FX gain decomposition calculator
How much of your offshore gain was the investment — and how much was just the rand?
A rand figure on an offshore holding blends two different things: the asset moving in its own currency, and the rand moving against that currency. This calculator splits your return into its local-currency return, its currency (FX) return, and the interaction term between them — four lines that reconcile to your total, so the parts genuinely add up. Everything is computed in your browser; your numbers never leave this page.
Your return % shows one number but hides two distinct return numbers.
How this is computed
Your gain is a product, not a sum
When you hold an offshore asset, its value in your own currency is two things multiplied together: how many units of the foreign currency the asset is worth, and how many rand each of those units converts to. Because value is a product, the return is too — and a product does not split into a tidy sum.
Write the local-currency return as R_local and the currency return as R_fx. Your total return in rand is:
(1 + R_base) = (1 + R_local) × (1 + R_fx)
R_base = R_local + R_fx + (R_local × R_fx)
└─ interaction term ─┘
That last bracket — the interaction term, sometimes called the cross term — is R_local × R_fx. It exists because the closing exchange rate applies to the grown asset value, not just the capital you started with. It is small most of the time and several percentage points when both the asset and the currency move a lot. It is real, and dropping it is the single most common reason a do-it-yourself split fails to reconcile. This calculator always shows it as its own line. This is the standard decomposition in the CFA curriculum on currency management.
Quote direction: the error that flips the sign
The rate must be quoted as reporting currency per one unit of the foreign currency — rand per dollar, not dollars per rand. With that convention, a rise in the rate means the rand weakened and your dollar asset is worth more rand, so your FX return is positive. Quote it the other way and every currency figure changes sign. The calculator fixes the direction and states it on screen and in your result — so you can't get it backwards.
WORKED EXAMPLE
You buy a US asset for $10 000 when the rate is R18.00 per dollar. The asset rises to $11 000 and the rate moves to R18.90.
Local return: 11 000 / 10 000 − 1 = +10.00%
Currency return: 18.90 / 18.00 − 1 = +5.00%
Interaction: 10% × 5% = +0.50%
Total in rand: 1.10 × 1.05 − 1 = +15.50%
In money: you began with R180 000 and ended with R207 900 — a gain of R27 900, made of R18 000 investment, R9 000 currency, and an R900 interaction. That R900 is the currency move applied to your $1 000 investment gain — which is exactly why it can't be assigned purely to the asset.
When you add money mid-period, it gets harder
If you contributed partway through, the tranches entered at different exchange rates, so there is no single FX return for the whole holding. The honest method tracks each contribution at its own entry rate and sums the rand gains — that always reconciles. The shortcut of differencing two blended (Modified Dietz) returns in two different currencies does not give a clean split, and the calculator warns you when your inputs fall into the case where that shortcut misleads. Enter contributions above and it switches to the tranche method automatically.
Common questions
After reading this section, if you still have questions, feel free to contact us however you want.
How do I separate currency gain from investment gain?
Compute the asset's return in its own currency, compute how the exchange rate moved, then combine them multiplicatively. Expanding the product gives three parts — local return, FX return, and an interaction term — that add up to your total. The calculator does all three and shows them reconciling.
Why is there an interaction term at all?
Because the closing rate converts your grown asset, not just your starting capital. The extra bit — the currency move applied to your investment gain — is the interaction term. It is small in calm periods and material in volatile ones, and it belongs to neither the asset nor the currency alone, which is why it gets its own line.
Should I fold the interaction term into the currency bucket?
You can, for a compact two-bucket "investment vs currency" view — and the calculator offers it. But fold it into currency, never into investment, because the interaction is generated by the currency move acting on your gain. The four-line view keeps it separate so the parts visibly add up.
Do my numbers leave my computer?
No. Every calculation runs in your browser. Nothing is uploaded, stored, or sent to a server — which also keeps it clean under POPIA.
Who built this
Rian Cronje comes to personal finance from the outside. After 25 years in corporate finance — Group Financial Controller roles, multi-currency consolidations and digital transformation, the unglamorous rigour of making a business's accounts actually reconcile — he found almost none of that discipline had reached the way individuals track their own wealth. He is not an advisor; he has nothing to sell you about where to put your money. He built Mintelo to close that gap: to hold a person's wealth to the standard a company holds its own books, and to break down the jargon that keeps capable people — him once included — locked out of their own numbers.
The methods shown here are standard, textbook financial calculations — implemented faithfully, with every intermediate figure shown so you can check the result yourself. This is an educational tool, not financial advice. Mintelo is a wealth-tracking product in development — join the waitlist to track your wealth across every account automatically.
See your real net worth across every account — in rand or dollar.
One reconciled view, the same number whichever way it's checked. Mintelo is the personal wealth platform built like a real one.