King Mackerel Weight Calculator

No scale on board? Two tape measurements get you a solid estimate range. This isn't the generic all-purpose fish formula most calculators default to — it's calibrated against real weighed king mackerel, covered below.

How to measure

Fork length — tip of the nose (mouth closed) to the fork of the tail, in a straight line, not over the curve of the body. This is the standard measurement point for king mackerel, and it's also what North Carolina's citation program uses, covered below.

Girth — wrap the tape around the fish at its widest point, usually just behind the pectoral fins, not the middle of the body. Snug the tape without compressing the fish.

How this is figured

The formula is Weight (lb) = Length (in) × Girth (in)² ÷ 670. No state agency or the IGFA publishes an official girth formula for king mackerel, so instead of borrowing the generic all-purpose saltwater constant (usually cited as 800, sometimes 900 for slender freshwater fish like pike) we calibrated this one against four real weighed king mackerel: 55" x 25" at 52 lb, 47.5" x 23" at 33.73 lb, 58" x 21" at 48 lb, and 65" x 28½" at 75 lb.

The four reference fish don't all agree perfectly with each other — real fish vary in body condition (lean vs. deep-bellied, pre- vs. post-spawn), so the same length and girth can genuinely carry a different weight from one fish to the next. Fitting a single divisor across all four landed within about 1–11% of three of them, but the 58" x 21" at 48 lb was a real outlier, off by roughly 20% no matter how the constant is tuned. That spread is exactly why this shows a range instead of one falsely precise number — the low end covers a leaner fish, the high end a heavier one, and most real fish will land somewhere inside it.

What actually counts for citations and tournaments

If you're fishing for a state citation or tournament money, check the actual rules before you rely on a tape measure. North Carolina's citation program, for example, accepts a 45-inch fork length for a release citation on king mackerel — the same measurement this calculator uses — but a kept-fish citation still requires an actual weight on a certified scale, not a calculated estimate. Most king mackerel tournament circuits, including the major ones, require scale weight for calcutta payouts regardless of length. Use this calculator to satisfy your own curiosity at the dock; use a certified scale for anything with money or a citation on the line.