If you have some post tax money in there; go to the vanguard site, manage my money, loans and withdrawals (I think). Put in for a withdrawal and it will show you the estimated tax you will pay before you execute it. You may be surprised at how little the tax actually is, but either way you can make a good guess on the tax you will pay to convert it now. Yes I convert some every year, the year over year gains aren't that much.