Fly Line Weight Calculator

How it is calculated

The tool is a lookup, not a formula. Your line number must be a whole number and is matched against a fixed table of the standard weight of the first 30 ft of line, in grains:

1→60, 2→80, 3→100, 4→120, 5→140, 6→160, 7→185, 8→210, 9→240, 10→280, 11→330, 12→380

The gram figure is converted with grams = grains ÷ 15.432 (one grain is about 0.0648 g) and shown to two decimals. The tool also prints a manufacturing tolerance of ±6 grains. The table covers numbers 1 to 12 only; 0, anything above 12 and any fraction such as 5.5 return no result. The weight describes the head, the first 30 feet, not the full line or the whole spool.

Common mistakes

• Reading the grain weight as the weight of the whole line. It is only the first 30 feet; the running line behind it is not included.
• Typing a half size such as 5.5 and expecting a result for it. Line numbers are whole numbers, so the tool shows nothing until you enter 5 or 6.
• Entering the rod length or the tippet size (for example 5X) instead of the line weight number.
• Expecting numbers above 12. The table stops at 12, so 13 or 14 gives no result.
• Assuming two lines of the same number weigh exactly the same. The ±6 grain tolerance means measured weights differ a little between brands.