Rule of 72: How Long Your Money Takes to Double in FD, PPF and Equity

Every bank RM, LIC agent and finance reel has some version of the same line: "Your money will double." Very few say how long that takes. The rule of 72 tells you in your head, in about five seconds, with no app and no spreadsheet.

Divide 72 by the yearly return. The answer is roughly the number of years your money needs to double. A 6.5% FD? About 11 years. Equity at an assumed 12%? About 6. Further down: how close it really gets, what it says about inflation and tax, and where it breaks.

The whole rule: 72 ÷ rate

Years to double ≈ 72 ÷ yearly return (in %)

You use the rate as a plain number. 8% means 72 ÷ 8, which is 9 years. 6% means 12 years. 9% means 8 years.

It works backwards too. If you know the number of years, 72 ÷ years gives the yearly return you need. Want ₹2 lakh to become ₹4 lakh in 8 years? You need about 9% a year (72 ÷ 8). In 5 years? About 14.4% a year, which no FD in India pays.

The rule assumes compounding: interest earns interest, and nothing is added or taken out in between. A cumulative FD, KVP or a one-time mutual fund investment works like that. Simple interest and monthly SIPs don't. More on that later.

Why 72, and not 70 or 100

With simple interest, doubling takes 100 ÷ rate years. At 8% simple interest, that's 12.5 years. Compounding is faster because the interest itself starts earning, so the number you divide by drops from 100 to roughly 70.

The exact answer needs logarithms (log 2 divided by log of 1 + rate), which nobody does at a bank counter. For the rates Indians usually see, 4% to 15%, the exact years come out close to 72 ÷ rate. Why 72 and not 70? Mostly convenience. 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, so the mental maths is easy, and the rule is most accurate around 8%, which is right where FDs, PPF and small savings schemes sit.

It's an old trick. As the Wikipedia page on the rule of 72 notes, it already appears in Luca Pacioli's maths book printed in Venice in 1494. Some people use 70 or 69.3, which fit better at very low rates. For everyday use, 72 is fine.

How long FD, PPF, KVP and equity take to double

Here is the rule against the exact time, for the places most of us keep money.

KVP is the neatest proof. The post office sells it as a scheme that doubles your money, and its maturity of 115 months at 7.5% comes straight out of the same maths. The rule says 9.6 years; 115 months is 9.58 years. PPF at 7.1%, KVP at 7.5% and the 4% post office savings rate all stay unchanged for October–December 2026, as per the government's quarterly announcement.

Bank FD rates move more often. SBI's card rate for 5–10 years is 6.05% right now, so a long SBI FD doubles in about 12 years (72 ÷ 6.05). At these levels, every extra 1% of rate takes a year or more off the doubling time.

The plain reading: safe products double your money about once a decade. Faster than that means market risk, and market returns don't come in a straight line. For a goal 3–5 years away, doubling isn't the point; keeping the money intact is. For goals 15–20 years away, doubling every 6 years instead of every 10 makes a huge difference, as the next section shows.

Checking the rule on the calculator

Put ₹1 lakh in the lumpsum calculator at 12% for 6 years and you get ₹1,97,382. Not quite ₹2 lakh, but close. At 8% for 9 years it shows ₹1,99,900. At 15% for 5 years, ₹2,01,136, because the rule's 4.8 years got rounded up to 5.

On the FD calculator, ₹1 lakh at 6.5% with quarterly compounding becomes ₹1,99,997 after 10.75 years. The rule said 11.1 years. The FD is a little faster because quarterly compounding makes 6.5% work like 6.66% a year, and 72 ÷ 6.66 is 10.8.

What a few doublings add up to

Now leave that ₹1 lakh alone for 30 years at an assumed 12%.

The gap grows because the rule rounds 6.12 years down to 6, and that small error compounds too. For a feel, the rule is great. For a plan, run the calculator.

Look at the last row. Between year 24 and year 30 the money goes from ₹15.18 lakh to ₹29.96 lakh. That one last doubling adds ₹14.8 lakh, more than the first 24 years put together. Start six years late and you don't lose a sixth of the result, you lose half. This is the honest argument for starting at 25 instead of 31.

The same ₹1 lakh at PPF's 7.1% doubles roughly three times in 30 years and ends at ₹7.83 lakh. Safe, tax-free, and about a quarter of the equity figure, if equity actually delivers 12%. That "if" matters, and we'll get to it.

The inflation version: how fast prices double

The rule works on prices too. 72 ÷ inflation tells you how many years it takes for prices to double, which is the same as your rupee buying half as much.

Retail inflation was 4.82% in August 2026, as per MoSPI's CPI release, and RBI expects about 5.2% for this year. At 5%, prices double in about 14 years (72 ÷ 5 = 14.4). Many planners take 6% for a household, because school fees and medical costs rise faster than the CPI basket. At 6%, prices double every 12 years.

Say your family spends ₹50,000 a month today. Run it through the lumpsum calculator as if it were an investment at 6%, and the same lifestyle costs ₹1,00,610 a month in 12 years and ₹2,02,447 in 24 years. If you're 36 now and plan to retire at 60, you'll need about four times today's spend just to live the same way. The retirement calculator builds this in.

Real return, the quick way

Money that doubles in 12 years while prices also double in 12 years has bought you nothing extra. So subtract inflation from the return, then use 72. Equity at an assumed 12% with 6% inflation leaves about 6% real, so what your money can buy doubles in about 12 years (12.6 by exact maths). A 6.5% FD against 5% inflation leaves about 1.5% real. 72 ÷ 1.5 is 48 years. The exact answer is closer to 44. Either way, an FD barely moves your buying power, and that's before tax.

After tax, the doubling time stretches

FD interest is added to your income and taxed at your slab rate every year, even on a cumulative FD. In the 30% slab, with 4% cess, that's 31.2% of every rupee of interest. A 6.5% FD then earns about 4.47% after tax, and 72 ÷ 4.47 is about 16 years, not 11. The lumpsum calculator at 4.47% for 16 years shows ₹2,01,310. Tax alone adds five years.

Put inflation on top and it gets worse. 4.47% after tax against 6% inflation is a negative real return of about 1.4% a year. In buying-power terms, that money never doubles. It slowly shrinks.

Equity mutual funds are taxed only when you sell: 12.5% on long-term gains above ₹1.25 lakh in a year, 20% if held 12 months or less, plus cess. ₹1 lakh at 12% for 6 years gains ₹97,382, which is under ₹1.25 lakh, so there's no tax if that's your only long-term gain that year. The doubling time barely moves. With bigger amounts some tax will apply, but it's charged once at the end, not every year.

If you're in a low slab or pay no tax after the rebate, the FD's 11-year figure holds. Check where your interest lands on the income tax calculator, and see the post on how FD interest is taxed for TDS and Form 121.

Where it's handy: debt and "double your money" pitches

The rule works with any period, as long as the rate and the answer use the same one. Put in a monthly rate and you get months.

Credit card debt. Most cards charge somewhere around 3–3.75% a month on an unpaid balance (your statement shows it under finance charges). At 3.5% a month, 72 ÷ 3.5 is about 20 months. Leave ₹50,000 unpaid with nothing going in, and it heads towards ₹1 lakh in under two years, before late fees and GST on the interest. Clear card dues before you invest anything. If you can't do it in one go, a personal loan at a much lower rate is usually cheaper; the personal loan EMI calculator shows what that would cost a month.

Pitches that promise doubling. Turn the rule around. Doubling in 3 years needs about 24% a year by the rule (26% exactly). Doubling in 2 years needs about 36% (41% exactly). No bank FD or government scheme comes anywhere near that, so a "guaranteed" double in two or three years from a chit scheme, a trading tip group on WhatsApp or a chain plan should make you walk away. Check whether the entity is registered with SEBI or RBI before putting in a rupee.

It catches slow pitches too. A single-premium policy that "doubles your money in 20 years" works out to about 3.6% a year (72 ÷ 20). That's less than the post office savings account.

Tripling and four times: the rules of 114 and 144

Two cousins of the same rule:

• 114 ÷ rate = years to triple. At 12%, 9.5 years. The lumpsum calculator shows ₹3,10,585 after 10 years on ₹1 lakh, which is 3.1 times.
• 144 ÷ rate = years to become four times. That's just two doublings. At 12%, 12 years; the calculator shows ₹3,89,598, or 3.9 times.

At PPF's 7.1%, a single amount triples in about 16 years (114 ÷ 7.1). Handy when someone at a family function says their plot "has become three times in 15 years". That's roughly 7.6% a year, about the same as a post office scheme, before you count stamp duty and maintenance.

Where the rule falls short

• It's an approximation. Between about 6% and 10% it's almost spot on. At high rates it under-counts: at 24% it says 3 years when the truth is 3.2, at 36% it says 2 when it's 2.25. At very low rates it slightly over-counts.
• It's for one-time money, not SIPs. A ₹5,000 monthly SIP at an assumed 12% shows ₹5,28,785 after 6 years on the SIP calculator, against ₹3,60,000 put in. That's 1.47 times, because most of the money went in later. It takes about 10 years (₹11.62 lakh on ₹6 lakh) to roughly double what you put in. A policy where you pay a premium every year has the same problem. A SIP's actual return is measured with XIRR, which the post on how SIP returns are measured explains.
• Market returns aren't steady. 12% is an average assumption. A bad year just before you need the money can push the doubling out by years. The rule tells you what happens if the average holds, not that it will. And don't feed it last year's number: a fund that gave 25% last year does not "double every 3 years". Plan with 10–12% for equity, and know it can come in lower.
• Rates change. An FD renewed at a lower rate, or a revised PPF rate, changes the doubling time from then on.
• It knows nothing about tax and inflation unless you feed it the rate you actually keep.

Four doubling times worth remembering at 30

If you keep only four numbers from all this, keep these. Three are where a salaried person's long-term money usually ends up. The fourth is what works against all of them.

Read the last row first, because it changes the other three. Over 30 years at 5% inflation, prices go up about 4.3 times. The taxed FD grows 3.7 times. So someone at 30 in the top slab who keeps retirement money in FDs and renews them every few years reaches 60 with more rupees and less buying power than they started with. Nobody at the bank counter will put it that way, but that's the arithmetic.

PPF ends up about 1.8 times ahead of prices (₹7.83 lakh against ₹4.32 lakh), tax-free and with no market risk. Take 6% inflation instead, as many planners do for household costs, and today's ₹1 lakh of spending costs ₹5.74 lakh, so PPF's lead shrinks to about 1.4 times. A decent floor for the safe part of long-term money, not a way to get rich. Equity, if 12% actually holds, ends about 7 times ahead of prices. That "if" is why the table says assumed, and why you should plan on a range, not a promise.

What the table hides is time. All four rows assume 30 years. Start at 36 and you get 24, which is one equity doubling fewer: ₹15.18 lakh instead of ₹29.96 lakh on the same ₹1 lakh. Six years of waiting costs half the result. That's the whole case for starting at 30 rather than "once the salary is a bit better".

So for money you won't touch for decades, the split is fairly plain. FDs for what you need in the next few years, PPF for the steady long-term part, and equity for most of what is meant for 60, keeping in mind that its 12% is a guess. Before you commit real money, look at the actual rupees side by side. The SIP vs FD comparison does that, SIP against RD and lumpsum against FD, which a division by 72 can't.

Written by the Gainly team for general information, not as investment or tax advice. Rates and rules change, so check the latest before you decide; tell us if something here is out of date. Contact