To calculate an extended warranty refund, you need to first determine the remaining coverage period. This can usually be found in the terms and conditions of the warranty agreement. Next, you will need to calculate the pro-rata refund amount based on the remaining coverage period. This involves dividing the remaining coverage period by the total coverage period and multiplying it by the original warranty cost. It is recommended to carefully review the terms and conditions of the warranty agreement or contact the warranty provider for specific instructions on calculating the refund.