An independent set of 43 marine calculators and guides for people who run, maintain and trailer their own boats.
Owners and skippers who would rather understand a number than take it on faith: the sailor checking a running fix against the GPS, the trawler owner working out whether the fuel will reach the next port with a reserve, the person in the yard deciding how many tubes of seam compound to buy. The site is not affiliated with any builder, engine maker, chandlery or training body.
A calculator belongs here when the answer comes from something you can measure on the boat or read off a chart, and the method can be written in a few lines that you could repeat with a pocket calculator. If the honest answer depends on a manufacturer’s own chart, such as a particular engine’s fuel curve or a rope maker’s load table, the page says so and points you there instead of guessing.
The library has been cut back to the 43 calculators that meet that bar. Pages that were thin or too similar to one another have been retired from the index so that each remaining page answers a distinct question properly.
Each formula is written on its page. The worked examples and the reference tables in the guides are checked by running the page’s own calculator code on the same inputs, so the prose, the tables and the live calculator agree. Where a formula is a rule of thumb, like the pitch-to-RPM swap or the VHF range estimate, the page calls it that and explains where it breaks down.
Inputs are validated for the slips that happen on a moving deck: a blank field, a second bearing smaller than the first, a reserve of 100 percent, a load bigger than the cable can carry. The page shows a message rather than a confident wrong number.
Navigation pages use nautical miles, knots and degrees true. Tank, paint and resin pages use U.S. gallons and fluid ounces, with metric equivalents where they help. Lengths on deck are in feet and inches. Results are rounded for reading, and the step-by-step block under each result shows the unrounded working.
The condition of your gear, the holding ground, the sea state, how your particular engine burns fuel at your load, the age of your batteries, the true shape of your tanks. Seamanship and local rules still decide the call. These pages are not an engine or electronics manual, not a surveyor’s report, not navigation advice and not a substitute for charts, tide tables or COLREGS.
If you find a wrong number, an unclear explanation or a case a page should cover, write to millsites.contact@gmail.com. Corrections are checked against the formula and published with the page’s examples regenerated.
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Shop and jobsite calculators live on The Tool Mill. The Boat Mill sticks to boats.