If you are single with no dependents, not able to be claimed as a dependent by someone else, and meet income requirements, and did not receive the first or second payment, then your refund would be $1800.
Your $1800 refund is the total of your two stimulus payments. TurboTax starts you with $1800 and then takes away the amount of stimulus payments that you have already received.
Since you did not receive any stimulus payments, your refund stayed at $1800. Have you received only the first payment, then your refund would have dropped to $600. If you had received the first and second refund payments, your refund would have dropped to zero.
