What it means
Under a clean float, supply and demand set the exchange rate and the central bank stays out of the market entirely. Under a dirty float, the central bank keeps the option to buy or sell its own currency when it judges that moves have become disorderly, too rapid or damaging to inflation and to companies with foreign currency debt.
The attraction is that it offers some of the shock absorption of a floating rate while limiting the volatility that makes business planning difficult. A manufacturer with imported inputs can live with a currency that drifts 5% over a year, but a 15% move in a fortnight can destroy a season's margin before prices can be adjusted.
Intervention works mechanically. To slow a depreciation, the central bank sells foreign currency reserves and buys its own currency, which supports the rate but drains reserves.
To slow an appreciation it does the reverse, buying foreign currency and issuing domestic currency, which is easier to sustain but can add to inflation unless the effect is offset by other operations. The critical nuance is that reserves are finite.
A central bank defending a level the market genuinely disagrees with will eventually run out of ammunition, and traders can calculate roughly how long that will take, which invites speculative pressure. Successful managed floats therefore lean against short-term disorder rather than trying to defend a particular number.
For businesses the practical implication is that a dirty float provides less protection than it appears to. Intervention smooths the path but does not remove the destination, so companies with foreign currency exposure still need hedges rather than faith in the central bank.
In practice
Real-world examples.
Example
An exporter of processed foods budgets its next season assuming the currency will drift no more than 4%, because the central bank has intervened twice in the past year to slow sharper moves. The treasurer still hedges 70% of expected foreign currency receipts, because managed does not mean fixed.
Example
A commercial bank's trading desk notices unusually large local currency buying in the market late in the session, at a level the currency has bounced off three times. The desk concludes the central bank is smoothing rather than defending, and adjusts its overnight position accordingly.
Example
A mining company with dollar revenue and local currency costs finds that central bank intervention to prevent appreciation has quietly improved its margins. It flags to its board that this benefit is a policy choice rather than an operational gain, and should not be built into long-term forecasts.
Formula
Calculation
Percentage currency move = (new rate - old rate) / old rate
Weeks of intervention sustainable = usable foreign exchange reserves / intervention per week
A central bank operating a dirty float sees its currency weaken from 15.00 to 15.30 per US dollar in a single week. The move is (15.30 - 15.00) / 15.00 = 0.30 / 15.00 = 2.0% in one week, which annualised would be a far larger depreciation than the bank considers orderly.
The bank decides to intervene by selling $600 million of reserves per week to buy local currency. Usable reserves stand at $36 billion.
At that rate, intervention could be sustained for $36,000 million / $600 million = 60 weeks, or roughly fourteen months, before usable reserves were exhausted.
If the pressure intensifies and weekly intervention rises to $1.5 billion, the same reserves last $36,000 million / $1,500 million = 24 weeks. That arithmetic is exactly what currency traders perform, and it explains why a central bank that signals a specific defended level often attracts more pressure rather than less.Case study
Seen in the real world.
Vantera Beverages is a fictional bottling company presented here as an illustrative example. It operated in a country with a long-standing dirty float and had never hedged its dollar-denominated concentrate purchases, on the reasoning that the central bank had kept the currency within a narrow band for six consecutive years.
When a commodity price shock hit the country's main export, the central bank spent heavily to slow the depreciation, but ultimately allowed the currency to fall by around a quarter over four months. Vantera's input costs in local currency rose accordingly, and because retail prices were contested by supermarket chains, it recovered only part of the increase in the first year.
The illustrative lesson the finance director drew was that intervention had changed the speed of the move but not its direction. The company adopted a rolling twelve-month hedging programme covering 60% of forecast dollar purchases, accepting a modest cost in normal years in exchange for surviving the abnormal ones.
Watch out
Common mistakes.
- Treating a dirty float as effectively a peg. Intervention manages the pace of change, and a currency under a managed float can still move a long way over a year.
- Assuming intervention always defends a weakening currency. Central banks intervene against appreciation too, particularly where exporters are politically important.
- Ignoring headline reserve figures without checking how much is actually usable. Reserves committed to swap lines or already sold forward are not available for intervention, so the true firepower is often smaller than the published number.
Questions
People also ask.
What is the difference between a dirty float and a clean float?
Under a clean float the central bank does not intervene at all, whereas under a dirty float it retains and uses the option to buy or sell its currency.
Does intervention actually work?
It can slow disorderly moves and buy time, but sustained intervention against a fundamental trend usually fails once reserves become visibly finite.
Should a business hedge if its currency is managed?
Yes, because managed floats reduce day-to-day volatility without preventing large cumulative moves, and hedging decisions should reflect the size of the exposure rather than recent calm.
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