This is a short, practical exercise, not a deep dive into monetary policy. It takes about ten minutes with a calculator open in another tab, and the output is a specific number you can actually use in a decision, rather than a vague sense that "things get more expensive over time."
Whatever a recurring cost is worth today, it isn't going to stay that price. Inflation raises the nominal cost of nearly everything over time, which means a weekly expense that feels manageable now is quietly getting more expensive every year even if the underlying habit never changes.
Here's a practical walkthrough for pricing that out properly instead of guessing.
Step 1: Get an Honest Current Number
Start with the actual current weekly or monthly spend, not a rounded-down guess. If it's a recurring purchase, check a recent receipt or statement rather than estimating from memory. Underestimating the starting number throws off every projection that follows it.
Step 2: Pick a Realistic Inflation Rate
Historical average inflation rates are publicly tracked, and the Bureau of Labor Statistics publishes the Consumer Price Index data that most inflation estimates are built on. A long-run average is a reasonable default if you don't have a strong reason to expect something different, but it's worth checking recent trends rather than assuming a single fixed number applies forever.
Step 3: Choose a Timeframe That Matches the Question You're Actually Asking
A 5-year projection answers a different question than a 20-year one. If you're deciding whether to change a habit now, a shorter timeframe shows the near-term impact. If you're thinking about long-term financial planning, a longer timeframe captures how much the compounding of both price increases and lost investment opportunity actually adds up to.
Step 4: Run the Numbers With the Calculator
Manual inflation math is straightforward in theory but easy to get wrong in practice, especially over longer timeframes where small rate differences compound into large gaps. The free inflation calculator by EvvyTools takes a current cost, a rate, and a timeframe, and returns the actual projected future cost instead of a rough mental estimate.
Plug in the number from Step 1, the rate from Step 2, and the timeframe from Step 3, and you'll get a real projected cost for the same habit a decade or two from now, assuming nothing else changes.
Step 5: Compare It Against What the Money Could Have Earned Instead
Inflation tells you what the cost will be. It doesn't tell you what you gave up by spending the money instead of investing it. Those are two separate calculations, and combining them gives the fullest picture: a habit's future price tag, plus the future value of the money if it had gone into an investment account instead.
Investor.gov has educational material on how compounding investment returns work over long timeframes, which pairs well with an inflation projection once you have both numbers in front of you.
Step 6: Sanity-Check the Result Against a Second Source
Any single projection is only as good as the assumptions behind it, so it's worth cross-checking a big or surprising result before treating it as settled. Investopedia publishes plain-language explanations of how inflation calculations work and what a reasonable range of assumptions looks like, which is a useful second opinion if a projected number seems unusually high or low relative to what you expected going in.
If two independent methods land in a similar range, that's a good sign the number is reasonably solid. If they diverge significantly, it usually means one of the underlying assumptions, the rate, the timeframe, or the starting number, needs a second look before you rely on the projection for an actual decision.
A Worked Example
Say a recurring cost currently runs $35 a week. At a long-run average inflation rate, that same weekly cost could realistically be somewhere in the neighborhood of $45 to $50 a week a decade out, purely from price increases, before accounting for anything else. Separately, if that same $35 a week had gone into an investment account instead at a modest average return, the accumulated total after ten years would likely be several thousand dollars, not because $35 a week sounds impressive on its own, but because of how many weeks are in a decade and how compounding works over that many contributions.
Neither of those numbers is a guess once you've actually run them through a calculator instead of estimating in your head. That's the entire value of doing this exercise with real tools instead of a rough mental approximation.
Why This Exercise Is Worth Doing at All
Most people never run this math because it requires two separate calculations chained together, and neither one is intuitive to do in your head. But the combined number, future cost plus lost investment growth, is a much more honest picture of what a recurring habit actually costs than the sticker price today.
EvvyTools recently published a longer piece walking through exactly this kind of layered cost calculation for a specific everyday habit, covering insurance, resale value, and opportunity cost on top of the obvious direct spend. It's a useful reference for anyone applying this same method to a different recurring cost of their own.
Step 7: Decide What Timeframe Actually Matters for Your Decision
Not every decision needs a 20-year projection. If you're deciding whether to trim a recurring expense this year to hit a shorter-term goal, a 3-to-5-year projection is more relevant to the actual decision in front of you than a distant retirement-scale timeline. Matching the projection window to the actual decision keeps the exercise useful instead of turning into an abstract number disconnected from anything you're actually deciding.
On the other hand, if the question is genuinely about long-term financial planning, like whether a habit is worth keeping through your working years, the longer timeframe is the one that actually answers that question. Running both a short and a long projection side by side, when the decision genuinely spans both timeframes, gives you the clearest picture of how the same choice plays out differently depending on how far out you're looking.
Revisiting the Projection Periodically
A projection made once and never revisited slowly drifts out of date, since both the actual spending amount and the inflation rate assumption can shift over a few years. Treating this as a periodic exercise, revisited every year or two rather than done once and filed away, keeps the number relevant to your current situation instead of anchored to assumptions from years earlier that may no longer hold.
This matters more the longer the projection window is. A 5-year projection made today is reasonably reliable for most of its span. A 20-year projection made today is a much rougher estimate by year fifteen, simply because more time has passed for the underlying assumptions to drift from reality.
Try It With Your Own Numbers
The fastest way to see whether this matters for your own situation is to actually run it. Start with the inflation calculator, or browse EvvyTools for the full set of free financial calculators covering everything from compound interest to debt payoff.
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