The yearly average is a convenience, not the rule

Report in dollars. The IRS states that you must express amounts on a US return in US dollars, and that the dollar is your functional currency unless a business unit requires otherwise. Everything else follows from that.

The one at the time of the item. The guidance says that where the dollar is your functional currency, you must immediately translate into dollars all items of income and expense using the rate prevailing when you receive, pay or accrue them.

That is a transaction-by-transaction default rather than an annual one. It matters most for single events: a bonus, a property sale, a pension payment.

The same page adds a tie-breaker. Where more than one rate exists, use the one that most properly reflects your income.

It comes from a table the IRS publishes as a convenience. The yearly average currency exchange rates page explains the arithmetic: divide the foreign amount by the average rate to reach dollars.

Crucially, the same page points to other options. It refers readers to government and external sources, or to any other posted exchange rate used consistently.

So the average is permitted rather than mandated. It suits a salary paid evenly through the year and suits a one-off disposal far less.

Not necessarily, and that surprises people. The FBAR is a FinCEN filing rather than an IRS one, so its own instructions govern how to convert a maximum account balance. Because the two forms answer to different authorities, one year can properly use different methods for income and for balances.

So a year can legitimately use one approach for income and another for account balances. The two filings answer to different authorities.

Read the current instructions each year rather than trusting a summary, including this one. That is the only method that survives a change in guidance.

Because the guidance asks for it explicitly, and because a reviewer can see it. A file that uses the yearly average for salary every year is coherent. One that switches sources whenever the rate moves favourably is not.

Consistency also protects the foreign tax credit. Income and the tax paid on it should be converted on the same basis, or the credit stops matching the income it relieves.

In practice we fix the method in writing at the start of a client relationship. It takes one line and removes an argument that would otherwise recur annually.

Currency movement can create income in its own right. Repaying a foreign mortgage is the classic case, because the dollar value of the debt moves between drawdown and repayment.

That is a separate rule from the translation question, and it catches people who never thought of themselves as currency traders.

Our article on foreign currency mortgage gains works through how that arises and when it bites.

Personal transactions have their own small carve-out, and business ones do not. Either way, the point is that currency movement is capable of producing a taxable amount on its own, separately from whatever the underlying asset did.

Pay in dollars. The IRS notes that tax payments must reach it in US dollars, and that a foreign currency payment converts at the rate the processing bank uses on the day it converts.

That detail matters for anyone paying from a UK account. The rate you calculated the liability on is not the rate that will settle it.

Leave a margin, or pay from a dollar account where you have one.

The IRS names governmental resources including the Treasury Department's rate and the Federal Reserve Bank, alongside external commercial sources, and Publication 54 covers the wider position for citizens abroad. That list is unusually permissive.

The requirement attaches to the use rather than the source. A commercial rate applied consistently is acceptable; a hand-picked rate chosen after the fact is not.

Keep a note of which source you used and a copy of the figures. Screenshots age better than memory.

Foreign tax paid converts too, and the timing question repeats on Form 1116. Tax deducted through PAYE across the year fits an average; a balancing payment in January does not.

Because Britain runs to 5 April and America to 31 December, the two systems rarely line up neatly anyway. Converting each payment on its own date makes the reconciliation easier, not harder.

Our comparison of the exclusion and the foreign tax credit covers the wider choice between the two reliefs.

Then the average starts to distort things, sometimes badly. Consider a contractor who invoices sporadically, or an employee whose bonus dwarfs the salary. Averaging a year in which most of the money arrived in one quarter produces a figure that never existed at any point during that year.

Instead, convert the large items separately and average only the rest. Nothing in the guidance requires a single method for everything.

Similarly, a year with a move in it splits naturally. Income earned before and after a relocation often deserves separate treatment anyway.

Three things make a conversion defensible later. First, the source you used. Second, the date you applied. Third, a copy of the published figure on that date.

Because published rates move and websites change, a screenshot is worth more than a link. Additionally, a short note explaining why a particular item used a spot rate saves reconstructing the logic years later.

Typically we keep a single sheet per year with one row per converted item. It has never taken more than twenty minutes to maintain.

Consequently, a query about a five-year-old return becomes a filing exercise rather than an investigation.

Almost everything about the calculation. Before the move your income was probably in dollars, and afterwards it is in pounds, so a single yearly average makes little sense across the boundary.

Furthermore, the move usually brings one-off amounts with it: a final US paycheque, a relocation payment, a first UK salary part way through a month.

In short, treat the year of arrival as two periods rather than one. Then apply the ordinary method to each of them.

Half an hour now saves an afternoon every April.

Take an illustrative year. An American in Leeds earns £64,000 through PAYE and sells a flat in June, realising a gain in pounds.

The salary converts sensibly at the yearly average, because it arrived in twelve roughly equal pieces. The disposal does not, since a gain depends on the acquisition and disposal dates rather than on the year as a whole.

Using one method for both would misstate the gain, sometimes by thousands. The figures are illustrative, though the mismatch is entirely real.

Sometimes more than people assume. A gain calculated in pounds and a gain calculated in dollars are different numbers, because the currency moved between purchase and sale. That is not an error in the arithmetic; it is the US measuring your position in its own currency throughout.

Consequently a property that barely rose in pounds can show a dollar gain, or the reverse.

The same effect reaches salary in a volatile year. Nothing about your work changed, yet the dollar figure did.

Rarely anyone, until something else prompts a look. A large refund claim, an amended return or a query about the foreign tax credit will all surface the conversion method as a side effect.

That is why the method matters more than the rate. An explicable approach applied consistently answers the question in one sentence.

Our clients who kept a conversion sheet have never spent more than an email on it. The rest rebuild a year from bank statements and published tables.

A sale of a home or a pension drawdown tends to prompt the look as well, simply because the amounts are larger than anything else on the return that year.

First, treating the yearly average as compulsory. Second, converting income and the tax paid on it by different methods. Third, changing source each year and keeping no record of why.

Fourth, using the year-end rate for everything, which is neither the default nor the published average.

Fifth, forgetting that the FBAR follows its own instructions. Our clients who separate the two filings in their heads make far fewer errors.

Sixth, converting a whole UK tax year as though it were a US one. The two years overlap by nine months and differ by three, so a single figure lifted from a P60 rarely belongs on a US return without adjustment.

We set the method once, apply it across every year we prepare, and document it in the file. That removes a recurring decision and makes each year comparable with the last.

Our US federal return service covers the conversions, the credit matching and the FBAR alongside. Where a single large transaction sits in the year, we convert it on its own dates rather than folding it into an average.

If you have already filed on a mixed basis, send us the returns and we will tell you whether it needs correcting or simply tidying from here.