Stable Numbers
UK Inflation Calculator
Convert a price from one date to another using CPI. Choose monthly or annual CPI from a JSON file, or enter a custom cumulative inflation if you already have a headline figure.
/data/uk-cpi.json on your server to enable this mode.
Loaded entries: . Choose specific months for monthly CPI or years for annual CPI.
For example, if prices rose by 24.3% between the two dates, enter 24.3.
Add CPI data
Create /data/uk-cpi.json with either or both of these keys:
{
"monthly": { "2020-01": 108.3, "2020-02": 108.5, "...": 123.4 },
"annual": { "2020": 109.0, "2021": 113.0, "...": 130.9 }
}
Values are index points. The calculator uses Adjusted = Amount × (Index(to) ÷ Index(from)).
About this calculator
Inflation adjusts money for changes in general price levels. The UK Consumer Prices Index (CPI) is a widely used measure of inflation for day-to-day comparisons. This page lets you move a figure from one date to another in two ways. In CPI index mode you pick monthly or annual index points from a simple JSON file. The tool multiplies your amount by the ratio of the target index to the starting index. In custom rate mode you can apply your own cumulative inflation percentage for the period if you already have a headline figure from a report.
Monthly CPI is best when you know the exact months. Annual CPI is the calendar-year average, which is often used for long spans. Either way, the formula is straightforward: Adjusted = Amount × (Index(to) ÷ Index(from)). If the index at your target date is 130.0 and the index at your start date is 100.0, then the factor is 1.3, which means prices are 30% higher and £100 becomes £130 in today’s terms.
Keep in mind that CPI reflects the average basket. Specific categories such as energy or rents can move very differently. For investment analysis, separate the concepts of nominal and real returns: nominal figures include inflation, while real figures remove it. To compare options over time, convert everything to the same year’s prices first, then look at the real differences. If you need to adjust wages, maintenance budgets or multi-year project costs, this small step stops inflation from muddying the picture.
The share link captures your inputs so you can send a setup or revisit it later. The CSV export records the source and factor used so your notes are auditable.
Explore more in the finance tools hub.