Fathom to Span Converter

The Six-Foot Maritime Standard

The fathom is defined as exactly 6 feet in the imperial and U.S. customary measurement systems.

Precise equivalents:

• 6 feet (by definition)
• 2 yards (6 ft ÷ 3 ft/yd)
• 72 inches (6 ft × 12 in/ft)
• 1.8288 meters (exactly, using 1 ft = 0.3048 m)
• 182.88 centimeters

Historical basis: The arm span of an average man with arms fully outstretched, measured from fingertip to fingertip.

Arm Span Origins

Old English "fæthm":

• Primary meaning: To embrace, encircle with outstretched arms
• Secondary meaning: The distance between fingertips when arms are extended

Practical measurement: Sailors hauling in sounding lines (weighted ropes for measuring depth) would pull hand-over-hand, with each arm span representing one fathom. This created a natural counting method:

1. Drop weighted line overboard
2. Haul in, counting arm spans
3. Number of arm spans = depth in fathoms

Standardization necessity: Since arm spans varied (5.5-6.5 feet typically), maritime commerce required a fixed standard. The British settled on exactly 6 feet, matching the standardized foot of 12 inches.

Nautical Charts and Depth Contours

Fathom lines: Nautical charts show depth contours (lines connecting points of equal depth) traditionally measured in fathoms.

Common contour intervals:

• 1, 2, 3, 5, 10 fathoms: Shallow coastal waters
• 20, 50, 100 fathoms: Coastal navigation
• 500, 1,000 fathoms: Deep ocean

Chart notation: Depths written as plain numbers on charts (e.g., "45") indicate 45 fathoms unless otherwise specified. Modern charts often include a note: "Depths in fathoms" or "Depths in meters."

Anchor Cable and Chain

Shackle: One "shackle" of anchor chain traditionally equals 15 fathoms (90 feet / 27.43 m) in the Royal Navy and many navies worldwide.

Anchoring depth rule: Ships typically anchor with a scope (ratio of chain length to water depth) of 5:1 to 7:1 for safety.

Example:

• Water depth: 10 fathoms (60 feet)
• Required chain: 50-70 fathoms (300-420 feet)
• That's 3.3 to 4.7 shackles

Ancient Maritime Practices (Pre-9th Century)

Mediterranean and Northern European sailors: Ancient mariners measured rope and depth using body-based units:

• Cubit: Elbow to fingertip (~18 inches)
• Pace: Two steps (~5 feet)
• Arm span: Outstretched arms (~6 feet)

Sounding lead: A heavy weight (lead sinker) attached to a marked line, dropped overboard to measure depth. Sailors counted arm spans as they hauled the line back aboard.

Old English Documentation (9th-11th Centuries)

Earliest references: Anglo-Saxon texts use "fæthm" for measuring rope lengths and describing distances.

Beowulf (8th-11th century): The epic poem mentions "fæthmas" in describing ocean depths and ship measurements.

Viking influence: Old Norse "faðmr" (similar arm-span measurement) influenced English usage through Viking contact and trade.

Medieval Standardization (13th-15th Centuries)

Edward I (1272-1307): English law under Edward I began standardizing measurements, including the fathom at 6 feet.

Admiralty regulations: The emerging Royal Navy needed consistent rope, sail, and depth measurements for shipbuilding and navigation.

Rope making: British rope makers sold cordage by the fathom, with standard lengths for anchor cables (120 fathoms = 1 cable length in some contexts).

Age of Exploration (15th-17th Centuries)

Navigation charts: Early nautical charts (portolan charts) began incorporating depth soundings in fathoms.

Captain James Cook (1768-1779): Cook's Pacific voyages produced meticulous charts with fathom-based depth measurements. His charts became templates for British Admiralty standards.

Example - HMS Endeavour soundings: Cook's logs record depths like "15 fathoms, sandy bottom" or "No bottom at 100 fathoms" (indicating depths exceeding 600 feet).

British Admiralty Charts (19th Century)

Hydrographic Office (founded 1795): The British Admiralty Hydrographic Office systematized global nautical chart production, standardizing fathoms for depth.

Matthew Fontaine Maury (1806-1873): American oceanographer Maury collaborated with the British to create standardized depth charts using fathoms, mapping ocean currents and depths.

Cable-laying expeditions: Transatlantic telegraph cable projects (1850s-1860s) required precise fathom-based depth surveys. HMS Agamemnon and USS Niagara charted the Atlantic floor in fathoms before laying the 1858 cable.

U.S. Navy Adoption (19th-20th Centuries)

Inherited British standards: The U.S. Navy adopted British maritime practices, including fathom-based charts and anchor cable measurements.

U.S. Coast and Geodetic Survey: Founded in 1807 (originally "Survey of the Coast"), it produced nautical charts in fathoms for American waters.

World War II: Submarine warfare and amphibious operations relied heavily on fathom-based depth charts. USS submarines operated in waters charted in fathoms.

Metrication Movement (20th Century-Present)

International Hydrographic Organization (IHO, founded 1921): Recommended global adoption of metric system for nautical charts.

Gradual transition:

• 1970s-1980s: Most nations began publishing new charts in meters
• UK Admiralty: Converted most charts to meters by the 1990s
• U.S. NOAA: Many American charts still use fathoms, particularly for coastal waters

Mixed usage today: Modern electronic chart systems (ECDIS) allow display in either fathoms or meters, accommodating mariners accustomed to either system.

1. Nautical Charts and Hydrography

Depth soundings: Nautical charts mark depths in fathoms, particularly on U.S. and older British charts.

Contour lines: Lines connecting equal depths (e.g., the 10-fathom line) help mariners avoid shallow areas.

Chart abbreviations:

• fms: Fathoms
• fm: Fathom
• No bottom at 100 fms: Depth exceeds 100 fathoms (600 feet)

2. Anchoring and Mooring

Anchor scope: Mariners calculate how much anchor chain to deploy based on water depth in fathoms.

Rule of thumb: Deploy 5-7 times the water depth in calm conditions, 7-10 times in storms.

Example:

• Depth: 8 fathoms
• Calm weather scope (5:1): 40 fathoms of chain
• Storm scope (10:1): 80 fathoms of chain

3. Commercial Fishing

Net depth: Fishermen describe trawl net depths in fathoms.

Example: "Running trawl at 50 fathoms" (300 feet deep)

Fishing line: Deep-sea fishing lines measured in fathoms to target specific depths.

4. Recreational Boating and Diving

Depth sounders: Many recreational boat depth finders display fathoms (though meters and feet are increasingly common).

Dive planning: Divers reference depth in fathoms on nautical charts when planning dive sites.

5. Submarine Operations

Periscope depth: Submarines traditionally use fathoms for depth control.

Example: "Dive to 20 fathoms" (120 feet)

Historical note: WWII submarine logs recorded depths in fathoms; modern submarines use meters.

6. Maritime Literature and Tradition

Nautical expressions:

• "To fathom something" = to understand its depth (metaphorically)
• "Unfathomable" = too deep to measure or comprehend

Sailing instructions: Traditional pilot books use fathoms for approach depths and anchorage recommendations.