41 C.F.R. § 301-11.20 – Meals and incidental expenses (M&IE) reimbursement amounts.

(a) Except as provided in paragraph (b) of this section, when travel is more than 12 but less than 24 hours, employees receive a per diem allowance of 75 percent of the applicable M&IE rate for each calendar day they are in a travel status. If their travel is 24 hours or more, on the first day of departure and last day of travel, they receive 75 percent of the applicable M&IE rate. Full days of travel are reimbursed at 100 percent of the applicable M&IE rate.
(b) For travel by ship, whether commercial or Government, the agency will determine an appropriate rate within the applicable M&IE rate.