The following is information about the financial aspects regarding witnesses. Under 28 U.S.C. § 1821, a $40 per day attendance fee is established for a witness in a court of the United States. It also entitles a witness to reimbursement of travel and subsistence expenses in the same amounts payable to a federal government employee for official travel. You may view the complete statute at https://www.law.cornell.edu/uscode/text/28/1821.
The $40 attendance fee is a daily fee. If attendance is required for multiple days, the witness is entitled to the attendance fee for each day their appearance is required.
In addition to the attendance fee, a witness is also entitled to receive compensation for their travel. Title 5 United States Code § 5704 authorizes a mileage fee equal to the mileage allowance which the Administrator of General Services has prescribed. Current mileage rates can be obtained from the United States General Services Administration website at https://www.gsa.gov/mileage.
In addition to mileage, a witness is also entitled to reimbursement of any parking fees incurred.