You are living on a round planet mapped in angles. That’s why, when we navigate at sea, we use nautical miles: one nautical mile equals one minute of latitude, so chart distances correspond directly to the graticule on your plotter and paper charts. And yes, I had to look it up! A graticule is a network of lines on a chart representing lines of constant latitude and longitude, used to depict geographic coordinates. The modern nautical mile is defined as exactly 1,852 meters, and the derived unit, the knot, is one nautical mile per hour. This geometry-friendly pairing (nm + knots) makes dead reckoning, ETA math, and current calculations clean and quick without constant conversions. This is part of A boater’s guide to nautical miles, fathoms, litres, gallons, and more.
Depth has its own tradition. The fathom, 6 feet or 1.8288 meters, came from the span of outstretched arms, a practical choice when sailors measured depth with a lead line marked at fathom intervals. Even though we all carry sounders now, “five fathoms” still rolls off the tongue and connects to how anchor rode is marked and handled. In short, fathoms persisted because they matched the human-scale way depth was taken (and shouted to the bridge) for centuries.
“So why are so many modern charts in metres?” Because that’s the international standard for digital charts: the IHO ENC product specification requires depth and height units to be in metres. Hydrographic offices build electronic charts (ENCs) to that spec; software can display soundings in feet or fathoms if you prefer, but the underlying data are metric. In the U.S., NOAA’s ENC design guidance explicitly specifies that soundings be stored and rendered in metres (often to decimetre precision in shallow water). Its Custom Chart tool lets you output paper-style charts while choosing metres, feet, or fathoms for display. Canada’s Chart 1 reminds you that the unit of soundings (metres, fathoms, or feet) is stated on the chart, so check the title block before you assume.
NM ⇄ Miles ⇄ Kilometres (and why the nm is still your friend)
A nautical mile is about 1.1508 statute miles or 1.852 kilometers. That handy link between minutes of latitude and distance means you can pace out runs and set safety margins right off the chart’s latitude scale, no calculator required. Knots (nm/h), then keep your time-distance-speed triangle consistent: if you’re making 6 kn, you’ll travel roughly 6 nm in one hour (less whatever the tide steals).
Fathoms, feet, or metres for depth?
Pick what your crew reads fastest. Many of us think in fathoms for anchoring (rode math!), but metres rule in modern charting and in most of the world. The U.S. still publishes products in feet and fathoms, so if you cruise cross-border, be bilingual. Whatever you choose, set your instruments and chart viewer to the same unit to avoid “I thought that was feet” moments.
Litres, gallons… and the myth of “US mL” vs “UK mL”
Let’s clear a common confusion: a millilitre (mL) is metric and identical everywhere; 1 mL is 1 cubic centimetre. What differs are the customary units that we often convert into mL: US fluid ounces, US pints/quarts/gallons, vs. Imperial (UK/Canadian historical) fluid ounces, pints/quarts/gallons. The U.S. has legally defined the gallon as 231 cubic inches (≈3.785 L), which is derived from the old wine gallon. Finally, math I understand! Britain’s 1824 reform established the Imperial gallon as the volume equivalent to 10 pounds of water (now exactly 4.54609 litres). Because those base gallons differ, so do the derived units: an Imperial pint (568 mL) is ~20% larger than a US pint (473 mL); the US fluid ounce (29.57 mL) is slightly larger than the Imperial fluid ounce (28.41 mL). Kitchen cups differ too (US “legal” cup: 240 mL; metric recipe cup: commonly 250 mL). Bottom line: mL is universal; it’s the cups/ounces/pints/quarts/gallons that wander.
Why the Split Happened (history in 60 seconds) – Stay With Me!
After independence, the United States retained earlier English measures, including the 231 in³ wine gallon for fluids. A few decades later, Britain standardized with the Weights and Measures Act (1824), redefining capacity on a water-mass basis and creating the Imperial family. That fork in the road is why the US gallon of diesel and the Canadian old Imperial jerrycan don’t match. However, Canada now predominantly uses litres in trade, the Imperial heritage still explains older references and some recipe books.
Wouldn’t it be great if all of this information was in one place??? Well, of course, I did that for you. Download my simple conversion table to keep handy. Print in landscape for best visibility.
Practical takeaways for cruisers
- Navigation: Work in nm/knots for plotting, currents, and ETAs. Your chart’s latitude scale is a built-in ruler.
- Depth: Choose meters if you want to align with charts and ENCs, or fathoms and rods if your crew prefers them. Be consistent across all instruments and charts.
- Fuel & water: Label jugs and logs in litres and add a small conversion note (1 US gal = 3.785 L; 1 Imp gal = 4.546 L depending on whether you are fueling up in the US or Canada).
- Galley: When a recipe says “cup,” check the origin. A US cup won’t equal a metric 250 mL or a UK historical cup; metric measures keep peace on a small stove.
If this all seems fussy, remember the philosophy: fewer, better conversions that keep the crew safe and the day simple. Use nautical miles and knots to stay true to the chart, metres to read depth the way your ENC does, litres to tame tank math, and metric mL in the galley so your brownies (and your guests) don’t suffer. The sea throws enough variables at us; units don’t need to be one of them.
