Fathom (fath) - Unit Information & Conversion
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What is a Fathom?
The fathom (symbol: fath or fth) is a unit of length in the imperial and U.S. customary systems, defined as exactly 6 feet (1.8288 meters), primarily used for measuring water depth in maritime contexts. The term derives from the Old English "fæthm" (outstretched arms), reflecting the ancient practice of sailors measuring rope length by stretching it between their extended arms. Throughout maritime history, fathoms have been the standard unit for depth soundings on nautical charts, anchor chain measurements, and submarine operations. While international hydrography has largely transitioned to meters, fathoms remain prevalent on U.S. and U.K. nautical charts, in recreational boating, commercial fishing, and naval traditions. The fathom is immortalized in maritime literature, most famously in Shakespeare's "The Tempest" ("Full fathom five thy father lies"), and the verb "to fathom" (to understand deeply) derives from this nautical measurement, reflecting the metaphorical exploration of ocean depths.
History of the Fathom
The fathom originated in ancient maritime practices where sailors measured rope and depth by arm spans—the distance between fingertips of outstretched arms. Old English "fæthm" (embrace, outstretched arms) appears in texts from the 9th century. Medieval European navies adopted similar arm-span measurements, but the English standardized the fathom at exactly 6 feet during the reign of Edward I (1272-1307), making it consistent with the emerging foot standard. By the 16th century, British Royal Navy regulations formalized the fathom for depth soundings, anchor cable measurements, and ship's rope inventory. Captain James Cook's Pacific voyages (1768-1779) used fathom-based charts that became models for global navigation. The British Admiralty standardized nautical charts in fathoms throughout the 19th century, spreading the unit across the British Empire and international maritime trade. The U.S. Navy adopted the fathom from British practice, and it became the official U.S. depth measurement unit. The International Hydrographic Organization (founded 1921) recommended metrication, but U.S. and U.K. charts retained fathoms. Modern sonar and GPS technology measure in meters, yet legacy charts and maritime tradition preserve fathom usage, particularly in American and British waters.
Quick Answer
1 fathom = exactly 6 feet = 2 yards = 1.8288 meters
Common depth references:
- 5 fathoms = 30 feet = 9.14 m (shallow coastal waters)
- 10 fathoms = 60 feet = 18.29 m (typical harbor depth)
- 50 fathoms = 300 feet = 91.44 m (coastal navigation)
- 100 fathoms = 600 feet = 182.88 m (deep coastal waters)
- 1,000 fathoms = 6,000 feet = 1,828.8 m (oceanic depths)
Literary reference: "Full fathom five thy father lies" (Shakespeare, The Tempest) = 30 feet deep
Quick Comparison Table
| Fathoms | Feet | Yards | Meters | Nautical Context |
|---|---|---|---|---|
| 1 | 6 | 2 | 1.83 | Shallow water, wading depth |
| 2 | 12 | 4 | 3.66 | Typical swimming pool depth |
| 5 | 30 | 10 | 9.14 | Recreational diving limit |
| 10 | 60 | 20 | 18.29 | Safe harbor depth for large vessels |
| 20 | 120 | 40 | 36.58 | Coastal navigation depth |
| 50 | 300 | 100 | 91.44 | Deep coastal waters |
| 100 | 600 | 200 | 182.88 | Continental shelf edge |
| 500 | 3,000 | 1,000 | 914.4 | Abyssal plain approach |
| 1,000 | 6,000 | 2,000 | 1,828.8 | Deep ocean floor |
| 2,000 | 12,000 | 4,000 | 3,657.6 | Mid-ocean ridge depths |
Definition
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:
- Drop weighted line overboard
- Haul in, counting arm spans
- 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
History
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.
Real-World Examples
Shallow Waters (0-10 Fathoms / 0-60 Feet)
- 0-1 fathom (0-6 ft): Wading depth, beaches, tidal flats
- 2 fathoms (12 ft): Swimming pool depth, shallow harbors
- 3 fathoms (18 ft): Small boat anchorage, recreational diving
- 5 fathoms (30 ft): Recreational scuba diving limit (without certification)
- 10 fathoms (60 ft): Large yacht anchoring, harbor entrances
Coastal Waters (10-100 Fathoms / 60-600 Feet)
- 20 fathoms (120 ft): Typical coastal navigation depth
- 30 fathoms (180 ft): Commercial shipping lanes
- 50 fathoms (300 ft): Recreational deep diving limit
- 100 fathoms (600 ft): Continental shelf edge (average)
Deep Waters (100-1,000 Fathoms / 600-6,000 Feet)
- 200 fathoms (1,200 ft): Submarine operating depth (historical WWII submarines)
- 500 fathoms (3,000 ft): Deep ocean floor approach
- 1,000 fathoms (6,000 ft): Abyssal plain depths
Extreme Depths (1,000+ Fathoms / 6,000+ Feet)
- 2,000 fathoms (12,000 ft / 3,657 m): Average ocean depth
- 6,000 fathoms (36,000 ft / 10,973 m): Challenger Deep, Mariana Trench (deepest point on Earth)
Literary and Cultural References
Shakespeare, The Tempest (1611):
"Full fathom five thy father lies; Of his bones are coral made"
Ariel's song describes a drowned man lying 5 fathoms (30 feet) below the surface.
Herman Melville, Moby-Dick (1851): Frequent fathom references for whale sightings and ocean depths.
Jules Verne, Twenty Thousand Leagues Under the Sea (1870): Though the title uses "leagues," depth measurements throughout the novel use fathoms.
Common Uses
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.
Conversion Guide
Fathoms to Feet
Formula: Feet = Fathoms × 6
Examples:
- 5 fathoms = 30 feet
- 10 fathoms = 60 feet
- 100 fathoms = 600 feet
Fathoms to Meters
Formula: Meters = Fathoms × 1.8288
Examples:
- 1 fathom = 1.8288 m
- 10 fathoms = 18.288 m
- 50 fathoms = 91.44 m
- 100 fathoms = 182.88 m
Meters to Fathoms
Formula: Fathoms = Meters ÷ 1.8288
Examples:
- 10 m ÷ 1.8288 = 5.47 fathoms
- 50 m ÷ 1.8288 = 27.34 fathoms
- 100 m ÷ 1.8288 = 54.68 fathoms
Fathoms to Yards
Formula: Yards = Fathoms × 2
Examples:
- 1 fathom = 2 yards
- 5 fathoms = 10 yards
- 10 fathoms = 20 yards
Nautical Miles to Fathoms
Formula: Fathoms = Nautical Miles × 1,012.686...
(1 nautical mile = 6,076.12 feet ÷ 6 feet/fathom)
Example:
- 1 nautical mile ≈ 1,013 fathoms
Common Conversion Mistakes
1. Confusing Fathoms with Feet
The Mistake: Assuming depth values on charts are in feet when they're actually fathoms
Why It's Dangerous:
- Chart shows "10" → Sailor assumes 10 feet
- Actually: 10 fathoms = 60 feet
- Result: Ship runs aground in "deeper than expected" water
The Fix: Always check chart notation: "Depths in fathoms" vs. "Depths in feet" or "Depths in meters"
2. Mixing Fathoms and Meters
The Mistake: Using fathom values when the chart is in meters (or vice versa)
Example error:
- Chart (in meters) shows depth "20"
- Sailor thinks 20 fathoms = 120 feet
- Actually: 20 meters = 65.6 feet = 10.9 fathoms
The Fix: Modern charts clearly indicate units. Check legend and use electronic chartplotters set to preferred units.
3. Incorrect Anchor Scope Calculation
The Mistake: Deploying chain length equal to depth instead of 5-7 times depth
Example:
- Depth: 10 fathoms (60 feet)
- Wrong: Deploy 10 fathoms of chain (60 feet)
- Correct: Deploy 50-70 fathoms (300-420 feet) for proper holding
Why it matters: Insufficient scope causes anchor to drag, risking collision or grounding.
4. Assuming All Fathom Charts Are Outdated
The Mistake: Believing fathom-based charts are obsolete and unreliable
The Truth: Many modern U.S. NOAA charts still use fathoms and are fully updated with current data. Electronic systems can display legacy charts in either fathoms or meters.
5. Ignoring Tidal Correction
The Mistake: Reading chart depth without accounting for tidal height
Example:
- Chart shows 5 fathoms (30 feet) at mean low water
- Low tide: Actual depth = 30 feet
- High tide (+12 feet): Actual depth = 42 feet = 7 fathoms
The Fix: Always correct charted depths for current tide using tide tables.
Fathom Conversion Formulas
To Meter:
To Kilometer:
To Hectometer:
To Decimeter:
To Centimeter:
To Millimeter:
To Inch:
To Foot:
To Yard:
To Mile:
To Nautical Mile:
To Micrometer:
To Nanometer:
To Light Year:
To Astronomical Unit:
To Parsec:
To Angstrom:
To Point (Typography):
To Mil/Thou:
To Furlong:
To Link (Gunter's):
To Pace:
To Span:
To Digit:
To Cable Length:
To Ell:
To Finger:
To Roman Mile:
To Stadion:
To Chi (Chinese):
To Shaku (Japanese):
To Li (Chinese):
To Toise:
To Bolt:
To Rope:
To Smoot:
To Sajene:
To Ken:
To Wa:
To Vara:
To Aln:
To Cubit (Royal/Egyptian):
To Versta:
To Arpent:
To Ri (Japanese):
To Klafter:
To Yojana:
To Skein:
Frequently Asked Questions
Exactly 6 feet = 1 fathom. This is the defining relationship. The fathom was standardized to 6 feet during medieval English measurement standardization.
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