The Rule of 72: How Long It Takes Your Money to Double
The Rule of 72 is a quick way to estimate how many years it takes your money to double. Divide 72 by the annual interest rate: at 12% interest, your money doubles in about 6 years (72 ÷ 12 = 6). It works in reverse too. Divide 72 by the inflation rate to see how quickly your money loses half its value.
Imagine you plant a mango seed. At first it's tiny, but every year the tree grows bigger. Then it drops seeds, and those seeds grow trees of their own. Before long, one seed has become a small orchard.
Your savings can work the same way, thanks to something called compound interest. The Rule of 72 is a simple trick that tells you how fast your "money tree" will double, and you can do it in your head.
What is the Rule of 72?
The Rule of 72 is a shortcut for working out how long it takes money to double at a given interest rate. You don't need a calculator or a finance degree, just one division:
Years to double = 72 ÷ interest rate
That's it. If your savings earn 8% a year, 72 ÷ 8 = 9, so your money doubles in about 9 years.
Why does it work?
It works because of compound interest, which means earning interest on your interest.
Say you save ₦100,000 at 10% a year. In year one, you earn ₦10,000, so you have ₦110,000. In year two you earn 10% on ₦110,000, which is ₦11,000, not ₦10,000. Each year the interest gets a little bigger because it's calculated on a bigger pile.
That snowball effect is what makes money double faster than you'd expect. The Rule of 72 simply gives you a quick estimate of when it happens.
How fast does money double at different rates?
Here's the rule in action with rates you might come across in Nigeria:
Interest rate per year | Years to double (72 ÷ rate) |
|---|---|
5% | about 14 years |
10% | about 7 years |
15% | about 5 years |
16% | about 4.5 years |
22% | about 3.3 years |
The higher the rate, the faster your money doubles. Going from 5% to 15% doesn't just give you a bit more. It cuts the waiting time from 14 years to about 5. That's why it pays to compare the best savings apps in Nigeria before you decide where to keep your money.
A real example with Renmoney savings
Let's say Amaka saves ₦500,000.
In an account paying 5%, it takes about 14 years to grow to ₦1 million.
With RenFlex at up to 15% p.a., it takes about 5 years, and she can still withdraw anytime with a savings account with no withdrawal charges.
With RenVault at up to 22% p.a., it takes about 3 to 3.5 years, if she rolls over her savings and interest each time a plan matures. See how RenVault works in our guide to high interest savings accounts in Nigeria.
Same ₦500,000. The only difference is where she keeps it.
The Rule of 72 and inflation: the other side of the coin
Here's the part many people miss. The Rule of 72 also shows how fast inflation eats your money.
Inflation is when prices go up over time, so the same ₦1,000 buys less than it used to. Remember when ₦100 could buy a decent plate of food? That's inflation at work.
Divide 72 by the inflation rate to see how long it takes your money to lose half its buying power:
At 15% inflation, your money's value halves in about 5 years.
At 20% inflation, it halves in under 4 years.
So if your savings earn less than inflation, your money is quietly shrinking even though the number in your account looks the same. That's why choosing a savings account with a strong interest rate matters so much, and why a solid budget that sets aside money for savings every month is your first line of defence.
Is the Rule of 72 exact?
Not quite. It's an estimate, and it's most accurate for rates between about 6% and 10%. At higher rates it slightly underestimates the time: at 22%, the exact figure is closer to 3.5 years than 3.3.
For everyday planning, though, it's close enough to help you compare options and make better choices quickly.
How to use the Rule of 72 to grow your money
Compare savings options. Before you open an account, run the rule on its interest rate. The difference in years is often eye-opening.
Check your savings against inflation. If your rate is lower than inflation, your money is losing value. Look for a better option.
Set realistic goals. Want ₦2 million for rent or school fees? The rule tells you roughly how long your current savings will take to get there. If fees come due before your savings are ready, here's how parents can finance tuition, plus the back-to-school costs to plan for beyond fees.
Start early. Every doubling period you add makes a huge difference, so the sooner you start, the more doublings you get. All you need to open a Renmoney savings plan is your BVN; here's how to check your BVN if you don't have it handy.
Frequently asked questions
What is the Rule of 72 in simple terms?
It's a shortcut to estimate how many years it takes money to double. Divide 72 by the yearly interest rate. At 12%, money doubles in about 6 years.
Does the Rule of 72 work for inflation?
Yes. Divide 72 by the inflation rate to estimate how long it takes your money to lose half its buying power.
Why is it 72 and not another number?
72 gives close answers for common interest rates and divides neatly by many numbers (2, 3, 4, 6, 8, 9 and 12), which makes the maths easy to do in your head.
Does the Rule of 72 only work with compound interest?
Yes. It assumes your interest is added to your savings and earns interest too. If you withdraw your interest, your money won't double as fast.
Let your money work harder
The Rule of 72 shows a simple truth: where you keep your money matters as much as how much you save. A higher interest rate means your money doubles sooner and stays ahead of inflation.
Ready to put the rule to work? Download the Renmoney app and start saving with RenFlex or RenVault today.
