Do you fetch CPI automatically?

No. This tool is privacy-first and runs fully in your browser. Paste index values you want to use; nothing is uploaded or stored.

Do you use the exact or approximate Fisher formula?

Both are shown. Exact: (1+i)/(1+r)−1. Approximation: i−r, good when rates are small.

Inflation Calculator – Fisher & Forward Rates

Inflation Calculator – Fisher (Ex-ante), Forwards, & Amount Adjuster

Bring your own CPI or price index values—everything runs locally in your browser for privacy.

Nominal interest rate (i, % per year)

Real interest rate (r, % per year)

Method

Results will appear here.

Forward (ex-ante) via Fisher: exact \(\displaystyle \pi = \frac{1+i}{1+r}-1\); approximation \(\pi \approx i-r\).

Start year (m, years from today)

End year (n, years from today, n>m)

Spot inputs

π₀,m (annual %, m-year spot inflation)

π₀,n (annual %, n-year spot inflation)

G₀→m = (1+π₀,m)^m

G₀→n = (1+π₀,n)^n

Results will appear here.

Forward inflation between years \(m\) and \(n\) (with \(n>m\)) from spot inflation:
\(\displaystyle (1+ \pi_{0,n})^n = (1+\pi_{0,m})^m \cdot (1+ f_{m,n})^{\,n-m}\Rightarrow f_{m,n}=\left(\frac{(1+\pi_{0,n})^n}{(1+\pi_{0,m})^m}\right)^{\!\frac{1}{n-m}}-1\).

Amount at start date (in currency)

CPI / Index at start

CPI / Index at end

Results will appear here.

Inflating/deflating an amount: \(\displaystyle \text{Amount}_{end} = \text{Amount}_{start}\times\frac{\text{Index}_{end}}{\text{Index}_{start}}\).

Formulas

1) Expected (Forward / Ex-ante) via Fisher

Exact Fisher: \(\displaystyle \pi = \tfrac{1+i}{1+r}-1\). Approximation: \(\pi \approx i-r\).

2) Forward Inflation for Future Years

Given spot inflation rates \(\pi_{0,m}\) and \(\pi_{0,n}\) (annualized, compounded): \(\displaystyle f_{m,n}=\left(\dfrac{(1+\pi_{0,n})^n}{(1+\pi_{0,m})^m}\right)^{\!\frac{1}{n-m}}-1\).

3) Inflate / Deflate Amounts Using Indexes

\(\displaystyle \text{Amount}_{end}=\text{Amount}_{start}\cdot \dfrac{\text{Index}_{end}}{\text{Index}_{start}}\).

You can use CPI, RPI, PCE, a sector-specific deflator, or even a price of a basket you define—just keep the index consistent across inputs.

Data sources

United States CPI-U, All Items (CPIAUCSL), via FRED. Local copy: /assets/tools/inflationUS.csv.
United Kingdom CPIH Index, All Items (L522), via ONS. Local copy: /assets/tools/inflationUK.csv.

This tool reads small CSV files hosted on this site. Everything runs in your browser.