Compare matching periods
Choose a start date and an end date. Three months of income cannot be compared directly with a year-on-year price change. First calculate the change in the amount for the chosen period, then find or specify the price change for that same period.
Do not mix indicators with different bases: month-on-month, December-to-date and year-on-year price growth answer different questions. Record the comparison base with the percentage. An official basket describes aggregate consumption, not one family’s exact purchases.
A formula without double counting
If nominal return for a period is r and price growth is π, the change in purchasing power is (1 + r) / (1 + π) − 1. Enter rates as decimals: eight percent is 0.08. Division converts the final monetary amount into prices at the starting date.
In the example, 100,000 roubles rise by 8% to 108,000. With 6% price growth, the final amount at starting-date prices is 108,000 / 1.06 = 101,886.79 roubles. Real return is approximately 1.89%, not exactly two percent. With 10% price growth, it is about −1.82%.
What belongs in the numerator
For a result after expenses, use the amount that can actually be used, including applicable deductions and fees. Do not subtract the same cost twice. Tax rules depend on income type and jurisdiction and are not specified here.
Top-ups require care: a higher balance caused by a new contribution is not income. For precise comparisons, model cash flows with their dates rather than comparing only opening and closing balances.
Working with future inflation
Future price changes are unknown. Prepare several scenarios instead of one confident number. “Mild”, “base” and “stress” describe the modeller’s assumptions, not an official forecast.
Record the period, nominal result, selected price change, included costs and formula. If a future purchase uses another currency or a specific good, assess that price separately. Aggregate inflation provides context; it does not guarantee the price of a needed purchase or a real return.