewers know it.
Say a stock grows 100% in year one then falls 50% in year two. The simple average of those rates is plus 25%. But you ended exactly where you started: $100 doubled to $200, then halved back to $100. The true CAGR is 0%. The simple average claimed you got rich while you actually went nowhere.
This happens because growth compounds and losses hit a different base. A simple average treats each year as independent, ignoring that year two builds on year one's result. CAGR respects compounding by working only from the true start and end points. When an interviewer hands you a string of growth rates, never average them. Go back to the first and last values.
How to Estimate CAGR Without a Calculator
You will not get a calculator in a case, so you need shortcuts. The most useful one is the Rule of 72.
The Rule of 72 says that if something doubles over N years, its CAGR is roughly 72 divided by N. Revenue that doubled in 6 years grew at about 12% a year. Doubled in 9 years, about 8%. This works in reverse too: at 10% growth, anything doubles in about 7 years. Memorize a few anchors and you can approximate most doubling cases instantly.
For cases that are not clean doublings, anchor to the nearest multiple. If something grew 3x, that is one doubling plus half of another, so the CAGR lands a bit above the doubling rate for that period. You will not be exact, but in a case the interviewer wants a fast, defensible estimate, not four decimal places.
Spotify's 2018 direct listing is a clean place to practice this thinking: investors were pricing years of subscriber and revenue growth into a single valuation, and CAGR was the lens that made those projections comparable. Practice this framework on a real case → Spotify 2018: The Direct Listing Bet on BoardroomIQ puts you in the room.
Practice this framework
Work through the Spotify 2018: The Direct Listing Bet case with AI coaching.
Where CAGR Misleads You
CAGR is honest about the destination and silent about the journey, and that silence can fool you.
Because it only uses the first and last values, CAGR completely hides volatility. Two companies can show an identical 15% CAGR while one grew steadily and the other swung wildly and got lucky in the final year. If you are assessing risk, CAGR alone will mislead you. Always ask what happened in between.
It is also dangerously sensitive to the endpoints you pick. Start your measurement in a recession year and the CAGR looks heroic; start it at a peak and the same company looks stagnant. When someone quotes a CAGR in a case, your first question should be "from which year to which year?" The choice of endpoints can tell almost any story.
How to Practice CAGR Before Your Interviews
Memorize the Rule of 72 anchors. Drill the common ones until they are reflex: doubling in 7 years is about 10%, in 10 years about 7%, in 5 years about 14%. When a case mentions a doubling, the rate should appear in your head before the interviewer finishes the sentence.
Catch the averaging trap. Take any series of yearly growth rates, compute the lazy simple average, then compute the true CAGR from start and end values. Watch how far apart they drift when the numbers are volatile. Train yourself to flinch at simple averages.
Interrogate every CAGR you read. Next time an article quotes a growth rate, find the start and end years and ask whether the endpoints were cherry-picked. This habit is exactly what separates a candidate who reports numbers from one who pressure-tests them.
The best way to practice CAGR is under realistic pressure, with a case that fights back.