You do not say what state you are in, but if you are in a community property state, everything is considered to be 50/50.
If you are not in a community property state, your separate tax liability would be a ratio related to your income.
For example, let's pretend that you earned $100,000 and your spouse earned $50,000 for a total income of $150,000.
Your percentage of the income would be 100,000 divided by 150,000 or 67%, and your spouse's percentage of your total income would be 50,000 divided by 150,000 or 33%.
Therefore, if your tax bill was $10,000 then 67% or $6,700 is your share of the tax and 33% or $3,300 would be your spouse's share.
