On a Loop or Guideline rod’s spec sheet, you might see something like “#4 · 10 g”. Pick up an American fly line and you’ll find “WF4F” on the box, with a weight in grains somewhere in the small print. Converting the units is easy. The key is knowing which part of the line each weight refers to. Here’s how to read those figures and compare them with your rod manufacturer’s recommendation.

What the AFFTA rating tells you

The numbered line weight system (#1 to #15) was introduced in 1961 by the American manufacturers’ association, now AFFTA. The weight table AFFTA publishes today classifies lines by the weight of the first 30 ft (9.14 m), excluding the level tip. This is the short section of constant diameter at the very front of the line, where the leader attaches.

For the most common trout line sizes, the figures are:

Line size Nominal weight over 30 ft (grains) Accepted range (grains)
#3 100 94 to 106
#4 120 114 to 126
#5 140 134 to 146
#6 160 152 to 168

You can find these figures in the American Casting Association’s line standards appendix.

Many modern lines are sold as “half a size heavy”, meaning they weigh half a line size more than the nominal AFFTA figure. The extra weight can help load some rods or make casting easier with less line outside the tip.

What head weight tells you

Brands such as Guideline and Loop also recommend a line weight for their rods. Guideline calls this the recommended head weight, usually given in grams.

Head weight is the weight of the whole head, from the front tip to the start of the running line. The running line is the thin, level section behind the head. Head weight tells you how much line weight you’re carrying when the whole head is outside the rod tip.

The recommendation may be a range or a single figure. Here are a few examples for 9 ft #4 rods:

Rod Manufacturer’s recommendation
Loop Q #4 10 g / 155 grains
Guideline Elevation #4 9 to 11 g / 140 to 170 grains
Guideline NT11 Trout #4 10 to 12 g / 155 to 185 grains

These figures, including the rounding, are quoted as published by the manufacturers. Loop describes its figure as a line weight recommendation. Guideline specifically refers to head weight.

To convert between the two units: 1 g ≈ 15.43 grains.

Why the two weights differ

With a long head, the total head weight includes line beyond the section measured by AFFTA. If the head is shorter than that reference section, the 30 ft measurement includes some running line too. That is why the two weights aren’t interchangeable.

For example, take a line with a 12.4 m head weighing 152 grains (9.85 g). Its first 30 ft weigh 126 grains (8.16 g), which puts it in the AFFTA #4 range. But the whole head is still lighter than the 10 to 12 g recommended for some #4 rods.

The reverse can happen too. A line with a short head that is half a size heavy can weigh more over the first 30 ft than another line, yet have a lighter head overall. Which line is heavier depends on how much of it you compare. To understand how it loads a rod, you need to consider both the weight outside the rod tip and how that weight is distributed along the line.

How to match a line to your rod

  1. Start with the rod manufacturer’s recommended line weight. If it specifies head weight, compare it with the line’s head weight. The same rule applies whether the figures are given in grams or grains.

  2. Look up the line’s head weight as well as its size. Guideline, for example, publishes it for each line size. Its Fario CDC WF4F has a 12 m head weighing 10.5 g. Its Classic Trout WF4F also has a 12 m head, but weighs 12 g. Both are #4 lines from the same brand, yet their heads differ by 1.5 g.

  3. Check which part of the line each published weight covers. “Head weight” refers to the whole head. Check the head length too, along with any notes about the first 30 ft. A figure that falls within the AFFTA range doesn’t prove how it was measured. If the spec sheet is unclear, ask the manufacturer.

  4. If your rod only shows a line rating, such as #4, start with the AFFTA weight. Look at head length and weight too. As you carry more line beyond the rod tip, you’re casting more weight.

  5. Think about how much line you have outside the rod tip. With only a little line out, you’re casting part of the head. The full head weight only applies once the entire head is outside the tip. The distance to your fly isn’t the same as the amount of fly line outside the tip, because the rod and leader contribute to that distance too.

How we label our lines

On our packaging and website, we list head length and head weight alongside the AFFTA weight over the first 30 ft. We keep the two weights separate so you can compare them with your rod’s recommendation without having to guess.

We explain how we work with these weights in our development diary. To hear about our lines and their launch, sign up for updates.