Retirement planning: How the Rule of 70 shows the real impact of inflation

New business data points to the fact that When people plan for retirement, the first question is often, “How much money will I need?” The answer is not as simple as picking a round figure. A retirement corpus of Rs. 1 crore may sound substantial today, but what that money can buy 15 or 20 years from now could be very different.

The Rule of 70 is a quick way to get a sense of how inflation affects prices over time. The calculation is simple: divide 70 by the annual inflation rate. The result gives an approximate number of years in which prices could double.

Take inflation at 5 percent. Dividing 70 by 5 gives 14 years. So, if something costs Rs. 1 lakh today and prices climb at an average rate of 5 percent a year, it could cost roughly Rs. 2 lakh after 14 years.

At 6 percent inflation, the same calculation gives around 11.7 years. In other words, prices would roughly double in less than 12 years.

The numbers are useful for retirement planning because household expenses do not stay frozen. Suppose a 40-year-old spends Rs. 60,000 every month today and anticipates to retire at 60. If inflation averages 5 percent over those 20 years, the monthly spending requirement could be around Rs. 2.4 lakh by the time retirement arrives.

That does not mean the person will definitely spend Rs. 2.4 lakh a month. It is an illustration of what happens if the same basket of goods and services becomes twice as expensive every 14 years or so. Actual expenses could be elevated or softer.

There is another catch. The inflation rate used in the calculation is an assumption. A household spending heavily on healthcare, education or rent may see its expenses climb faster than the general inflation rate. Someone who owns a home and has fewer education-related expenses could have a different experience.

The Rule of 70 can additionally be useful when looking at savings kept in low-return investments. Suppose money earns 6 percent a year while inflation averages 5 percent. The investment is growing in nominal terms, but its purchasing power is increasing much more slowly. Taxes on interest income can reduce the real gain further.

This is particularly relevant for people who are close to retirement. Keeping money in safer instruments can reduce market risk, but putting the entire retirement corpus into investments that barely beat inflation can create another problem: the money may not last as long as anticipated.

For someone still several years away from retirement, inflation is one reason to review the savings amount regularly rather than fixing a target once and forgetting around it. If income rises, increasing retirement contributions can help keep the plan aligned with future expenses.

The Rule of 70 is not a retirement calculator. It does not tell you how much to invest every month or what return you will earn. It is simply a quick way to make inflation easier to visualise.

A useful starting point is to take your current annual household expenses, apply a reasonable inflation assumption and see what they might look like at retirement. That one calculation can make a retirement target look very different from the figure that initially seemed enough.