Reading the code210XL037 is a 21.0″ pitch length belt (the first number is the length in tenths of an inch), XL pitch, 3/8″ wide. The last three digits are the width in hundredths of an inch. Common widths are 025 (1/4″), 031 (5/16″) and 037 (3/8″).
Counting teethPitch length divided by the pitch gives the tooth count. A 21.0″ XL belt has 105 teeth. If the code has worn off, counting the teeth is the most reliable way to get the length.
XL or T5?An XL belt has a 5.08mm pitch; a T5 or 5M belt is exactly 5mm. Over a long belt that small difference adds up, so they can’t be mixed. Count the teeth and measure over ten pitches to be sure.