Proportional and risk-based copier sizing both read one number off each account: equity. That number carries a unit. A copier that divides follower equity by master equity without checking that both are in the same currency is dividing euros by dollars and printing the answer as a ratio. The error does not shrink as accounts get bigger. It is a units error, and its magnitude is the exchange rate itself.
When a trade copier sizes positions from account equity and the master and follower accounts are denominated in different currencies, comparing the raw equity numbers produces a sizing error equal to the exchange rate between those two currencies. A USD master with a EUR follower undersizes the follower; a EUR master with a USD follower oversizes it, and oversizing is the direction that breaches drawdown limits. A second, separate exposure survives even after equity is converted correctly, because an instrument such as an S&P 500 future settles its profit and loss in USD no matter what currency the account is denominated in.
Why does a currency mismatch break copier sizing?
Because equity is a number plus a unit, and the copier reads only the number. Proportional sizing reduces to follower equity divided by master equity, times the master's contract count. Risk-based sizing reduces to a risk budget divided by a per-contract stop cost. Both assume their inputs share a unit. Nothing inside either formula checks that they do.
Platform documentation is explicit about denomination, which makes the omission easy to spot once you look. MetaQuotes defines the account state fields in the MQL5 account properties reference as balance in the deposit currency, equity in the deposit currency, profit in the deposit currency, and margin used in the deposit currency, with account currency exposed as a separate string property. The API hands you the unit. It does not force any consumer of that data to honor it.
This is a configuration problem before it is a software problem. Prop-firm accounts inherit whatever denomination the firm chose, which is not always the trader's home currency and not always consistent across programs at the same firm. A trader running a personal USD account as master and two funded EUR accounts as followers has a mismatch on both legs, and typically no screen anywhere in the stack that says so.
The error factor is exactly the exchange rate
The error is not roughly the exchange rate. It is the exchange rate, exactly, with account size cancelling out. Write R for the number of follower-currency units per one master-currency unit. The correct ratio is (follower equity / R) divided by master equity. The naive ratio is follower equity divided by master equity. Divide naive by correct and both equity terms vanish, leaving R.
Take the standard case. The master holds 100,000 USD, the follower holds 100,000 EUR. The raw numbers match, so the copier computes 100,000 / 100,000 = 1.0 and copies size for size. At an illustrative 1 EUR = 1.10 USD the follower's equity is 110,000 USD and the correct ratio is 110,000 / 100,000 = 1.10. The copier therefore trades 1.0 / 1.10 = 0.909090 of intended size, a 9.0909% shortfall. Concretely: the master trades 10 ES contracts, the correct follower size is exactly 11, and the copier sends 10.
That 1.10 is a round figure chosen for readable arithmetic, not a live quote. The European Central Bank states on its euro reference rate page that the rates are usually updated at around 16:00 CET every working day except TARGET closing days, are published for information purposes only, and that using them for transaction purposes is strongly discouraged. Check the current rate, and expect the rate your broker actually applies to differ from any published reference.
Here is why the bug survives testing: the percentage error does not depend on account size. A 250,000 USD master with an 80,000 EUR follower gives a naive 0.32 against a correct 0.352, and 0.32 / 0.352 = 0.909090. A 125,000 USD master with a 37,500 EUR follower gives a naive 0.30 against a correct 0.33, and 0.30 / 0.33 = 0.909090. Identical. Change the account sizes all you like and the error never moves, because it was never a function of them.
| Master equity | Follower equity | Naive ratio | Correct ratio | Error factor | Effect on follower |
|---|---|---|---|---|---|
| 100,000 USD | 100,000 EUR | 1.000 | 1.100 | 0.909 | 9.09% undersize |
| 250,000 USD | 80,000 EUR | 0.320 | 0.352 | 0.909 | 9.09% undersize |
| 100,000 EUR | 100,000 USD | 1.000 | 0.909 | 1.100 | 10% oversize |
| 100,000 USD | 10,000,000 JPY | 100.00 | 0.667 | 150.0 | 150x oversize |
All four rows use illustrative round rates, and the last one is the same bug with the volume turned up. At an illustrative 150 JPY per USD, a JPY 10,000,000 follower is worth 66,666.67 USD, so the correct ratio is 0.6667 against a naive 100.0. The oversize factor is 150.0, which is the rate. Nobody ships that bug, because it detonates on the first trade. The EUR/USD version is the identical defect with the factor set to 1.10, and it degrades performance silently for months.
The error is not approximately the exchange rate. It is the exchange rate, and account size never enters the calculation.
The break has one specific location in the pipeline. Trace it: the master broker feed emits equity in USD, the follower broker feed emits equity in EUR, both flow into a ratio node, the ratio node feeds a lot-size node, and the lot-size node feeds the outbound order. A correct configuration inserts an FX conversion box on the follower leg between the feed and the ratio node, so the ratio node only ever sees one currency. A broken configuration has that box missing, and every number downstream of the ratio node is off by the exchange rate. The two setups differ by exactly one hop, which is why reading the config screen never reveals the problem: the missing element is not a wrong setting, it is an absent step.
Which direction breaches a drawdown limit?
The dangerous direction is a follower whose currency unit is worth less than the master's, because naive equality then oversizes it. USD master with EUR follower undersizes and costs performance. EUR master with USD follower oversizes and costs the account.
Reverse the earlier example. The master holds 100,000 EUR, worth 110,000 USD at 1.10. The follower holds 100,000 USD. The raw numbers match again, so the copier again sends size for size, but the correct ratio is 100,000 / 110,000 = 0.909090. The oversize factor is 1.0 / 0.909090 = 1.10, so the follower trades roughly 10% more than intended on every trade, permanently.
Ten percent sounds survivable until it lands on a hard limit. Say the follower is a 100,000 USD funded account with a 2,000 USD daily loss limit, the master trades 10 ES contracts, and the position takes a 4-point adverse move. ES is $50 per index point, so 4 points is $200 per contract. The intended follower size is 10 x 0.909090 = 9.0909 contracts, which a whole-lot copier rounds to 9, for a loss of 9 x 4 x 50 = $1,800.00. The uncorrected copy sends 10, for 10 x 4 x 50 = $2,000.00, landing exactly on the limit. Two hundred dollars of sizing that nobody configured and no log records separates an ordinary red day from a breach.
The second mechanism: the instrument's P&L currency
Converting equity correctly fixes the ratio and leaves a separate exposure untouched. The currency an instrument settles profit in is a different property from the currency the account is denominated in. MetaTrader exposes both as symbol fields, defined in the MetaTrader 5 symbol specification help as a profit currency in which the profit of the symbol trading is calculated and a margin currency in which the margin requirements are calculated, neither of which has to match the account's deposit currency. The margin leg carries its own FX, which is worth knowing when a euro account posts margin against a USD-margined contract.
CME-listed US index futures accrue profit and loss in USD regardless of where the account is booked. A EUR-denominated account trading ES generates USD gains and losses that pass through a conversion before they reach the equity figure. That conversion happens inside the platform whether the trader thinks about it or not, at a rate the trader did not choose.
Watch what that produces with the futures position frozen. A EUR account holds 5,000 USD of unrealized ES profit. At 1.10 that is 5,000 / 1.10 = EUR 4,545.45. The euro strengthens to 1.15 and the same 5,000 USD becomes 5,000 / 1.15 = EUR 4,347.83, a loss of EUR 197.63, or 4.3478%, which 1 minus (1.10 / 1.15) = 0.043478 confirms. The index did not move, the contract count did not change, no order was sent. Run it the other way: at 1.05 the position is worth 5,000 / 1.05 = EUR 4,761.90, a gain of EUR 216.45, or 4.7619%. The trader is long the dollar and never placed an FX trade. The SEC makes the general point on Investor.gov: when the exchange rate between the U.S. dollar and the currency of an international investment changes, it can increase or reduce your investment return.
Whether a flat account still carries this exposure is broker-dependent, so ask your broker directly. Some auto-convert realized profit into the account currency at the close, which ends the exposure there. Others hold a separate cash sub-balance in the instrument's currency until you convert manually, which leaves it open indefinitely, including overnight, over weekends, and while you are flat.
How a rate move eats drawdown budget with no trade open
If your firm computes drawdown in the account's denomination, an adverse rate move spends drawdown budget with no trading loss. Take a EUR account carrying a 20,000 USD cash sub-balance from realized ES profits the broker has not converted. At 1.10 that is EUR 18,181.82. At 1.15 it is EUR 17,391.30, a drop of EUR 790.51. Against a EUR 2,000 daily loss limit measured on equity, one rate move has consumed 39.53% of the day's budget before the trader places an order.
The reason it arrives unseen is structural. Follow the chain: an ES fill fires, the $50 per point multiplier turns index movement into a USD unrealized number, that USD number passes through an FX conversion node fed by the live rate, and the euro figure leaving that node is what the firm's drawdown gate reads. The conversion node sits between the futures P&L and the gate. So the rate input can move on its own, with the entire futures side of the chain frozen, and the gate reading still moves. A trader watching only the platform's P&L window is watching the upstream side of the conversion node.
Drawdown rules vary on three axes that all matter here: the currency the limit is computed in, whether it reads equity or closed balance, and whether it is evaluated intraday or at end of day. Treat these examples as mechanism, then read your own firm's rules to see which combination applies to you.
Risk-based sizing has the same hole in a different place
Switching from proportional to risk-based sizing relocates the problem rather than escaping it. The formula is contracts = risk amount / (stop points x point value). The risk amount is denominated in the account currency. The point value is denominated in the instrument's profit currency. If the account is EUR and the instrument is ES, that division mixes units and is invalid before it is inaccurate.
Worked: a EUR account with EUR 100,000 equity risks 2% on ES with a 4-point stop. The stop is worth 4 x 50 = $200 per contract. Done correctly, the EUR 2,000 budget converts once at 1.10 into $2,200, and $2,200 / $200 = 11 contracts. Treating the euro budget as if it were dollars gives 2,000 / 200 = 10 contracts. That is the same 9.09% undersize as the proportional example, produced by a different formula, which is why fixing one sizing path in a copier does not fix the other.
Convert the budget, not the point value. Compute the risk budget in the account currency, convert it once into the instrument's profit currency, then divide by stop points times point value in that same currency. Point value is a fixed contract constant that never needs touching, while the risk budget is the quantity you actually control, so putting the single conversion at the front keeps one moving part instead of two. If you are choosing between sizing models at all, the tradeoffs in copy trading position sizing methods sit on top of this, and the account-level framing in position sizing for funded accounts assumes the budget is already stated in the currency you think it is.
How to test your copier with a single trade
One live trade settles what documentation will not. Whether a copier converts equity before computing ratios is product-specific and often undocumented, but the two possible behaviors predict follower sizes that differ by exactly the exchange rate. Put a known equity gap between master and follower, send one small trade, and compare the filled follower size against both the raw ratio and the converted ratio. Only one will match.
Three things to establish first. Confirm what currency each account is actually denominated in by reading it from the platform rather than from the firm's website or your own bank. Ask your broker whether realized profit is auto-converted at close or parked in a currency sub-balance. Find out what conversion spread you are charged. At an illustrative 0.5% spread, converting 5,000 USD into a EUR account gives an effective rate of 1.10 x 1.005 = 1.1055, and 5,000 / 1.1055 = EUR 4,522.84 instead of the EUR 4,545.45 the mid rate implies, a cost of EUR 22.61. Spreads vary widely by broker and tier, so get your own number. The charge lands on gross converted flow rather than net profit, so it scales with turnover and hits high-frequency scalpers far harder than swing traders.
Check one more thing while you are in there: any manual multiplier already sitting on the follower. A multiplier a trader nudged to some odd non-round value months ago because it mysteriously worked may be an accidental FX correction. Adding a proper conversion on top without removing it applies the correction twice.
When this fix is not worth making
The cleanest fix is to delete the problem: run master and followers in the same denomination and none of the above applies. That is available more often than traders assume, since many firms offer USD-denominated programs and many brokers let you choose a deposit currency at account opening. Choose once, correctly, and skip the whole class of failure.
Where that is impossible, be honest about what a conversion-aware copier buys. It makes the sizing correct. It does not make the exposure disappear. A euro account trading dollar-settled futures holds an FX position with or without a copier, and no copier setting closes it. The setting accounts for the exposure; only a currency choice or a deliberate hedge removes it.
On small accounts, skip the precision. Contract rounding dominates the FX error at low size. A copier that truncates a 1.1 contract target down to 1 loses 9.09% on that order alone, and truncation can discard almost a full contract, so below roughly eleven contracts of intended size the worst-case rounding error matches or exceeds the 9.09% currency error. For a follower trading one or two contracts, fixed-lot mode with a size you chose deliberately is more honest than a precise ratio that rounding destroys on every order. Note too that this exposure attaches to the instrument's settlement currency rather than to the copier, so it looks different on CFD products whose contract terms are set by the broker rather than an exchange, which is one more line item in the futures versus CFD prop firm comparison.
Frequently asked questions
Does a trade copier automatically convert currencies between accounts?
Some do and some do not, and it is not safe to assume either way. Whether a copier converts equity into a common currency before computing the sizing ratio is a product-specific implementation detail that is often undocumented. Test it empirically: create a known equity gap, send one trade, and check whether the filled follower size matches the raw ratio or the converted ratio.
How big is the sizing error if my master is USD and my follower is EUR?
The error equals the exchange rate exactly, so at an illustrative 1 EUR = 1.10 USD the follower is undersized by 9.09%. A naive ratio of 1.0 against a correct ratio of 1.10 means the copier trades 0.909090 of intended size on every order. Account sizes cancel out of the math entirely, so the percentage is identical on a 25,000 account and a 250,000 account.
Which currency mismatch direction is dangerous for a funded account?
A follower denominated in the weaker currency unit is the dangerous direction, because naive equality oversizes it. A EUR master with a USD follower produces roughly 10% oversize at an illustrative 1.10 rate, and that extra size is what turns a normal losing trade into a daily-loss breach. The reverse pairing only costs performance.
Can an exchange rate move breach my drawdown limit with no position open?
Yes, if your account holds unconverted profit in a foreign currency and your firm computes limits in the account denomination. A EUR account holding a 20,000 USD sub-balance loses EUR 790.51 when the rate moves from 1.10 to 1.15, consuming 39.53% of a EUR 2,000 daily budget with no trade placed. Confirm with your broker whether realized profit is auto-converted at close or held in a currency sub-balance.
What is the difference between profit currency and deposit currency?
Profit currency is the currency an instrument settles its gains and losses in, and deposit currency is the currency the account itself is denominated in. MetaTrader exposes them as separate symbol and account properties because they frequently differ, for example a EUR account trading a USD-settled index future. The platform converts between them at a rate you did not select.
Does risk-based sizing avoid the currency problem?
No, it moves the problem into the point-value division. The formula divides a risk budget denominated in the account currency by a stop cost denominated in the instrument's profit currency, which is invalid unless one side is converted first. A EUR 2,000 budget on ES with a 4-point stop yields 11 contracts when converted correctly and 10 when the euro budget is treated as dollars.
Should I convert the risk budget or the point value?
Convert the risk budget, once, at the front of the calculation. Point value is a fixed contract constant that never changes, while the risk budget is the quantity you actively control, so converting it keeps a single moving part in the formula. Compute the budget in the account currency, convert it into the instrument's profit currency, then divide by stop points times point value.
Is it better to just open all accounts in the same currency?
Yes, matching denominations across master and followers is the cleanest fix and removes the entire class of problem. Many prop firms offer USD-denominated programs and many brokers let you pick a deposit currency at account opening. Where matching is impossible, a conversion-aware copier corrects the sizing but does not remove the underlying FX exposure.
Does the currency error matter on a small account?
No, contract rounding dominates the FX error at small size. Truncating a 1.1 contract target down to 1 is a 9.09% shortfall on its own, the same magnitude as the currency error at an illustrative 1.10 rate. Below roughly eleven contracts of intended size, fixed-lot sizing chosen deliberately is more honest than a precise ratio that rounding discards.
What conversion cost should I expect on realized profit?
Expect a broker-specific conversion spread charged on gross converted flow rather than on net profit, so verify your own broker's figure. At an illustrative 0.5% spread, converting 5,000 USD into a EUR account gives EUR 4,522.84 instead of the EUR 4,545.45 a 1.10 mid rate implies, a cost of EUR 22.61. Because the charge applies to every conversion, traders who convert on each close pay it on winners and losers alike.