Every calculator on TackleCalc shows its formula on the page. This page goes one level deeper: for each of the 19 tools, what goes in, where the parameters come from, and how far off the answer can be.
The short version: nothing here is invented, and nothing here was measured in a laboratory by us. Three kinds of input feed every calculation, and each tool says which ones it uses.
Where a number is a rule of thumb rather than a law, we say so on the page that uses it. Drag as one third of line strength is a convention with a wide safe margin, not a material property. We would rather label it honestly than dress it up.
The table below lists each calculator, its core relation, and the practical error to expect. Error bands are what the method itself implies, not a claim about any individual fishing situation.
| Calculator | Core relation | Parameters from | Expect roughly |
|---|---|---|---|
| Lure weight | Overlap of the rod power window and the species lure window, then shifted for presentation style | Manufacturer rod power ratings; species lure ranges | One weight step either way |
| Jig head weight | Depth band table, adjusted for current and lure profile | Trade convention for jig fishing by depth | One size either way in wind |
| Reel line capacity | Rated capacity times the square of the diameter ratio | Manufacturer spool rating and published diameters | Within about 10 percent; thin braid runs optimistic |
| Braid to mono converter | Diameter lookup across materials at equal breaking strain | Published diameter tables | Brand to brand spread of 10 to 15 percent |
| Hook size selector | Bait size and target species mapped to hook size and gap | Trade size numbering | One size either way by hook pattern |
| Fish weight | Length times girth squared, divided by 800, times a condition multiplier | Girth-to-length ratios back-solved from the North Dakota Game and Fish Department length-weight table; muskellunge from the Wisconsin DNR average growth curve. 800 is the field constant used with the standard length-times-girth-squared formula. | Within about 5 percent of the state table at common sizes, plus or minus 20 percent for an individual fish |
| Line diameter converter | Pound test to millimeters and back, per material, with PE equivalents | Published diameter tables and the PE size scale | Tight for one brand, loose across brands |
| Sinker weight converter | Ounces, fractions and grams interconverted | Exact unit conversion | Exact |
| Drag and leader | Weakest link in the system times a fraction between one quarter and one third | Line ratings plus the drag convention | Conservative by design; the low end is the safe one |
| Knot strength | Line rating times the retention share typical for that knot on that material | Published knot testing across mono, fluoro and braid | Plus or minus 10 percent; tie quality dominates |
| Reel gear ratio | Ratio times spool circumference gives inches per turn | Manufacturer ratio and spool diameter | Accurate when the spool diameter is right |
| Fly line weight | Line weight designation against grain windows | AFTMA line weight convention | Manufacturers vary by up to half a weight |
| Fillet yield | Live weight times a species yield share | Published yield figures by species | Wide; fish condition changes it a lot |
| Shock leader | Leader class and length set from main line and cast load | Casting load convention | A class either way |
| Trolling depth | Line out, speed, sinker weight and diameter combined | Field convention plus line drag behavior | Plus or minus 15 percent; speed matters most |
| Downrigger depth | Cable angle solved from ball drag and cable drag, giving true depth from counter reading | Physics: buoyancy-corrected ball weight, drag as half rho Cd d v squared, solved as a quadratic | Good for the inputs given; current is not modeled |
| Fly leader length | Rod weight and fly size mapped to leader and tippet length | Fly fishing convention | A foot either way |
| Casting distance | Lure weight, line diameter and rod length against an empirical distance model | Field data behind the model | Plus or minus 20 percent; wind and technique are not modeled |
| Lure sink rate | Terminal velocity scaling with the square root of weight over drag area | Physics, with line drag included | Close in still water; current changes it |
We have not measured line breaking strains, knot retention or sink rates in a laboratory. We have not tested every product named on this site. Where a figure comes from published testing, we use it as published; where it comes from a manufacturer, we use it as stated; where it is a convention, we call it a convention.
What we do claim is that every calculation is reproducible. The formula is printed on the page, the inputs are visible, and you can check the arithmetic by hand. If a result looks wrong for your situation, the reason will be one of the inputs, not a hidden constant.
For record-keeping rules and fish measurement conventions, the International Game Fish Association publishes the standards most anglers end up following. For tackle manufacturing conventions, the American Sportfishing Association maintains the line weight and tackle standards that the trade works from. Manufacturer specification sheets remain the primary source for any individual rod, reel or line, and we treat them as authoritative over our own tables.
If a formula here is wrong, or a parameter does not match the source we cite, tell us and we will fix the page. Use the contact page and include the calculator, the input you used and the source you are working from. Corrections are made to the page itself.
Estimates only: the values shown are a starting point, not a guarantee. Products of the same stated size differ between makers, so your own result will vary. TackleCalc is not affiliated with any manufacturer, and these figures are a guide rather than a guarantee.