A reserve currency is a currency that monetary authorities hold through assets denominated in that currency as part of their official foreign-exchange reserves. For quant work, the useful question is how much reserve value is exposed to each currency, and why that value changes. The currency used to display a dataset does not answer that question.
Reserve use is one part of a currency's international role. Trade invoicing, cross-border payments, borrowing and use as an exchange-rate anchor are related but distinct activities. A country can invoice trade in a currency without pegging to it, and a private firm's foreign-currency deposit is not an official reserve holding. Deep markets, liquid assets and confidence in institutions can support international use; the label guarantees neither permanent dominance nor freedom from exchange-rate, credit or interest-rate risk. The Federal Reserve's review treats these roles separately.
Denomination survives conversion
Suppose a fictional monetary authority holds the following eligible reserve assets. Values have already been translated into millions of US dollars at the same valuation date:
| Underlying denomination | Value in the common reporting currency |
|---|---|
| US dollar | 60 |
| Euro | 25 |
| Other currencies combined | 15 |
| Total | 100 |
The dollar-denominated share is
Reporting the euro assets as USD 25 does not turn them into dollar-denominated assets. Denomination identifies the asset's currency; reporting currency supplies a common measuring unit. Nor does denomination by itself establish reserve eligibility: an asset must qualify for the official-reserves classification before it enters this portfolio.
Now hold every asset quantity fixed. Suppose exchange-rate translation alone raises the euro component's dollar value from 25 to 30, while the other components stay at 60 and 15. The portfolio becomes worth 105 and its dollar share becomes
The dollar share falls even though no dollar asset was sold. More generally, currency shares can change through transactions, exchange-rate revaluation, asset-price changes and changes in statistical coverage. A share chart alone cannot identify reserve managers' trading decisions.
Original fictional comparison. All bars use US dollars as the reporting unit; the segments retain their underlying currency denomination. Only the euro translation value changes, with quantities held fixed.
For conversion practice, the owned FX lesson explains currency-pair units and appreciation. It is free with sign-in.
Reading an actual reserve-currency dataset
The IMF's Currency Composition of Official Foreign Exchange Reserves (COFER) is a quarterly aggregate dataset. It reports values in US dollars using end-of-period market exchange rates, while retaining currency-of-denomination categories. Its coverage excludes monetary gold, SDR holdings and the reserve position in the IMF; these belong to broader official reserve assets. Individual-country COFER reports are confidential. See the dataset definition and FAQ.
Check the methodology vintage before comparing shares. Starting with 2025Q3, the IMF removed the separate “unallocated” component and revised the series back to 2000Q1. The revised currency composition covers the full foreign-exchange-reserve total using imputation where needed; the IMF also publishes the imputed share. A historical download's share of allocated reserves therefore need not have the same denominator or estimation basis as a current download. The November 2025 technical note explains the imputation. Save the release vintage and definitions with a backtest rather than treating the latest revised series as information available at every past date.
Sources
- Federal Reserve — The International Role of the U.S. Dollar, 2025 Edition: reserve holdings, exchange-rate anchors and transaction uses as separate dimensions.
- IMF — COFER dataset: coverage, quarterly reporting and the revised series.
- IMF — COFER frequently asked questions: conversion rates, confidentiality and reserve-asset boundaries.
- IMF — Improving the Analytical Usefulness of the IMF’s COFER Data: November 2025 methodology change and imputation.