Put on work gloves to prevent acquiring splinters from the wooden deck-ledger board.
Hook the edge of measuring tape onto one end of the ledger board. Extend the measuring tape along the length of the ledger board until you reach the opposite end of the board from where the tape is hooked.
Hold the measuring tape against the ledger board. Line up the numbers on the measuring tape with the edge of the board, so that you can gather a precise measurement of the board.
Read the numbers on the measuring tape to determine the length of the ledger board. Measure the board to the nearest ⅛-inch to ensure you’re completely accurate.