Economics / worked example
Inflation and purchasing power: a fictional basket you can calculate
Calculate a fully fictional weighted basket, distinguish an item price from inflation and purchasing power, and check every number without market data.
You will learn
- Distinguish an individual price, a weighted basket change and purchasing power.
- Recalculate a fixed basket’s percent change from its quantities, prices and weights.
- Explain why this invented example is not an official statistic or investment result.
Before you begin
Comfort adding and dividing simple numbers; no economics knowledge, account, trading or investment is needed.
Price, basket change and purchasing power answer different questions
A price is the amount attached to one item at one time. Inflation in a defined basket is a percentage change in the cost of a selected group of goods or services between two periods. One item can rise while another falls, so an individual price change is not by itself the basket’s result.
The U.S. Bureau of Labor Statistics describes CPI as the average change over time in prices paid for a representative consumer basket, and cautions that the result need not match one person’s experience. This lesson uses that distinction and percentage-change method only. It does not report BLS data or estimate inflation in any country.
Evidence: BLS CPI basket and limitations
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
Set one fictional basket and keep its quantities fixed
Everything below is invented for practice. Period 0 and Period 1 are hypothetical periods exactly one year apart; MU means fictional money units, not dollars or a real currency. The quantities stay fixed. Baseline spending also defines each item’s weight: base-period item cost divided by the 1,000 MU starting basket cost.
The table represents one simplified basket, not a sampled statistical index, a particular household or a claim about Binance users. The item prices do not come from BLS or another live dataset.
| Item | Fixed quantity | Period 0 unit price | Period 0 cost / weight | Period 1 unit price | Period 1 cost | Individual price change |
|---|---|---|---|---|---|---|
| Housing service | 1 monthly unit | 500 MU | 500 MU / 50% | 550 MU | 550 MU | +10% |
| Grocery basket | 1 fixed basket | 300 MU | 300 MU / 30% | 360 MU | 360 MU | +20% |
| Transit pass | 1 pass | 150 MU | 150 MU / 15% | 135 MU | 135 MU | −10% |
| Communication plan | 1 plan | 50 MU | 50 MU / 5% | 60 MU | 60 MU | +20% |
Evidence: BLS CPI basket and limitations
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
Reproduce the percentage change
Add the cost of the same quantities in each period. Period 0: 500 + 300 + 150 + 50 = 1,000 MU. Period 1: 550 + 360 + 135 + 60 = 1,105 MU. The basket’s percentage change is (1,105 − 1,000) ÷ 1,000 × 100 = 10.5%. The unit is percent over the stated one-year interval.
A cross-check uses the base-period weights and each item’s price change: 50% × 10% + 30% × 20% + 15% × (−10%) + 5% × 20% = 10.5%. The weighted result matches the cost ratio because quantities and base weights were held fixed. This is an arithmetic teaching model, not the BLS CPI’s full sampling and index-construction procedure.
Evidence: BLS percentage-change formula · BLS CPI basket and limitations
U.S. BLS method reference for percent-change calculations and CPI series. The numbers and weights in this lesson are entirely fictional and are not BLS data, a current inflation estimate or an official index.
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
Translate the basket cost into fixed-money purchasing power
Suppose the same nominal amount, 1,000 MU, is held for Period 1. It buys 1,000 ÷ 1,105 = 0.904977 of the invented basket, or about 90.50% of one basket. The purchasing-power decrease for this fixed basket is (1 − 1,000 ÷ 1,105) × 100 ≈ 9.50%.
The 10.5% basket-price increase and roughly 9.50% reduction in what a fixed 1,000 MU buys are not equal percentages: the second calculation divides by the later basket cost. In this made-up example, 1,105 MU would buy exactly one basket. This describes only the units in the exercise; it says nothing about wages, savings, a cryptoasset, investment returns or what anyone should buy.
Evidence: BLS CPI basket and limitations · BLS percentage-change formula
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
U.S. BLS method reference for percent-change calculations and CPI series. The numbers and weights in this lesson are entirely fictional and are not BLS data, a current inflation estimate or an official index.
Limits: this exercise is not an official CPI or personal forecast
The Bureau’s CPI covers specified U.S. consumer populations and uses a representative basket; it is not a personal inflation calculator or a complete cost-of-living measure. A different country, time period, population or basket can produce a different result. The BLS reference supports the concepts and calculation method, not the fictitious values above.
A basket result depends on its quantities, weights, item definitions and comparison dates. Changing those choices changes the result. Price levels in two places cannot be compared by subtracting unrelated index values. No BNB, Binance, crypto price or portfolio return is included. Inflation arithmetic is not a forecast or financial recommendation.
Evidence: BLS CPI basket and limitations
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
Exercise and worked solution
Try one controlled change: replace only the Period 1 grocery-basket cost of 360 MU with 330 MU; keep every other Period 1 entry and all base quantities unchanged. Calculate the new basket cost, its percentage change from 1,000 MU and how much of that basket a fixed 1,000 MU could buy.
Solved: 550 + 330 + 135 + 60 = 1,075 MU. The basket change is (1,075 − 1,000) ÷ 1,000 × 100 = 7.5%. A fixed 1,000 MU buys 1,000 ÷ 1,075 ≈ 0.930233 of the basket. The corresponding purchasing-power decrease is about 6.98%. The answer uses only the stated fictional quantities.
Evidence: BLS percentage-change formula · BLS CPI basket and limitations
U.S. BLS method reference for percent-change calculations and CPI series. The numbers and weights in this lesson are entirely fictional and are not BLS data, a current inflation estimate or an official index.
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.
Check your understanding
In the solved basket, why is the change 10.5%, and what fraction of it can a fixed 1,000 MU buy in Period 1?
Show explanation
The fictional cost rises from 1,000 to 1,105 MU, so (1,105 − 1,000) ÷ 1,000 × 100 = 10.5%. The fixed amount buys 1,000 ÷ 1,105 ≈ 0.904977 of the basket; its purchasing-power decline is about 9.50%, not 10.5%.
Primary sources
- Calculating percent changes: Consumer Price IndexU.S. Bureau of Labor Statistics · 2026-10-26
U.S. BLS method reference for percent-change calculations and CPI series. The numbers and weights in this lesson are entirely fictional and are not BLS data, a current inflation estimate or an official index.
- Consumer Price Index Frequently Asked QuestionsU.S. Bureau of Labor Statistics · 2026-10-26
The BLS FAQ describes U.S. CPI populations, representative baskets, weighting and limits: CPI does not describe every individual’s prices or a complete cost of living. This U.S. reference is not a universal measure. The teaching basket here is synthetic.