Naval Rope Length Fathoms Calculator

How fathoms convert to metres

The fathom (English "fathom", Portuguese "braça") is a traditional nautical unit equal to 6 feet = 1.8288 m, and it goes back to the span of a sailor's outstretched arms. The formula is about as simple as they come: metres = fathoms × 1.8288. That makes 20 fathoms 36.58 m, and 100 fathoms = 182.88 m. Up to the 1960s, most nautical charts in English-speaking countries plotted depth in fathoms. Modern charts have largely moved to metres, but legacy paper charts and the old sounding lines still work in fathoms.

In Brazil the terrestrial "braça" used to be 2.2 m, but the naval braça follows the international fathom of 1.8288 m so it stays compatible with charts and sonar scales.

Real-world applications

Commercial fishing crews reading sonar off older gear still call out depths in fathoms. Technical divers planning decompression stops lean on the conversion to line up their metric dive-computer depths with legacy site charts. And underwater archaeology surveys, shipwreck mapping in particular, often have to cross-reference original logbook entries that recorded depths in fathoms.

FAQ

Why 6 feet exactly? The fathom was set on the average arm span of an adult sailor, which made it handy for measuring rope and depth by hauling the line in hand over hand.

Is the Brazilian "braça" the same? In old land surveys the terrestrial braça was 2.2 m, but the naval braça on Brazilian Navy charts equals the international fathom (1.8288 m).

Do modern charts still use fathoms? Most international charts have switched to metres, yet US NOAA charts and plenty of British Admiralty editions still ship in fathoms, so check the legend before you plot anything.