Method, example and interpretation · Revised 16 October 2026
Dates, percentage changes and purchasing power
These everyday calculations solve three different interpretation problems: elapsed calendar age, the base used in a percentage, and the difference between nominal money and real purchasing power. Small definition changes can produce large differences in an answer.
Age is a calendar calculation
Choose a valid birth date and an as-of date. Calendar years and month lengths are used rather than treating every month as thirty days. The total-day count uses UTC dates to avoid daylight-saving differences. A leap-day birthday is treated as 28 February in a non-leap year; organisations can apply different eligibility conventions.
Always identify the percentage base
Percentage change is (new − old) ÷ old × 100. The denominator is the original amount. An increase followed by the same percentage decrease does not return to the starting point because the second percentage uses a new base. The “what percent” mode answers a ratio question and should not be confused with percentage-point change.
Inflation works in both directions
Future cost multiplies a current price by (1 + inflation)^years. Purchasing power divides a future nominal amount by the same factor. The assumption is a constant rate; your household’s expenses may rise at a different pace from a published consumer-price index. Use separate rates for sensitivity checks rather than treating one projection as a forecast.
Worked example
A basket costs ₹1,00,000 today and prices rise 6% annually for ten years.
Future cost: ₹1,79,085; purchasing power of a future ₹1,00,000: ₹55,839.
The same nominal sum buys less, while funding the same basket requires a larger future sum.
Questions about this tool
Why is a 20% gain followed by a 20% loss a loss overall?
₹100 becomes ₹120, then 20% of ₹120 is ₹24. The final ₹96 is 4% below the original amount.