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Inflation risk

Inflation risk is uncertainty about future purchasing power. Compare nominal gains with the exact basket-adjusted return and see why fixed dollars can buy less.

Inflation risk is the uncertainty that rising prices will reduce the purchasing power of money, investment proceeds or promised payments. A portfolio can gain in dollars while falling short of the goods and services it needs to fund. For a fixed nominal payment, unexpectedly high inflation means less purchasing power than the recipient planned for.
The exposure is to a price basket over a particular horizon. Inflation in a broad consumer index is not necessarily the increase in an individual household's spending costs. The RBA's explanation of inflation measurement describes both the CPI basket and this coverage limitation.

Count what the money can buy

Consider an original, synthetic one-year example in USD. There are no taxes, fees, contributions or withdrawals. Wealth grows from USD 100 to USD 110, a 10% nominal return. Over the same year, an index for a fixed-composition consumption basket rises from 100 to 105, or 5% inflation.
To make the units tangible, let one basket unit cost USD 1 initially and USD 1.05 at year-end. Starting wealth buys 100 basket units. The ending balance buys
110 USD1.05 USD per basket unit=104.7619… basket units.\frac{110\text{ USD}}{1.05\text{ USD per basket unit}} =104.7619\ldots\text{ basket units}.
Purchasing power has grown, but by less than the dollar balance. Its exact return is
rreal=104.7619…100−1=1.101.05−1=121≈4.7619%.r_{\mathrm{real}} =\frac{104.7619\ldots}{100}-1 =\frac{1.10}{1.05}-1 =\frac{1}{21} \approx4.7619\%.
An unchanged USD 100 balance instead buys 95.2381 basket units, rounded: it loses purchasing power without losing any nominal dollars.
Synthetic one-year purchasing-power scenarios. With 5% inflation, USD 110 buys 104.7619 basket units and unchanged USD 100 buys 95.2381. Starting wealth buys 100 units. The 10% nominal-growth scenario breaks even in purchasing power when inflation is 10%.
Synthetic one-year purchasing-power scenarios. With 5% inflation, USD 110 buys 104.7619 basket units and unchanged USD 100 buys 95.2381. Starting wealth buys 100 units. The 10% nominal-growth scenario breaks even in purchasing power when inflation is 10%.
Original synthetic scenarios for one year, the same currency and the same fixed-composition basket. Both start with USD 100 and a basket price of USD 1. Curves vary inflation while holding each nominal ending balance fixed; filled points mark the 5% case. No inflation forecast or probability is implied.

Divide growth factors before subtracting one

For starting and ending nominal wealth W0,W1W_0,W_1 and positive basket-price index levels I0,I1I_0,I_1, the purchasing-power growth factor is

1+rreal=W1/W0I1/I0=1+rnominal1+π,1+r_{\mathrm{real}} =\frac{W_1/W_0}{I_1/I_0} =\frac{1+r_{\mathrm{nominal}}}{1+\pi},
where π=I1/I0−1\pi=I_1/I_0-1 is inflation over the same period. This wealth-ratio calculation assumes no external cash flows; otherwise, first measure an appropriate nominal investment return. Use a total-return balance, including distributions consistently, rather than comparing a price-only gain with a spending objective.

Rearranging gives

rreal=rnominal−π1+π.r_{\mathrm{real}} =\frac{r_{\mathrm{nominal}}-\pi}{1+\pi}.
Simply subtracting inflation is an approximation. In the example, 10%−5%=5%10\%-5\%=5\% overstates the exact real return by about 0.2381 percentage points. With a positive price level, the exact formula also makes the break-even condition transparent: nominal return equals inflation. The figure's 10% nominal-growth balance preserves its initial purchasing power at 10% inflation.

Erosion, surprise and the hedge you actually need

Realised inflation describes a price change that has occurred; inflation risk concerns uncertainty before it is known. Even an anticipated price increase erodes a fixed nominal balance. The additional planning problem is that realised inflation may exceed the assumption used to set savings, payments or required returns. The RBA explains why higher-than-expected inflation reduces a lender's planned purchasing-power return.
Fixed nominal bond payments illustrate the exposure: receiving every promised dollar does not guarantee the planned real spending power. That is distinct from credit risk, where the promised payment may not arrive, and from a bond's market-price loss when yields change. The Bonds and Yields lesson develops the cash-flow, price and yield distinctions; its full lesson requires membership, with a public preview available.
Indexation can connect a payment to a specified inflation measure. For example, U.S. Treasury Inflation-Protected Securities adjust principal with their specified consumer price index. That link is to the contract's index, rather than a promise to cover every holder's personal cost of living. Matching the spending basket, payment dates and holding horizon remains essential. Selling a bond before its payments are due also introduces market-price uncertainty; index-linked cash flows alone do not establish a fixed sale value.
The useful risk question is therefore concrete: how many units of the intended spending basket will the future proceeds buy, and how uncertain is that number?

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