Let’s be clear about this: no serious fund manager closes a $200 million deal based solely on dividing 72 by 18. Yet, during initial screenings, coffee-break chats, or pitch prep, the Rule of 72 surfaces more than you’d think. We’re far from it being obsolete. If anything, its simplicity makes it dangerously useful—especially when misapplied.
Understanding the Basics: How the Rule of 72 Actually Works
At its core, the Rule of 72 is arithmetic alchemy. You take a compound annual growth rate—say, 9%—and divide 72 by that number. The result? Eight years to double your money. Simple. Clean. Wrong in the details, but close enough for government work.
Compound interest is the engine here. Unlike simple interest, which only grows on principal, compound interest earns on both principal and accumulated gains. That’s where the magic happens. And that’s exactly where people underestimate how fast (or slow) wealth builds.
The thing is, 72 isn’t some divine number carved into financial stone. It’s a rounded approximation. The actual mathematical constant closer to reality is closer to 69.3—derived from natural logarithms. So why 72? Because it plays nice with mental math. It has more divisors—2, 3, 4, 6, 8, 9, 12—making it easier to use without a calculator.
Where It Gets Tricky: Assumptions Behind the Math
The rule assumes a fixed rate of return. That’s rare in private equity. Returns aren’t linear. A fund might lose money in year one, triple capital in year five after an exit, then plateau. Yet, LPs (limited partners) still ask, “At what IRR does my money double?” And advisors whisper, “Just use 72 divided by the target IRR.”
And that’s the trap. Because real-world returns are lumpy. Cash flows come in waves. Management fees, carry structures, and waterfall allocations distort the clean doubling narrative. But because humans crave simplicity, the rule persists.
A Quick Reality Check: Accuracy Across Return Rates
Take a 6% return. Rule of 72 says 12 years to double. Actual time? About 11.9 years. Not bad. Now try 24%. Rule says 3 years. Reality? Closer to 3.28. The higher the rate, the more the rule overestimates speed. At 36%, it suggests 2 years—but math says 2.64. That changes everything if you're modeling exits in volatile sectors like biotech or deep tech.
Which explains why seasoned investors treat it as a gut check, not gospel.
Private Equity Realities: How Funds Use the Rule (and When They Shouldn’t)
Private equity deals typically target internal rates of return (IRR) between 15% and 25%. Using the Rule of 72, that’s a doubling time of roughly 3 to 5 years. Ambitious? Absolutely. Achievable? Sometimes. Consistently? Hardly.
Fund managers often cite these multiples in pitch decks. “We target 2x MOIC over six years with a 20% IRR.” Plug that into the Rule of 72—72 ÷ 20 = 3.6 years to double. But MOIC (multiple on invested capital) and IRR aren’t the same. MOIC is raw return. IRR factors in timing. A fund could hit 2x MOIC but spread over nine years, dragging IRR down to 8%. The rule doesn’t account for that distortion.
And yet, junior analysts still use it during due diligence sprints. They’ll glance at a portfolio company’s projected EBITDA growth, assume a 20% CAGR, and mutter, “So we’re looking at a 3.6-year double.” Senior partners usually nod—then run the LBO model anyway.
It’s a bit like using a sundial when you own a Rolex. You know it’s imprecise. But it’s fast, and everyone understands it.
The Timing Illusion: Why IRR Can Be Misleading
Because private equity is as much about timing as magnitude. A 2x return in four years beats 3x over ten. The Rule of 72 captures this intuitively—higher rate, faster double. But it doesn’t reflect cash drag, vintage year risk, or the fact that most funds don’t return capital evenly.
Consider KKR’s 2007 takeover of TXU. Huge leverage. Poor timing. The fund didn’t double capital for over a decade. Yet early models likely used optimistic growth rates and quick exit assumptions—perfect fodder for Rule of 72 abuse.
Fee Structures Distort the Doubling Game
Management fees (typically 2%) and carried interest (20%) eat into net returns. The Rule of 72 usually applies to gross returns. But LPs care about net. So if a fund projects 20% gross IRR, net might be 16%. Now the doubling time jumps from 3.6 to 4.5 years. That’s a whole extra year lost to fees—a detail the rule ignores.
Experts disagree on how much this skews perception. Some argue LPs should apply the rule to net returns only. Others say it’s too crude to matter either way.
Rule of 72 vs. Real Modeling: When Back-of-the-Envelope Fails
You can’t build a portfolio on napkin math. Yet, in early-stage conversations, the Rule of 72 opens doors. “If we grow EBITDA at 15% and exit at 10x, that’s a 2.5x in five years.” Quick division: 72 ÷ 15 = 4.8 years. Close enough.
But because real LBO models include debt paydown, tax shields, capex cycles, and exit multiples that fluctuate with markets, the rule is just a starting point. A 2023 study of 47 mid-market buyouts found that initial IRR projections averaged 23%, but actuals landed at 17.4%—a gap large enough to turn a “double in three years” dream into a six-year grind.
That said, the rule remains embedded in training programs. Blackstone, Apollo, and Ares all use simplified frameworks in associate boot camps. Why? Because before you model 147 line items, you need a story. And the Rule of 72 helps craft it.
Monte Carlo Simulations: The Anti-Rule of 72
Some firms now use probabilistic models that simulate thousands of outcomes. These show not just a single IRR, but a distribution—say, a 30% chance of doubling in five years, 50% in seven. Compared to the Rule of 72, it’s overkill. For internal analysis? Invaluable. For explaining to a pension board? Forget it. No one wants a histogram when asking “When do I get paid?”
Alternative Doubling Rules: 70, 69.3, or Even 73?
Some prefer the Rule of 70—it’s slightly more accurate for lower rates. Others use 69.3 for continuous compounding. A few quants swear by 73 for high-inflation environments. Honestly, it is unclear whether any offer meaningful advantage. The differences are marginal. The Rule of 72 endures because it’s memorable. Not because it’s right.
Alternatives to the Rule of 72 in Modern Fund Analysis
To give a sense of scale: a $100 million fund returning 2x MOIC in seven years delivers about a 10.4% IRR. The Rule of 72 would suggest 72 ÷ 10.4 ≈ 6.9 years—deceptively close. But change the timing of distributions, and IRR swings wildly. That’s why many allocators now weight TVPI (total value to paid-in) and DPI (distributions to paid-in) equally with IRR.
And that’s exactly where the rule fails—it can’t measure liquidity. A fund might have a stellar IRR but haven’t returned a dime. LPs get nervous. They want cash, not theoretical growth.
IRR vs. Multiple: Which Tells the Truer Story?
IRR rewards speed. Multiples reward scale. A 1.5x in three years beats a 2x over nine, even if the latter feels bigger. The Rule of 72 leans toward IRR thinking. But institutional investors increasingly distrust IRR’s sensitivity to timing. A last-minute dividend recap can inflate it overnight.
In short, the rule favors speed demons. But long-term wealth builders often win quietly.
Persistence Analysis: Do Top Quartile Funds Stay Top?
Data from Preqin shows only 35% of top-quartile private equity funds repeat the feat in the next vintage. Which explains why some allocators ignore past IRR entirely. If performance doesn’t persist, why fetishize a doubling timeline? Yet the Rule of 72 still shapes expectations. Because narrative beats data every time.
Frequently Asked Questions
People don’t think about this enough: the Rule of 72 isn’t exclusive to private equity. It applies to any compounding asset. But private equity’s high-return aspirations make it especially relevant. Here’s what clients and analysts get wrong most often.
Can the Rule of 72 Be Used for Losses?
Sure. Divide 72 by the negative return rate to estimate how long until value halves. A portfolio losing 8% annually? Half gone in nine years. Not a fun calculation, but useful for stress testing.
Does Inflation Affect the Rule of 72?
It does, indirectly. A 6% return with 3% inflation delivers ~3% real growth. So while nominal doubling takes 12 years (72 ÷ 6), real doubling takes 24 (72 ÷ 3). Most users forget this. Suffice to say, inflation erodes the purchasing power of that "doubled" capital.
Is the Rule Useful for Early-Stage Venture Capital?
Less so. VC returns are binary—home runs or write-offs. The rule assumes steady compounding. In venture, you’re not doubling capital gradually. You’re waiting for a 10x or 20x exit. A 2021 study of Y Combinator startups found median time to exit was 6.2 years, with average returns skewed by outliers like Stripe. The rule? Not much help there.
The Bottom Line: A Flawed Tool That Still Earns Its Place
I find this overrated as a precision instrument but invaluable as a communication device. The Rule of 72 survives because it translates complex finance into human terms. We’re wired to understand doubling. We’re not wired to parse net present value formulas at 8 a.m. in a boardroom.
But let’s not pretend it’s rigorous. It ignores fees, taxes, volatility, and cash flow timing. It’s like judging a car by top speed alone—sure, 200 mph sounds great, but what about fuel efficiency, safety, or resale value?
Take position: use the Rule of 72 to spark conversation, not close deals. Pair it with MOIC, DPI, and realistic exit timelines. And when someone says, “This will double in three years,” ask, “Net or gross? And when do I actually get the money?”
Because in private equity, timing is everything. And that’s exactly where the rule lies.