Inflation math reduces to one compound formula: future cost equals today's amount multiplied by (1 + r)^n, where r is the annual inflation rate written as a decimal and n is the number of years you apply it. Run that calculation in reverse and you get future purchasing power, which is what a fixed sum of cash will actually buy once prices have risen. Worked example: $100 at 3% annual inflation for 10 years produces a future cost of about $134.39 and a future purchasing power of about $74.41. Excel handles both sides of this with two short formulas once your inputs sit in labelled cells, and the Inflation Calculator handles them as well, without any spreadsheet setup, updating the two outputs the moment any input changes. Both approaches share the same math and the same caveat: the model assumes one constant rate across the entire period, so it is a scenario tool, not a record of historical inflation.
The Compound Inflation Formula Behind Both Views
Inflation compounds the same way interest does, just in reverse. Instead of a balance growing, prices rise and each dollar buys a smaller share of them, which is why a fixed sum of cash quietly loses buying power even while its face value never changes. The whole model is one equation, presented two ways:
Future cost = amount × (1 + r)^n Future purchasing power = amount ÷ (1 + r)^n
Here r is the annual inflation rate as a decimal (3% becomes 0.03) and n is the number of years. The two operations are mathematical inverses, so you can read inflation as "what will this cost later" or "what will my cash actually buy later" depending on the question in front of you. Inflation, as a general rise in prices across an economy, is documented in sources like the Wikipedia overview of inflation, and the distinction between nominal and real values is laid out in Wikipedia's entry on real versus nominal value.
The single worked example worth keeping in mind is the one already shown: $100 today at 3% for 10 years gives $100 × (1.03)^10 = $100 × 1.343916... ≈ $134.39 future cost, and $100 ÷ 1.343916... ≈ $74.41 future purchasing power. Both numbers come from the same compound factor of about 1.344; they simply face opposite directions.
Calculating Inflation in Excel Step by Step
Setting this up in Excel takes about a minute once you know which cells hold which inputs. The model has only three inputs — amount, rate, years — and two outputs — future cost and future purchasing power — so a small block of cells is enough.
- Label three input cells. In A1 type "Amount", in B1 type "Rate (decimal)", and in C1 type "Years". Labels keep the formulas readable when you come back to the spreadsheet later.
- Enter your numbers directly below the labels. Put the dollar amount in A2 (for example, 100), the annual inflation rate as a decimal in B2 (for example, 0.03 for 3%), and the number of years in C2 (for example, 10).
- Add two output labels. In A4 type "Future cost" and in A5 type "Future purchasing power".
- Write the future-cost formula in B4. Click B4 and type =A2*(1+B2)^C2, then press Enter. The result is how much something costing $100 today will cost in 10 years at 3% — about $134.39.
- Write the purchasing-power formula in B5. Click B5 and type =A2/(1+B2)^C2, then press Enter. The result is what $100 of cash will actually buy in 10 years, measured in today's dollars — about $74.41.
- Change any input to refresh both outputs. Edit A2, B2, or C2 and Excel recalculates B4 and B5 immediately. That is the entire model.
A few refinements make the layout easier to reuse. If you prefer to type the rate as a percentage, store 3 in B2 and write =A2*(1+B2/100)^C2 instead; the same formula structure still produces both outputs. To compare scenarios down a column, leave the input cells as named or absolute references and copy B4 and B5 next to each new set of inputs. Deflation is built in: enter -0.02 in B2 and future cost falls below the starting amount while purchasing power rises above it, because prices are moving the other way.
The same compound approach that powers these cells also drives related planning tools, including the guide to calculating inflation-adjusted return for any investment, which extends the model to portfolio performance.
Skip the Spreadsheet With a Browser Calculator
For readers who would rather not lay out cells and write formulas, the Inflation Calculator runs the identical compound math and surfaces both outputs side by side. The workflow matches the Excel version one-to-one, except the inputs are typed into the page rather than a worksheet.
- Enter the amount of money you have today, in dollars.
- Enter the annual inflation rate you want to assume, using a negative number for deflation, plus the number of years.
- Read the future cost and the future purchasing power, which update instantly as you change any input.
Everything runs entirely in the browser: no data leaves the device, no sign-up is required, and there is nothing to install. Because both outputs are tied to the same live inputs, you can flip the rate between 2%, 4%, and 6% to see how a long horizon amplifies small rate differences, which is exactly the kind of scenario sweep an Excel column of formulas is designed to do.
Future Cost and Purchasing Power: Two Views of the Same Factor
Future cost and future purchasing power are not two different calculations. They are the same compound factor, (1 + r)^n, applied in opposite directions. Future cost multiplies today's amount by that factor and tells you what something priced today will cost later. Future purchasing power divides today's amount by the same factor and tells you what your cash will actually command later, expressed in today's money. Together they describe one economic story from both sides: prices rise, and a fixed pile of cash quietly buys less of them.
| Task | Excel layout | Inflation Calculator |
|---|---|---|
| Setting up inputs | Three labelled cells with one number each | Three typed values, no setup |
| Future cost formula | =A2*(1+B2)^C2 | Computed automatically from inputs |
| Future purchasing power formula | =A2/(1+B2)^C2 | Computed automatically from inputs |
| Updating as inputs change | Recalculates on Enter | Updates as you type |
| Scenario sweeps | Copy the formulas down a column | Retype inputs for each scenario |
| Deflation support | Negative rate works as-is | Negative rate works as-is |
| Data handling | Lives in your workbook | Runs entirely in your browser |
Excel wins when you want a permanent worksheet that compares many scenarios side by side or feeds results into a larger financial model. The calculator wins when you want a quick answer to one specific question, or when you want to nudge the rate up and down to build intuition without editing formulas.
What These Numbers Can and Cannot Tell You
Both approaches share the same limits, because both run the same compound model. The calculation assumes a single fixed annual inflation rate for the entire period, so it is best used to compare scenarios — for example, how 2%, 4%, and 6% each play out over 20 or 30 years — rather than to predict an exact future price. Real inflation, tracked by indexes like the Consumer Price Index, rises and falls every year, so the actual outcome will differ from any single-rate projection.
Deflation is fully supported in both tools. A negative annual rate produces a future cost below the starting amount and a future purchasing power above it, which mirrors what happens during periods when prices actually decline. The amount and number of years still must be zero or positive in either tool; the rate is the only input that can go negative.
Because inflation touches long-term financial planning, the output should be treated as general information rather than a forecast. The numbers are useful for stress-testing retirement savings, future expenses, salary expectations, and any figure you want to keep meaningful over time, but they should be confirmed with a licensed professional before they drive a real decision.