What it means
The problem it solves is that a dollar in one year and a dollar ten years later are not the same unit of measurement. Comparing them directly is like comparing a distance measured in metres with one measured in yards and pretending the numbers mean the same thing.
The conversion uses a price index, most often the consumer price index for household figures or a sector specific deflator for industry data. Every amount is scaled by the ratio between the index in the base year and the index in the year the amount was recorded.
Choosing the base year matters more than people expect. Any year can be the base, but comparisons are only meaningful when every figure in a table has been converted to the same base, and mixing bases quietly produces nonsense.
In business the technique exposes growth that is really just inflation. A sales line rising 5% a year during a period of 5% inflation is flat in constant dollars, which is a very different conversation with the board than a chart of rising nominal revenue.
The limit is that a general index reflects an average basket, not your basket. A construction firm facing steel and labour cost rises far above the consumer index will find a general deflator understates the squeeze on its real position.
In practice
Real-world examples.
Example
A government department reports that its budget rose from $400,000,000 to $480,000,000 over five years. Once restated in constant dollars against a 20% rise in the price index, the budget turns out to be flat, and the department's claim of extra investment does not survive the adjustment.
Example
A manufacturer compares average selling prices across fifteen years to decide whether it has been steadily giving away margin. Expressed in constant dollars, the price of its flagship product has fallen 18%, even though the sticker price has risen every single year.
Example
A pay negotiator prepares for talks by converting a decade of wage settlements into constant dollars. The exercise shows that nominal rises of 2% a year against 3% inflation left staff about 10% worse off in real terms, which reframes the whole discussion.
Formula
Calculation
Constant dollar amount = nominal amount x (price index in the base year / price index in the year of the amount)
A distributor had revenue of $8,000,000 in a base year when the consumer price index stood at 100. Ten years later revenue is $10,400,000 and the index has reached 130, meaning the same basket now costs 30% more.
Restating the later figure into base year dollars gives $10,400,000 x (100 / 130) = $8,000,000. Nominal revenue grew by ($10,400,000 - $8,000,000) / $8,000,000 = 30%, but in constant dollars the business is exactly where it started, with real growth of 0%.
Change one number and the story changes. Had revenue reached $11,700,000, the constant dollar figure would be $11,700,000 x (100 / 130) = $9,000,000, so real growth would be $9,000,000 / $8,000,000 - 1 = 12.5% across the decade, a little over 1% a year compounded.Case study
Seen in the real world.
Here is an illustrative and entirely fictional example. Pentworth Municipal Water, an invented utility, was accused by local councillors of overspending after its annual capital budget rose from $12,000,000 fifteen years earlier to $18,000,000, an apparent increase of 50%.
The finance team restated the figures in constant dollars using a construction cost index that had risen from 100 to 150 over the same period. The current budget converted to $18,000,000 x (100 / 150) = $12,000,000, precisely the level of fifteen years earlier, so in real terms nothing had been added at all.
The restated chart changed the debate entirely. Instead of arguing about whether spending had grown too fast, the fictional utility's board began discussing why a network fifteen years older was receiving exactly the same real investment as before.
Watch out
Common mistakes.
- Comparing figures from different years without adjusting them, then describing purely inflationary growth as real performance.
- Mixing base years inside a single table, which makes every comparison in it unreliable even though each individual conversion looked correct.
- Applying a general consumer price index to costs that behave nothing like consumer prices, such as energy, construction materials or specialist labour.
Questions
People also ask.
Is constant dollar the same thing as real terms?
Yes, the two phrases mean the same idea, with constant dollar more common in accounting and budgeting and real terms more common in economics.
Which index should be used?
Use the one that matches the thing being measured, so consumer prices for household spending, a construction index for building costs and a sector deflator for industry output.
Does the choice of base year change the growth rate?
No, the percentage change between two restated figures is the same whichever base year is used, though the absolute dollar amounts shown will differ.
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