Quote from Hurricane:
Let me give you some simple example that works for me, even though most of my trading involves cross currencies (i.e. don't involve USD).
+ Beginning year cash in foreign currencies (valued in USD) = $5000
+ Year end cash in foreign currencies (valued in USD) = $2000
+ Foreign currency trading gains (both cross trades and those involving USD) = X - You'll see in a moment that the value of X doesn't get directly input into the year end gain calculation.
+ Commissions on foreign currency trades = $1000 - This is always paid in USD at IB for a USD account.
+ Net purchases of USD = $25,000 - This would include gains from forex trades involving USD (e.g. buy/sell AUD.USD) and periodic purchases of USD to clear currency gains from forex trades not involving USD (e.g. buy/sell EUR.CHF).
+ NET P/L for tax reporting = Net purchases of USD - commissions - beginning year foreign cash + year end foreign cash = 25,000 -1000 - 5000 + 2000 = 21,000
It gets a bit more complicated if you have foreign dividends (you need to subtract these), foreign stock purchases (you need to add these in), foreign stock sales (you need to subtract these), open trades at year end, etc. But it will give you a very accurate figure that takes into account not only the P/L from your direct trades but also the P/L resulting from currency changes that occur from the time you close a trade until you convert the foreign currency into USD. And all of this data is available on IB's year end statement.
I do track each of my trades in Excel and use day end exchange rates to monitor my progress but the above eliminates any errors I might have made along the way and takes into account differences between day end and actual rates.
Comments/questions?