Since everyone's tax situation is unique to them, I will leave the specific amount of the taxes you will owe the IRS to your tax preparer. As a general rule, pension plans are usually considered tax deferred money. If the funds you are referring are coming from a 401(k) or a lump sum pension, the amount that you move into a Roth would be considered taxable income for the year. You would have this as extra income for the year and most likely will have to pay more in taxes the year you do this. Once the funds are in the Roth you will have the opportunity to grow the money and pull the funds in the future without taxes since you have already paid the taxes when you put it into the Roth.
To correctly answer the question, we'll need to know what kind of pension you're talking about. Is it a 401K plan, a defined benefit plan, a defined contribution plan, whether you're vested, your age, etc.
William,. If you are going to roll qualified pre-tax retirement plan assets into a Roth, it is all considered taxable income. You can, however do a partial rollover, and have 2 separate accounts.