How do I calculate the tax liability if last year we filed joint and this year is separate?

If you file a separate return in 2021, but you filed a joint return with your spouse for 2020, your 2020 tax is your share of the tax on the joint return. You are filing a separate return if you file as single, head of household, or married filing separately.

To figure your share of the taxes on a joint return, first figure the tax both you and your spouse would have paid had you filed separate returns for 2020 using the same filing status as for 2021. 

Then multiply the tax on the actual joint return by the following fraction: (The tax you would have paid had you filed a separate return) divided by (The total tax you and your spouse would have paid had you both filed separate returns).

2020 tax you would have paid filing MFS                    $80

2020 tax your spouse would have paid filing MFS    $100

Total 2020 tax if both filed MFS                                     $180

$80          44%

$100        56%

$180        100%

2020 tax paid filing MFJ                    $150

                                                             X 44%

2020 tax presumed to be paid        $66