Every finance student learns the discounted cash flow model as a ten-tab spreadsheet. Terminal value, WACC build, sensitivity tables, the lot. Then they sit down in an interview, get handed a business with no laptop in front of them, and freeze.
The analysts who look sharp in the room are not building that spreadsheet in their head. They are running a much shorter mental shortcut, and it gets them within a few percent of the real answer in under a minute. The shortcut is the rule of 72, and it is the single highest-leverage piece of mental math you can walk into an interview with.
What is the rule of 72 and why does it matter for valuation?
The rule of 72 is a quick way to estimate how long it takes money to double or halve at a given rate of return, without doing the actual exponential math. You divide 72 by the discount rate to get the number of years.
At a 12% discount rate, 72 divided by 12 is 6. That means money halves in value roughly every 6 years once you discount it back to today. This single number is the entire trick. If you know that a dollar six years from now is worth about 50 cents today at a 12% discount rate, you can mentally value almost any stream of cash flows without a calculator.
How do you actually value a business in your head using this?
Take a business generating $2M AUD in revenue and $1M AUD in free cash flow, and you are valuing it over a 12-year horizon at a 12% discount rate. That is the exact setup analysts use to demonstrate this in an interview.
Here is the mental sequence, step by step:
- Ignore the discounting for a moment and just add up the raw cash flows. $1M a year for 12 years is $12M gross, undiscounted.
- Apply the rule of 72. At a 12% discount rate, money halves every 6 years. Over a 12-year period, that is two full 6-year halving periods.
- Average the discounting effect across the whole period. Because the early years are worth close to full value and the later years are worth much less, the cash flows average out to roughly 50 cents on the dollar across the full 12 years.
- Cut the gross figure in half. $12M gross, cut in half, lands you at approximately $6M.
That is the entire mental model. No spreadsheet, no calculator, one division and one halving.
How accurate is the mental shortcut compared to a real DCF?
This is the part that makes the trick worth learning properly rather than dismissing as a rough guess. The real, fully modelled DCF on that exact $2M revenue, $1M free cash flow, 12-year, 12% discount rate business comes out to $6.19M.
The mental shortcut landed at approximately $6M. That is within 3% of the real answer, done with zero spreadsheet and inside a minute.
| Method | Result |
|---|---|
| Raw undiscounted cash flow ($1M x 12 years) | $12M |
| Rule of 72 mental shortcut (halved for discounting) | ~$6M |
| Full modelled DCF | $6.19M |
| Variance of mental shortcut vs real DCF | ~3% |
For context on why this margin matters: in an interview, you are not being scored on hitting the number to the dollar. You are being scored on whether you can reason about value under pressure with no tools in front of you. Landing within 3% while your competitor is still trying to remember the terminal value formula is the entire game.
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Why does this gap between "looking sharp" and "freezing at the whiteboard" exist?
Because most students only ever practise the model, not the mental shortcut. They can build a DCF perfectly in Excel with unlimited time and no one watching, and then have nothing to offer when someone asks them to sanity-check a valuation out loud with a whiteboard marker in hand.
The rule of 72 shortcut is not a replacement for knowing how to build a real DCF. It is what you reach for in the thirty seconds before you start building one, or in the interview room where building one is not physically possible. Interviewers know the difference between a candidate who has internalised how discounting actually behaves and one who has only ever pressed a button in a spreadsheet to get the answer.
Does the rule of 72 shortcut work at other discount rates too?
Yes, and this is why it is worth learning properly rather than memorising the single 12% example. The rule scales with any discount rate you are given.
At a 12% discount rate, 72 divided by 12 gives you a 6-year halving period, which is what produces the roughly 50c-on-the-dollar average used above. If an interviewer hands you a different discount rate, the same division still applies: 72 divided by the rate gives you the halving period for that specific scenario, and you build the same halve-and-average logic from there.
The mechanics do not change. Only the halving period does. Once you have run it once on a real number, like the $2M revenue example above, the method sticks.
FAQ
What is the rule of 72 in finance?
The rule of 72 is a mental shortcut for estimating how long it takes an amount to double or halve at a given rate of return. You divide 72 by the rate to get the number of years. At a 12% discount rate, that gives 6 years, meaning money halves in present value roughly every 6 years.
How do you value a business without a calculator in an interview?
Add up the raw, undiscounted cash flows over the forecast period, then use the rule of 72 to work out how much value is lost to discounting across that period, and apply that as a rough average haircut to the gross figure. A $2M revenue, $1M free cash flow business over 12 years at a 12% discount rate produces $12M gross, which becomes approximately $6M once you apply the roughly 50% average discount from two 6-year halving periods.
How accurate is the mental DCF shortcut versus a real modelled DCF?
On the worked example above, the mental shortcut landed at approximately $6M against a real, fully modelled DCF result of $6.19M, a variance of about 3%. That is close enough to be useful for interview reasoning and sanity-checking, though it is not a substitute for a properly modelled valuation when the actual number matters.
Why do interviewers ask candidates to value a business in their head?
Because it tests whether you understand how discounting behaves, not just whether you can operate a spreadsheet. A candidate who can reason through value out loud, under time pressure, with no tools, is demonstrating a level of comprehension that building a DCF in Excel with unlimited time does not prove on its own.
Does the rule of 72 work for any discount rate, not just 12%?
Yes. Divide 72 by whatever discount rate you are given to find the halving period for that rate, then apply the same halve-and-average logic to your cash flows. The number 12% and the resulting 6-year halving period in the worked example is one specific case, not a fixed rule.
Is the rule of 72 shortcut a replacement for learning to build a real DCF model?
No. It is a mental tool for the moments a full model is not possible, an interview whiteboard, a live conversation, a quick sanity check, not a substitute for knowing how to build and defend a proper discounted cash flow model when the situation calls for one.
Where to go from here
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