Reading the code100MXL025 is a 10.0″ pitch length belt (the first number is the length in tenths of an inch), MXL pitch, 1/4″ wide. The last three digits are the width in hundredths of an inch. Common widths are 012 (1/8″), 019 (3/16″) and 025 (1/4″).
Counting teethPitch length divided by the pitch gives the tooth count. A 10.0″ MXL belt has 125 teeth. If the code has worn off, counting the teeth is the most reliable way to get the length.
MXL or 2M?MXL has a 2.032mm pitch and trapezoidal teeth. A 2M belt is 2mm pitch with round teeth. They look close but won’t mesh properly with each other’s pulleys, so check before you swap.