Equity compensation (stock options, RSUs, ESPP) is one of the most rewarding parts of working at a tech company. It is also one of the most painful tax situations to manage when you are working across borders. Here is a worked example of how the math actually works when you vest equity over a period that crosses multiple countries.
The setup: a real-world scenario
Consider a software engineer named Anna with the following situation:
- Granted 4,000 RSUs by a US tech employer when hired in Country A (US)
- Vesting schedule: 25% per year over 4 years (1,000 RSUs/year)
- Year 1 vest: still in Country A (US tax resident)
- Year 2 vest: relocated mid-year to Country B (Spain)
- Year 3 vest: still in Country B (Spain)
- Year 4 vest: relocated mid-year to Country C (Portugal)
Each year, 1,000 RSUs vest. The market value of the stock at each vest is different. Each country has its own rules for when and how to tax the RSUs.
The fundamental problem
Different countries tax equity compensation at different points in time and with different methodologies. The US taxes RSUs at vest (income equal to fair market value on vesting date). Spain typically also taxes at vest, but applies different rules to determine taxable amount when the vesting period spans multiple residencies. Portugal has its own approach.
The result: the same vesting event can produce DIFFERENT taxable income amounts in different countries, and you may owe tax in MULTIPLE countries on the same vest.
The proportional allocation method
Most tax treaties use a proportional allocation when an equity grant vests across multiple residences. The principle: the income is sourced to each country based on the percentage of the vesting period spent in that country.
For Anna’s Year 2 vest (1,000 RSUs vesting after she relocated mid-year):
- Vesting period for those specific RSUs: 1 year (the year between Year 1 vest and Year 2 vest)
- Time spent in US during that period: ~6 months
- Time spent in Spain during that period: ~6 months
- Allocation: 50% taxable in US, 50% taxable in Spain
Each country gets proportional taxing rights. The treaty foreign tax credit mechanism then prevents double tax — you pay tax in Country A on its allocation, claim credit for that against Country B’s tax on the same allocation, and vice versa.
The mistakes that catch people out
Three things I see go wrong repeatedly:
- Default employer reporting assumes single-country. Your US employer’s tax department reports your full vest as US-sourced income on your W-2. That is technically incorrect if you are no longer US tax resident — but the W-2 reflects what they think happened. You have to manually correct this on your tax returns by claiming the correct sourcing.
- Foreign country may demand the entire vest be taxed locally. Spain’s default position is often to tax the full vest as Spanish income if you were Spanish tax resident at vest, ignoring the US sourcing. You have to actively invoke the treaty and proportional allocation.
- Currency conversion timing differs. The US uses USD natively. Spain converts to EUR at a specific date (often vesting date official rate). Even if your taxable income is theoretically the same in both countries, the converted amounts can differ by 5-15% depending on exchange rate timing.
The practical math for Anna
Assume each year’s 1,000 RSUs vest at $100 fair market value (so $100,000 taxable income per year). Anna’s total US tax bracket is 32% federal + 5% state. Spain’s effective rate (assuming Beckham Law applies) is 24% on Spanish-source income up to threshold. Portugal IFICI rate would depend on her qualifying activity.
For Year 2 vest with 50/50 US/Spain allocation:
- $50,000 US-sourced: US tax owed $18,500. Spain treats this as treaty-exempt (foreign source).
- $50,000 Spain-sourced: Spain tax owed $12,000. US gives FTC for Spain tax paid.
- Total: $30,500 ($50k US allocated $18,500 + $50k Spain allocated $12,000)
If she failed to invoke proportional allocation and let each country tax the full vest: she would pay roughly 32% on full $100k in US ($32k) AND 24% on full $100k in Spain ($24k) = $56k total. The proportional allocation saves her about $25,000 on this single vest.
How to actually model this
For a multi-year, multi-country vesting schedule, the calculation gets complex fast. The approach I use (and that I model through Taixable as part of my advisory work there):
- List every vesting event with its grant date, vest date, and FMV at vest
- For each vest, determine the vesting period (date from grant to vest, or last vest to current vest for graded vests)
- Determine days in each country during that vesting period
- Allocate the vest income proportionally
- Apply each country’s tax to its allocation
- Apply treaty foreign tax credits to prevent double taxation
Spreadsheet works for simple cases. For 4-year vesting schedules across 3+ countries, the complexity quickly justifies a tool. Either way, the principle is the same: every vest needs sourcing, and the sourcing follows the proportional rule unless treaty or local law specifies otherwise.
What employers will not do for you
Your employer’s payroll and tax reporting will NOT do the proportional allocation for you. They will report based on their default systems — typically all in the country where the equity was granted. You have to manually adjust on your tax returns and provide documentation showing why the sourcing differs.
This is something to do BEFORE you relocate, not after. Talk to your employer’s stock plan administrator about how relocation will affect reporting. Get any letters or documentation that confirm your time-in-country during vesting periods.
Bottom line
Cross-border equity compensation is one of the highest-value-at-stake parts of expat tax planning. Get the proportional allocation right and you can save 20-40% on a single vest. Get it wrong and you are leaving real money with multiple tax authorities. Document your time in each country during each vesting period, manually allocate income on your tax returns, and use a tool or spreadsheet that can model the math correctly. Disclosure: I am a business advisor for Taixable, mentioned above as one example of a tool that handles multi-country equity comp modeling. There are others; the methodology matters more than the specific tool.