Enter the current price level (for example, a CPI value) and the past price level into the Deflation Rate Calculator. The calculator will evaluate the deflation rate (the percent decrease in the price level from the past period to the current period).
Deflation Rate Formula
The calculator uses three formulas, one for each mode.
CPI rate mode:
Deflation Rate = ((P_past - P_current) / P_past) * 100
Future price mode:
P_future = P_start * (1 - r)^t
Find rate or term mode:
r = 1 - (P_final / P_initial)^(1/t) t = ln(P_final / P_initial) / ln(1 - r)
- P_past: price level or CPI in the earlier period
- P_current: price level or CPI in the later period
- P_start: starting price or amount
- P_future: projected price after the time period
- P_initial, P_final: initial and final prices used to back out a rate or time
- r: annual deflation rate as a decimal (negative values mean inflation)
- t: time period in years
The CPI tab takes two price levels and returns the percent drop. A positive result is deflation, a negative result is inflation. The future price tab compounds a starting amount by (1 - r) each year to project a future price. The find tab solves either for the annual rate (given two prices and a time period) or for the years required (given two prices and a rate).
Reference Tables
Use these to sanity-check inputs and interpret the result.
| Annual rate | Label | What it usually signals |
|---|---|---|
| Above +2% | Strong deflation | Falling demand, debt stress |
| 0% to +2% | Mild deflation | Slow growth or productivity gains |
| 0% | Price stability | Flat price level |
| -1% to -3% | Normal inflation | Most central bank targets |
| Below -5% | High inflation | Overheating or supply shock |
| Annual deflation | Price after 5 years | Price after 10 years | Price after 20 years |
|---|---|---|---|
| 1% | $95.10 | $90.44 | $81.79 |
| 2% | $90.39 | $81.71 | $66.76 |
| 3% | $85.87 | $73.74 | $54.38 |
| 5% | $77.38 | $59.87 | $35.85 |
Example Problems
Example 1: CPI rate. Last year the CPI was 120. This year it is 114. Plug those into the CPI tab. The rate is ((120 - 114) / 120) × 100 = 5.00% deflation. Purchasing power rises by about 5.26%, so the same dollar buys roughly 1.0526 times as much.
Example 2: Future price. A product costs $100. You expect 2% annual deflation for 5 years. The future price is 100 × (1 - 0.02)^5 = $90.39. Total deflation over the period is 9.61%.
FAQ
What is a deflation rate? It is the percent drop in a price level or price index from one period to the next. A 3% deflation rate means prices fell by 3%.
How do I handle inflation in this calculator? Enter the rate as a negative number in the future price or find tab. The CPI tab handles it automatically: if the current level is higher than the past level, the result is reported as inflation.
Why use CPI instead of a single product price? CPI averages a basket of goods, so it reflects general price changes. A single product price can move for reasons unrelated to economy-wide deflation, like supply changes or a new model release.
Does the formula compound? Yes. The future price formula compounds annually using (1 - r)^t. If you enter months, the calculator converts to a fractional year before compounding.
