Editor's Note: This article will teach you all you need to know about knots, as in the nautical measurement for speed. If you're looking to learn more about marine tied knots (i.e. the kind involving ropes), check out our guide on the 8 Marine Knots Every Boater Should Know.


Whether you own a center console fishing boat, a jet boat, or a cruising sailboat, you’re officially a boater — a mariner, in nautical terms. And boating, unlike most modern recreational pastimes, comes with centuries of history, tradition, and its own unique language. That includes specialized systems of measurement, which can be confusing for newcomers.


Naturally, all of this can be a bit confusing for those new to the fold.


If you’re a seasoned sailor, feel free to skip ahead to our Boat Maintenance and DIY section or browse our How-To articles and videos for more advanced content. For beginner boaters, here’s a crash course in nautical measurements: speed, distance, depth, and wind force.


sextant

Above: Mariners have their own way of measuring things and describing what those measurements mean. Photo by US Navy.


Speed and Distance: What’s a Knot, Anyway?


While most people measure speed in miles or kilometers per hour, boaters use knots. One knot, abbreviated as kt, is equivalent to one nautical mile per hour. This works out to roughly 1.2 land miles per hour (MPH).




  • 1 knot = 1 nautical mile per hour = 1.151 statute (land) miles per hour




So, why nautical miles? Because navigation is based on latitude and longitude. One nautical mile equals one minute of latitude on a nautical chart — a useful nautical measurement standard long before GPS was invented.


Fun history fact: In the days of sail, mariners measured speed by tying knots at intervals (every 47’3”) in a rope attached to a wooden plank, called a common log or chip log. They tossed this plank overboard as the ship traveled and counted how many knots passed in 30 seconds while the whole thing unwound. This resulted in the term "knots" for speed.


Red and black Cigarette 59 Tirranna boat racing across the water

Is this boat doing 70 knots or is it going over 80 MPH? Yes. (Photo by Cigarette Racing.)


Even though knots are the traditional nautical measurement, most modern boaters use GPS devices set to MPH, which is why we often include both in our reviews. To convert MPH to knots, simply multiply the MPH by 0.87. To convert knots to MPH, multiply the knots by 1.151.


Depth: From Fathoms to Feet


A fathom is a traditional nautical unit equal to 6 feet. It originally represented the span of an average man’s outstretched arms — though that is obviously a somewhat variable length. Over time, the fathom was standardized to six feet because it's nearly one-thousandth of a nautical mile (6,080 feet), making for easy-ish calculations.


Today, most recreational boaters talk in feet, but you might still encounter fathoms on older charts or in deep-sea navigation. Common fathom-based depth lines on charts include:




  • 20 fathoms = 120 feet




  • 50 fathoms = 300 feet




  • 100 fathoms = 600 feet




You can usually customize your GPS or chartplotter to display depth in either feet or fathoms, depending on your preferences.


Wind Force: The Beaufort Scale


While wind is often reported in MPH or knots, mariners also use the Beaufort Scale to describe wind strength with descriptive terms. Developed in the early 1800s by Royal Navy officer Francis Beaufort, it remains in use today — especially in weather forecasts and sailing lingo. 



  • 0 knots of wind: calm

  • 1 to 3 knots: light air

  • 4 to 6 knots: light breeze

  • 7 to 10 knots: gentle breeze

  • 11 to 16 knots: moderate breeze

  • 17 to 21 knots: fresh breeze

  • 22 to 27 knots: strong breeze

  • 28 to 33 knots: near gale

  • 34 to 40 knots: gale

  • 41 to 47 knots: strong gale

  • 48 to 55 knots: storm

  • 56 to 63 knots: violent storm

  • 64 knots and above: hurricane


blowing a gale

Above: In sailing, there's a big difference between "blowing a gale" and a "fresh breeze." Thirteen knots, to be exact.


Even NOAA often uses Beaufort terms in its forecasts. A weather report might say: “Gale winds expected,” which translates to a nautical measurement of 34 to 40 knots.


Final Thoughts on Nautical Measurement


Learning nautical measurements is a key part of becoming a confident boater. Terms like knots and fathoms may seem old-fashioned, but they’re rooted in practical seamanship. With a little practice, they’ll feel second nature, helping you navigate safely and speak the language of the sea.


Want to learn more about the basics about boating? Be sure to also check out our guides on:


Maritime Rules of the Road: A Primer for Boaters
Knot Tying for Boaters: 5 Basic Knots
How to Tie a Line on a Cleat
and
Boat Winterizing Tips




Editor's Note: This article was originally published in November 2014 and most recently updated July 8, 2025.


Written by: Lenny Rudow
With over two decades of experience in marine journalism, Lenny Rudow has contributed to publications including YachtWorld, boats.com, Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design, and he has won numerous BWI and OWAA writing awards.