If you contribute 5500 to a retirement plan and you are able to claim 10% as a tax credit how do you calculate howmuch on the initial contribution is eligible?

Yes, if you make contributions to a retirement plan then you might be able to get the Retirement Savings Contributions Credit (Saver’s Credit). It will use the first $2,000 you contribute during the year to a retirement account to calculate the credit.

If you know you qualify for the 10% then your credit should be $200. Please be aware, this is a nonrefundable credit and can only reduce the tax you owe to zero, but it can't provide you with a tax refund. TurboTax will automatically calculate the credit for you when you enter your contributions.

Please see What Is the Savers Credit? for additional information.