Marine & Nautical Calculators
Anchor Chain Length Calculator
Calculate the anchor chain length needed at a given scope ratio, water depth, and bow height, with common scope ratios for different conditions.
Boat Draft Calculator
Calculate hull draft from displacement, waterline length, beam, and hull shape using Archimedes' principle, in either salt water or fresh water.
Boat Fuel Consumption Calculator
Work out the fuel needed for a planned trip, or the range a full tank provides at a given cruising speed and burn rate, in either direction.
Displacement Hull Speed Calculator
Estimate the practical speed ceiling of a displacement hull from its waterline length, the point where pushing a bow wave demands disproportionate power.
Knots to MPH Converter
Convert speed between knots and miles per hour using the exact 1.150779 ratio between the nautical mile and the statute mile, in either direction.
Marine Battery Capacity Calculator
Estimate battery runtime for a given load, or the battery capacity needed to hit a target runtime, accounting for voltage and depth of discharge.
Marine Distance Calculator
Calculate great-circle distance between two coordinates using the haversine formula, or find estimated time of arrival from distance and cruising speed.
Nautical Mile Converter
Convert precisely between nautical miles, kilometers, statute miles, and meters using the exact 1,852-meter nautical mile definition used at sea.
Sail Area Calculator
Calculate total sail area from simple triangle dimensions or full sloop-rig measurements, using the standard I, J, P, E rig dimension convention.
Tide Time Calculator
Project the next high or low tide time from the last observed tide, or estimate current tide height using the navigator's Rule of Twelfths.
The Numbers Every Boat Owner Checks Before Leaving the Dock
Boating math covers a wide range, from how much chain to let out at anchor to how fast a displacement hull can physically travel regardless of engine power. These calculators apply the standard formulas used in seamanship and boat design, whether the concern is safety at anchor or performance under sail or power.
Popular Marine & Nautical Calculators
Ten tools span anchoring, performance, and navigation:
- Anchor Chain Length Calculator — calculates recommended scope (chain length relative to water depth) for secure anchoring.
- Displacement Hull Speed Calculator — estimates the maximum practical speed of a displacement hull based on waterline length.
- Boat Draft Calculator — estimates how deep a boat's hull sits in the water based on load and hull characteristics.
- Nautical Mile Converter — converts between nautical miles, statute miles, and kilometers.
- Tide Time Calculator — estimates high and low tide timing for a given location and date.
Why a Bigger Engine Can't Beat Hull Speed
A displacement hull (one that pushes through water rather than planing on top of it) has a theoretical maximum speed determined almost entirely by its waterline length, not its engine power — because past that speed, the boat would need to climb over its own bow wave, which requires disproportionately more power for very little additional speed. This is why adding a bigger engine to a displacement-hull boat produces diminishing returns past hull speed, while a planing hull design (common in many powerboats) escapes this limit entirely by rising up and skimming across the water's surface instead of pushing through it.
Frequently Asked Questions
Why does anchor scope matter so much?
A shallower scope angle (more chain relative to depth) lets the anchor dig in and hold using its designed horizontal pull, while too steep an angle from insufficient scope can pull the anchor up and out rather than setting it.
Is a knot the same as a mile per hour?
No — a knot is one nautical mile per hour, and a nautical mile (based on Earth's circumference) is slightly longer than a statute mile, so a given knot value is always a bit faster than the same number in mph.
Explore More
Working on engine performance more broadly? See the Automotive Calculators, or check weather factors relevant to sailing in the Weather Calculators.