Investing in Foreign Currencies? How the Capital Gains Tax Becomes a Mathematical Nightmare (and How to Solve It)

Introduction

Since January 1, 2026, Belgium has imposed a 10% capital gains tax on realized gains from financial assets. At first glance, it sounds straightforward: you sell a stock at a profit, you pay 10% on that profit. But for the hundreds of thousands of Belgian investors who trade American stocks, British funds, or other foreign-currency investments through international brokers like Interactive Brokers, DEGIRO, or Trade Republic, the reality behind that simple rule is enormously complex.

The problem? Every movement on your foreign currency cash account is potentially a taxable event. Not just when you sell stocks, but also when you buy stocks, receive dividends, or even when your broker deducts the capital gains tax itself from your dollar account. The Belgian tax authorities treat your USD cash account as a financial instrument in its own right, and exchange rate fluctuations on that account are therefore taxable.

The Belgian financial newspaper De Tijd recently put it aptly: "Investing in other currencies threatens to become higher mathematics due to the capital gains tax." And that is no exaggeration. In this article, we walk you through the complexity step by step, provide concrete calculation examples, and show how our Belgian Tax Calculator handles these calculations automatically for you.

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Table of Contents

1. Why Currency Is So Complex for the Capital Gains Tax
2. Foreign Currency Cash Accounts: The Hidden Problem
3. The ECB Exchange Rate: Which Rate Counts When?
4. The FIFO Principle for Currency: Tracking Baskets
5. Complete Example: From Purchase to Settlement
6. The December 31, 2025 Snapshot Rule for Currency
7. Three Strategies to Manage the Complexity
8. How the Belgian Tax Calculator Handles This Automatically
9. Frequently Asked Questions
10. Official Sources

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1. Why Currency Is So Complex for the Capital Gains Tax

The Basic Principle

The 10% capital gains tax applies to realized gains from the sale of financial instruments. Simple enough so far. But the law also specifies that foreign currency held in cash accounts linked to securities accounts is treated as a financial instrument.

This concretely means that there are not one, but two layers of capital gains calculation when you invest in foreign currencies:

Layer 1: The capital gain on the stock itself. You buy Apple at $200 and sell at $220. The gain is calculated in euro, based on the ECB exchange rate on the purchase and sale dates. This is the part most investors expect.

Layer 2: The capital gain on the currency. Every time dollars move to or from your USD cash account, you potentially realize an exchange rate gain (or loss). This is the part most investors do not expect, and it makes the calculation exponentially more complex.

Source: "The problem is that all movements on those cash accounts also fall under the capital gains tax if you realize a gain through exchange rate movements." (De Tijd, February 12, 2026)

⚠️ Note: 10% or 33%? The examples in this article assume the standard rate of 10% for normal asset management. However, if the Belgian tax authorities determine that you are actively trading in foreign currencies, your exchange rate gains may be classified as speculative. In that case, a rate of 33% with no annual exemption applies.

Why your bank won't do this for you

A crucial point: your bank or broker does not calculate these currency gains automatically. Unlike the sale of stocks, where Belgian brokers can automatically withhold the 10%, exchange rate gains on cash accounts are entirely your own responsibility.

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2. Foreign Currency Cash Accounts: The Hidden Problem

Many investors who use Interactive Brokers, DEGIRO, or other international brokers have a USD cash account alongside their EUR account. These accounts offer practical advantages: you can easily and cheaply buy American stocks without your broker charging exchange fees each time.

The problem: every movement is a potential taxable event

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3. The ECB Exchange Rate: Which Rate Counts When?

For tax calculations in Belgium, you must always use the ECB reference rate. This is the official exchange rate published daily by the European Central Bank around 4:00 PM CET.

Weekends and holidays? The ECB does not publish rates on weekends and holidays. In that case, you use the rate from the most recent business day before.

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4. The FIFO Principle for Currency: Tracking Baskets

FIFO stands for First In, First Out: when you "sell" currency, the first acquired dollars are considered sold first. Every incoming flow to your cash account forms a separate "basket" with its own purchase value.

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5. Complete Example: From Purchase to Settlement

Starting position

On December 31, 2025, there is $5,000 in your USD cash account at your broker. On that date, $5,000 = €4,500 (exchange rate: 1 USD = 0.90 EUR).

Summary

Of the total tax of €57.61, only €44.40 is automatically withheld by your bank. The remaining €13.21 you must calculate and declare entirely yourself.

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Tired of the calculations? The Belgian Tax Calculator does all these calculations automatically.

👉 Try the Belgian Tax Calculator for free | TOB calculation is completely free, including CSV import and PDF export.

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6. The December 31, 2025 Snapshot Rule for Currency

Gains realized before January 1, 2026 remain tax-free. For currency, the balance on December 31, 2025 is recorded with the EUR/USD rate on that date. This is your reference point.

The "purchase value" is the maximum of the original cost and the value on 12/31/2025.

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7. Three Strategies to Manage the Complexity

Strategy 1: Don't maintain a USD cash account

You work exclusively from your EUR account.

Strategy 2: Keep a USD cash account and calculate yourself

You maintain a USD cash account, benefit from lower transaction costs, but track all exchange rate gains yourself.

Strategy 3: Automate it with a tool

You import your transaction history from your broker, and a specialized tool does all the calculations for you.

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8. How the Belgian Tax Calculator Handles This Automatically

Full currency support

• Automatic ECB rates for every transaction
• FIFO engine with dual tracking of stocks and currencies
• Hybrid cost basis calculation (weighted average before 2026, FIFO after)
• Broker import from Interactive Brokers, DEGIRO, Trade Republic, Bolero, and more

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9. Frequently Asked Questions

Is every dollar account taxable? No. Only cash accounts linked to a securities account.

Can I deduct currency losses? Yes, in the same tax year.

Does the €10,000 exemption also apply to exchange rate gains? Yes.

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10. Official Sources

• FPS Finance: financien.belgium.be
• ECB Reference Rates: ecb.europa.eu
• De Tijd: tijd.be

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Conclusion

The 10% capital gains tax sounds simple, but for investors who trade in foreign currencies, the reality is far from straightforward. The Belgian Tax Calculator is designed to handle exactly this complexity.

Smart investing in 2026 starts with smart administration.

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Disclaimer: This article is intended solely for informational and educational purposes. It does not constitute legal or tax advice. Always consult a qualified tax advisor for advice on your specific situation.

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