The sinker is not what the current is pulling on
Ask an angler why the rig will not hold and the answer is usually about the sinker. The arithmetic says otherwise. Drag on a cylinder in crossflow is roughly half the water density times a drag coefficient times the frontal area times the square of the speed. For a sinker, the frontal area is a couple of square inches. For fifty feet of line, the frontal area is the diameter multiplied by the whole wetted length, and for even thin line that comes out several times larger than the sinker.
So the force sweeping your rig downstream is mostly a line problem, distributed over its entire length. That reframes the whole exercise. Adding lead treats a symptom. Reducing the area the water has to push on removes the cause.
Two levers, and one is much stronger than the other
Frontal area scales directly with diameter, so halving the diameter halves the force and therefore halves the lead. Braided line at a given breaking strain is roughly a third to a half the diameter of monofilament at the same rating, which is why a rig that needs sixteen ounces on heavy mono holds with six or eight on braid. That is the practical lever, and the result table prices it directly at 50, 75, 125 and 150 percent of whatever diameter you entered.
The other lever runs the wrong way. Force scales with the square of current speed, so water moving twice as fast pulls four times as hard, and the lead required goes up fourfold too. Nothing about tackle selection fights that. The table showing half, one, one and a half and two times your current speed exists to make the shape of that curve obvious, because it is the reason a spot that fishes beautifully at the end of a tide becomes unfishable in the middle of it.
Grip, buoyancy and why the lead is more than the force
A sinker resists being dragged by friction against the bottom, and friction is a fraction of the weight pressing down. Two things reduce that weight. The first is buoyancy: lead has a specific gravity of about 11.3, so submerged it weighs roughly 91 percent of its dry weight, and the ounce marked on the sinker is a dry ounce. The second is the grip factor, which is doing a lot of work in one number.
| Grip factor | Roughly what it describes |
|---|---|
| 0.2 to 0.3 | Smooth round sinker on polished rock, hardpan or clean shell |
| 0.4 to 0.5 | Ordinary sinker on sand, gravel or mixed bottom |
| 0.6 to 0.8 | Flat or gripping shape bedded into sand or soft mud |
Grip is also unstable in a way friction on dry land is not. Once a sinker starts to move it stops being bedded, the grip drops, and it accelerates downstream until something stops it. This is the reason a rig either holds solidly or does not hold at all, with very little in between, and it is why rounding up matters more than precision in the input.
Reading the answer honestly
The model here covers drag on the line and on anything you entered for the rig. It knows nothing about the shape of your sinker, whether the bottom is scoured smooth or littered with structure, how much the rod tip is being worked by swell, or the fact that current is rarely uniform from surface to bottom. In a tidal river the surface can be running hard while the water near the bed is barely moving, which makes the honest answer lighter than this calculation. Near a headland or a reef edge the reverse happens.
Treat the output as the order of magnitude and the comparison tables as the real content. If you are fishing from an anchored boat, the same current is working on the boat and the rode with far more force than it applies to your line, and the anchor rode scope calculator deals with that side of it. If the rig has to be trolled rather than held, the trolling depth calculator is the page for how deep it runs. Before fishing a new water at all, check the local rules through the fishing rules guide, since what gear is permitted varies by state and by water body and is not something any calculator should be telling you.
Questions people ask
Why does thinner line need less weight?
Because the force the current applies to the line scales directly with the frontal area the line presents, and for a cylinder that area is diameter times length. Halve the diameter and you halve the area, the force and the lead needed to resist it. Nothing else about the line matters to this calculation, which is why the input asks for diameter in inches rather than pound test. Braid is the usual way people take advantage of it, since braided line at a given breaking strain is substantially thinner than monofilament at the same rating, but the mechanism is about thickness, not material.
Should I use boat speed or water speed for the current?
Water speed past the line, which is not the same as boat speed over the ground and not the same as the tide table figure at the nearest station. If you are anchored, the current past your line is the speed of the water relative to the fixed bottom, which you can estimate from how fast floating debris passes the boat. If you are drifting with the current, the water is barely moving past your line at all, which is exactly why a drifting rig holds bottom with far less weight than an anchored one in the same tide. If you are holding position against the current under power, the water speed past the line is the current itself plus whatever you are doing on top of it.
The number seems very high. Is it wrong?
Check three inputs before assuming it is. The scope multiplier is the most common culprit, because entering 2.0 doubles the wetted length and doubles the force, and while long bellies of line do happen, the number should reflect what is really in the water rather than the worst case. The current speed is the second, since the square law means an over-estimate of 50 percent produces an answer more than twice too big. The third is the grip factor, which most people set too low out of pessimism. Beyond that, high answers in fast water are simply true, and are the reason experienced anglers fish the slack rather than fight the flow.
Does sinker shape matter?
A great deal, and it is not in this model. A flat or pyramid shape bedded into sand grips far better than a round ball on the same bottom, and a shape with wires that dig in can hold in current that would roll anything else. Shape also changes the drag on the sinker itself, which is small compared with the line but not zero. The way to reflect a good gripping shape here is to raise the grip factor, and the way to reflect a bad one is to lower it. Getting the shape right is often worth more than adding ounces.
Does this work for a drop shot or a vertical jig?
The mechanism applies to any line in moving water, but the failure mode is different. A vertical presentation is not usually trying to grip the bottom, it is trying to stay under the boat, and the number that matters there is how far the line sweeps rather than whether the weight holds. The same drag force is what causes the sweep, so the comparison tables still tell you the right story: thinner line sweeps less at the same weight, and faster current sweeps far more. Use the result as a relative guide rather than a holding weight.