Phase 5 · Behavioral Magnets
Purchasing Power Decay Calculator
Cash doesn't lose value with a crash — it loses it quietly, a few percent a year, forever. See what your idle money will really be worth, and what it's costing you to sit still.
Under the hood
The math, fully exposed
We grow each scenario by its nominal rate, then deflate everything back into today's dollars:
Real value = nominal ÷ (1 + inflation)years
Mattress nominal = cash (no growth)
Savings nominal = cash × (1 + yield)years
Invested nominal = cash × (1 + return)years
Purchasing power lost = cash − mattress real value
Real yield = (1 + yield) ÷ (1 + inflation) − 1
- Nominal vs real: the dollar count on idle cash never falls, which is what makes the loss so easy to miss. In real, spendable terms it shrinks every single year.
- Beating inflation is the bar: if your savings yield is below inflation, your real yield is negative — you\'re losing slowly. Only a return above inflation grows real wealth.
- Not all cash is a mistake: an emergency fund and near-term money belong in cash regardless. This models the excess idle cash that has nowhere to be.
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Answers
Frequently asked questions
How does inflation erode the value of cash?
Inflation raises the price of goods, so the same dollar buys a little less each year. Cash sitting still keeps the same number on it but loses purchasing power: at 3.5% inflation, $100 buys about $96.50 worth a year later, and roughly $70 worth after a decade. The money didn't move — the goalposts did.
Isn't keeping my money in cash the safe choice?
It's safe in nominal terms — the balance can't fall — but it's a near-guaranteed loss in real terms whenever inflation outruns your interest rate. Cash protects you from market swings while quietly leaking value to inflation. The "safe" choice has a hidden, compounding cost.
What return do I need just to break even with inflation?
Your yield has to at least match the inflation rate to hold purchasing power steady. Earn less than inflation (most basic savings accounts, most of the time) and you're losing ground slowly; earn more and you're gaining real value. The break-even is simply the inflation rate itself.
So how much cash should I actually hold?
Enough to be safe, not so much that it bleeds. A common guideline is an emergency fund of 3–6 months of expenses plus any money you'll need within a few years — that belongs in cash or a high-yield savings account regardless of inflation. It's the excess idle cash, beyond what you need soon, that inflation punishes most.