The formula uses the standard compound-growth formula. Future cost of an item priced ₹X today, at inflation rate i for t years = X × (1+i)^t. Going the other way, an amount ₹Y t years from now is worth Y / (1+i)^t in today's purchasing power. The math is symmetric and exact - what's harder is picking the right inflation rate, since headline numbers, RBI targets and your personal experience often differ by 2-3 percentage points.
India publishes two main inflation indices: the Consumer Price Index (CPI), which the RBI uses for its 4% target, and the Wholesale Price Index (WPI), used for inflation-indexed bonds and some industrial contracts. CPI is the relevant number for personal financial planning. CPI breaks down further into Combined (national), Urban, Rural and category-wise (food, fuel, housing, etc.) - use the most relevant category for your specific goal.
The RBI inflation target is 4% with a ±2% band, and the central bank uses repo-rate adjustments to anchor expectations. In practice, periods of 6-7% inflation have been more common than periods at the 4% target. For long-term planning (retirement, education), assume 6% as a base case and 8% as a stress-test case.