What Size Jig for Bluefin Tuna? Use the Gram-to-Foot Rule
Last updated: August 2026
Match jig weight to depth at roughly a gram per foot: 300g covers about 150–300 ft, 400g covers about 200–400 ft. Current pushes that number up — the stronger the drift, the heavier you need to go to hold a vertical line. But going heavier than the mark actually calls for costs you time in the strike zone the same way going too light does. The rule works as a ceiling, not just a floor.
I built the Torpedo because too many nights on the rail I watched guys grab the heaviest jig in the box for every mark, tungsten or lead, on the assumption that heavier always wins. It doesn't. A jig that's already holding vertical at 300g isn't fishing any better at 400g — it's just getting to the bottom of a mark you weren't actually on.
How does the gram-to-foot rule actually work?
The rule of thumb is close to 1:1 — a gram of jig weight for roughly every foot of depth. A 250g jig is built for about 100–250 ft. A 300g jig covers 150–300 ft. A 350g jig holds 150–400 ft once the current picks up. It's not exact physics — current speed, line diameter, and how fast the boat is drifting all shift the number — but it's close enough to build a system around instead of guessing at the rail.
The mechanism is simple: a jig's fall speed is a function of weight versus drag. More weight overcomes water resistance faster and holds a straighter line down. Current adds a sideways force fighting that same drop, so the heavier the current, the more weight it takes to win that fight and keep the line close to vertical.
What happens if you fish too heavy?
The same thing that happens if you fish too light, just in reverse. If the captain calls a mark at 200 ft and you're fishing a 400g jig, you blow through the strike zone on the way down and spend most of the drop below the fish instead of in front of them. Overshooting costs the same time-in-zone that undershooting does — fewer looks at your jig per drift, more resets, less actual fishing per hour on deck.
When should you actually size up?
Three real reasons, not habit: the mark itself is deeper (300 ft or more), the current is strong enough that a lighter jig is visibly scoping out — drifting sideways instead of dropping straight — or wind and boat drift are adding lateral pull a lighter jig can't fight. Outside of those three, going heavier doesn't buy you anything the lighter jig wasn't already giving you.
There are two different ways to fix a jig that's scoping out, and they're not interchangeable. Adding weight is one. The other is density or shape — tungsten holds a smaller profile at the same weight, and a flat-back lead design holds vertical through shape instead of mass. I broke down that tradeoff in full in tungsten vs. lead tuna jigs.
Does braid diameter change the math?
Yes, and it's the part most anglers skip. Thicker braid has more surface area for current to grab, which works against a jig's weight the same way a weak current does. Two anglers fishing the identical 300g jig on 50lb versus 80lb braid are not fishing the same effective depth — the thicker line is quietly adding drag the jig has to fight. If your jig is scoping out and the weight looks right on paper, check the braid before you size up.
Matching weight to depth
| Depth you're marking | Slack to moderate current | Heavy current |
|---|---|---|
| 100–250 ft | 250g | 300g |
| 150–300 ft | 300g | 350g |
| 200–400 ft | 350g | 400g or more |
Most real trips cross more than one row in that table before the day is done — current builds through a tide change, or the boat moves to a deeper mark. That's the actual case for carrying more than one weight, not because heavier is inherently safer, but because the right weight changes as the day does. It's also why I still don't fish a hard tackle box packed with only one size — see the tackle storage breakdown for what actually needs to ride along on a 1.5-day trip.
FAQ
What size jig do I need for 300 feet of water?
About 300g in slack to moderate current. In heavy current, size up to 350g so the jig holds vertical instead of drifting off the mark.
Is a heavier jig always safer if I'm not sure?
No. It fixes undershooting but creates overshooting, which costs the same time in the strike zone. Heavy enough beats as heavy as possible.
How do I know if I'm too light instead of just impatient?
Watch your line angle on the drop. Holding close to vertical means you're fine. Angling off and drifting well past the boat before it hits bottom means the jig is scoping out — a weight problem, not a patience problem.
Does current change the rule?
Yes — current is the main thing that pushes the number up from the 1g-per-foot baseline. The rule is a starting point on the dock, not a fixed formula.
Does braid diameter matter as much as jig weight?
It matters more than most anglers think. Thicker braid adds drag that fights the jig's weight the same way current does — if a jig is scoping out at the "right" weight, check the line before adding grams.
I still fish the Torpedo 300g and 400g off this exact rule — 300g for anything inside 300 ft, 400g once the mark or the current calls for more. 
Shop the Torpedo 300g.