How Many Cables Make a Nautical Mile—and Why Old References Disagree
Modern international conversion
The direct answer: 10 cables equal 1 nautical mile
Modern international conversion
- 1 international cable = 0.1 international nautical mile
- 10 international cables = 1 international nautical mile
- 1 international nautical mile = exactly 1,852 metres
- 1 international cable = 185.2 metres
Under the modern international conversion convention, 10 cables make 1 nautical mile. The international nautical mile is fixed at exactly 1,852 metres; treating an international cable as one-tenth of that distance gives 185.2 metres per cable. This is an exact derivation under the stated convention, rather than a claim that every historical cable had the same length. NOAA confirms the fixed 1.852-kilometre international nautical mile, while cable references identify the international cable as its one-tenth subdivision.
In common imperial units, one international cable is approximately:
- 607.6 feet
- 202.5 yards
- 101.27 fathoms, using a six-foot fathom
These imperial equivalents are rounded conversions from 185.2 metres.
The word international matters. “Cable” has not had one universal length across every country, navy, period, or publication. Older references may mean 600 feet, 608 feet, or 720 feet. Those figures are not merely alternative roundings of 185.2 metres; they represent different conventions.
For that reason, this article uses international cable as the default for a general modern conversion. When interpreting a historical chart, logbook, naval manual, or archival account, determine what that source means by cable before converting.
Cable-to-nautical-mile formulas and conversion table
The following formulas apply specifically to international cables and the fixed international nautical mile. They use the convention that one international cable is one-tenth of a nautical mile, with the nautical mile fixed at 1,852 metres.
International cables to international nautical miles
international nautical miles = international cables ÷ 10
International nautical miles to international cables
international cables = international nautical miles × 10
International cables to metres
metres = international cables × 185.2
The cable ratio is given specifically for international units by an international cable conversion reference. The metre formula then follows directly from one-tenth of the 1,852-metre international nautical mile.
International cable conversion table
| International cables | International nautical miles | Metres |
|---|---|---|
| 1 | 0.1 | 185.2 |
| 2 | 0.2 | 370.4 |
| 5 | 0.5 | 926 |
| 10 | 1 | 1,852 |
| 20 | 2 | 3,704 |
| 25 | 2.5 | 4,630 |
| 50 | 5 | 9,260 |
| 100 | 10 | 18,520 |
Table basis: 1 international cable = 0.1 international nautical mile = 185.2 metres; the remaining entries are direct arithmetic from those inputs.
The conversion is simple because the international cable is treated as a decimal subdivision of the nautical mile. Dividing by 10 moves the decimal point one place left; multiplying by 10 moves it one place right.
Worked example: 5 international cables
To convert 5 international cables to nautical miles:
5 ÷ 10 = 0.5 international nautical mile
To convert the same distance to metres:
5 × 185.2 = 926 metres
Therefore:
5 international cables = 0.5 international nautical mile = 926 metres
This result is derived from the cited international-unit inputs of 0.1 nautical mile and 185.2 metres per cable.
Worked example: 10 international cables
10 ÷ 10 = 1 international nautical mile
10 × 185.2 = 1,852 metres
Therefore:
10 international cables = 1 international nautical mile = 1,852 metres
The result matches the fixed length of the international nautical mile.
Worked example: 25 international cables
25 ÷ 10 = 2.5 international nautical miles
25 × 185.2 = 4,630 metres
Therefore:
25 international cables = 2.5 international nautical miles = 4,630 metres
Again, these are direct calculations from the cited international conversion convention.
Reverse example: 3 nautical miles
To convert international nautical miles to international cables, multiply by 10:
3 × 10 = 30 international cables
Therefore:
3 international nautical miles = 30 international cables
Do not apply these formulas automatically to an old or otherwise qualified cable. For example, 25 cables of 720 feet each do not equal 2.5 modern international nautical miles. Conversion to the modern international nautical mile must instead proceed through the cable’s stated physical length.
How long is an international cable in metres, feet, yards, and fathoms?
One international cable can be expressed as follows:
| Unit | One international cable |
|---|---|
| Metres | 185.2 m |
| Feet | approximately 607.6 ft |
| Yards | approximately 202.5 yd |
| Fathoms | approximately 101.27 fathoms |
The exact metric value is derived as one-tenth of the 1,852-metre international nautical mile. The imperial values are rounded.
The fathom figure uses a six-foot fathom. A nautical-measurement reference defines a fathom as six feet and describes its historical association with lowering rope to measure water depth (fathoms and nautical measurement). Thus:
607.6115 feet ÷ 6 feet per fathom ≈ 101.2686 fathoms
Rounded to two decimal places:
1 international cable ≈ 101.27 fathoms
This helps explain an apparent contradiction among references. A cable is often described as 100 fathoms, but 100 six-foot fathoms equal 600 feet, not approximately 607.6 feet:
100 fathoms × 6 feet = 600 feet
Using the exact foot-to-metre conversion:
600 feet × 0.3048 = 182.88 metres
The 100-fathom figure therefore describes the ordinary or traditional 600-foot cable, not an exact conversion of the 185.2-metre international cable.
The difference is small but real:
185.2 metres − 182.88 metres = 2.32 metres
Or in feet:
approximately 607.6 feet − 600 feet = approximately 7.6 feet
Across 10 cable lengths, substituting 600 feet for 185.2 metres per cable creates a shortfall of 23.2 metres:
10 × 2.32 metres = 23.2 metres
That may not matter in a rough historical description, but it does matter when a conversion is presented as exact.
It also helps to keep the units’ roles separate. A fathom is a unit of length traditionally associated with water depth. A cable is a longer unit used for distance. The units frequently appear together because an ordinary cable was commonly treated as 100 fathoms, but they are not synonyms.
Why a cable may be listed as 600, 608, 607.6, or 720 feet
References disagree because cable has denoted several measurement conventions. The applicable value may depend on the country, naval service, period, or whether the writer meant an ordinary cable or a cable derived as one-tenth of the modern international nautical mile.
| Cable convention | Fathoms | Feet | Metres | How to interpret it |
|---|---|---|---|---|
| International cable | approximately 101.27 | approximately 607.6 | 185.2 | One-tenth of an international nautical mile |
| Ordinary or traditional cable | 100 | 600 | 182.88 | Traditional 100-fathom convention |
| Historical British Navy cable | approximately 101.33 | 608 | 185.3184 | Period-specific historical naval value |
| Old U.S. Navy cable | 120 | 720 | 219.456 | Period-specific old U.S. naval value |
The international and ordinary values, as well as the historical British and old U.S. variants, are compared in published cable-unit summaries. The naval labels require historical qualification rather than treatment as current standards.
The historical labels are essential. They do not establish present-day British or U.S. naval practice. A 1949 article in the U.S. Naval Institute’s Proceedings reported a British Navy cable of 608 feet and a U.S. Navy cable of 120 fathoms, or 720 feet, for that period (Alton B. Moody’s historical discussion).
Why 607.6 feet appears
The approximately 607.6-foot figure is the converted length of the international cable:
185.2 metres ÷ 0.3048 metre per foot ≈ 607.6115 feet
It is not a restatement of every historical cable. It follows specifically from treating the international cable as one-tenth of the fixed 1,852-metre international nautical mile.
Why 600 feet appears
The 600-foot cable comes from the traditional relationship:
100 fathoms × 6 feet = 600 feet
It is close to one-tenth of a modern international nautical mile, but it is not exact. The international nautical mile is approximately 6,076.1 feet, so one-tenth is approximately 607.6 feet rather than 600 feet.
Expressed through metres:
600 feet = 182.88 metres
182.88 ÷ 1,852 ≈ 0.09875 international nautical mile
A 600-foot cable is therefore approximately—but not exactly—0.1 international nautical mile.
Why 608 feet appears
The 608-foot value is associated with a historical British naval convention:
608 feet × 0.3048 = 185.3184 metres
That is extremely close to 185.2 metres, but the values are not identical:
185.3184 metres − 185.2 metres = 0.1184 metre
The difference is about 4.7 inches. Although small, an accurate transcription should preserve the source’s stated convention instead of silently replacing it with the international value.
Why 720 feet appears
The old U.S. Navy value used 120 fathoms:
120 fathoms × 6 feet = 720 feet
720 feet × 0.3048 = 219.456 metres
This is substantially longer than an international cable:
219.456 metres − 185.2 metres = 34.256 metres
Relative to 185.2 metres, it is about 18.5% longer. Consequently, 10 old U.S. Navy cables measure 2,194.56 metres, not 1,852 metres:
10 × 219.456 metres = 2,194.56 metres
2,194.56 ÷ 1,852 ≈ 1.185 international nautical miles
The international-cable formula of dividing the cable count by 10 cannot convert this 720-foot unit into modern international nautical miles. Conversion must proceed through the physical length. If a historical document defines its own relationship between cables and a historical nautical mile, preserve that relationship separately rather than equating its mile with today’s 1,852-metre international nautical mile.
The practical lesson is straightforward: do not treat 600 feet, 608 feet, approximately 607.6 feet, and 720 feet as interchangeable. The first three are close enough to invite careless substitution, while the fourth creates a much larger error. Each figure should retain its proper label.
Where the cable unit came from
The distance unit developed from a ship’s physical cable: the heavy rope or, later, chain associated with shipboard work. A cable length reflected the standard length in which such a ship’s cable came, so the physical object lent its name to the distance unit. This origin and the historical variation in cable length are summarized in the hydrographic reprint of Moody’s historical paper.
That leaves two related but distinct meanings:
- A cable can be a physical rope, wire, or chain used for anchoring, mooring, or towing.
- A cable or cable length can be a unit of distance.
Historically, the distance unit was often associated with roughly 100 fathoms. Using a six-foot fathom gives this convenient approximate hierarchy:
1 fathom = 6 feet
100 fathoms = 600 feet
1 ordinary cable = 100 fathoms
10 ordinary cables ≈ 1 modern nautical mile
The relationship looks like a deliberately designed decimal system: 100 fathoms to a cable and 10 cables to a nautical mile. The historical account is more complicated, however. Moody’s paper argues that the fathom, cable, and nautical mile developed independently and that their approximately decimal relationship was apparently coincidental.
That distinction explains why tidy arithmetic and historical practice do not always align. Physical ropes and chains followed maritime working conventions, while nautical miles developed from geographic and navigational concepts. Their lengths ended up close enough to create a useful approximate hierarchy, but not close enough to prevent competing definitions.
Using an international cable as one-tenth of the fixed international nautical mile produces an exact decimal conversion within that declared convention. Historical material may instead preserve an older physical, national, or naval convention. Both of these statements can therefore be true:
- Under the modern international conversion convention, 10 international cables equal exactly 1 international nautical mile.
- Under some historical conventions, 10 cable lengths were only approximately 1 modern international nautical mile—or, in the 720-foot case, were substantially more.
Cable, nautical mile, fathom, and knot are not the same thing
These terms belong to nautical vocabulary, but they do not measure the same quantity or operate on the same scale.
| Unit | What it measures | Relevant relationship |
|---|---|---|
| International cable | Distance | 0.1 international nautical mile; 185.2 metres |
| International nautical mile | Distance | Exactly 1,852 metres; 10 international cables under the declared convention |
| Fathom | Length, traditionally associated with water depth | 6 feet; an ordinary cable is commonly given as 100 fathoms |
| Knot | Speed | 1 nautical mile per hour |
A cable and a nautical mile measure distance. A fathom also measures length, but its traditional nautical role is measuring water depth. A knot is different: it measures speed, not distance.
NOAA states that the international nautical mile is exactly 1.852 kilometres and that one knot equals one nautical mile per hour. It also distinguishes the nautical mile as a distance unit from the knot as a speed unit.
This means:
A vessel travels 1 nautical mile.
A vessel’s speed is 1 knot.
It would be incorrect to say that a destination is “three knots away,” because knots do not express distance. If a destination is three nautical miles away and a vessel maintains three knots, simple distance divided by speed gives a one-hour travel time. Actual passage time may differ because of route and operating conditions.
The latitude connection—and its limit
The nautical mile has a historical connection with one minute of latitude. That relationship made nautical miles useful with latitude and longitude, but it should not be turned into the claim that one minute of latitude has exactly the same physical length everywhere.
Earth is not a perfect sphere, so a minute of latitude varies by location. The historical hydrographic account gives a range from about 6,046 feet at the equator to 6,108 feet at the poles. That variation illustrates why a fixed standard is needed when a precise, consistent length is required.
The modern international nautical mile supplies that fixed length:
1 international nautical mile = exactly 1,852 metres
The safest summary is:
- The nautical mile has a historical and geographic connection to one minute of latitude.
- The modern international nautical mile is a fixed 1,852-metre unit.
- An actual minute of latitude should not be assumed to measure exactly 1,852 metres at every location.
How to interpret an unqualified cable safely
For a general modern calculation, this article uses the international cable of 185.2 metres as its declared default:
10 international cables = 1 international nautical mile
That default is a transparent editorial choice, not proof of what another author intended. If you are interpreting a historical chart, naval document, logbook, engineering record, or maritime narrative, establish the source’s convention before converting.
1. Check the date
A mid-20th-century naval publication may use a service-specific value different from the modern international cable convention. A much older log may reflect a traditional rope length, local usage, or a definition supplied elsewhere in the volume.
The date also helps prevent a historical statement from being misread as a current standard. In particular, the 608-foot British and 720-foot U.S. naval figures discussed above are period-specific historical values, not evidence of present-day official practice.
2. Check the country and service
Look for the issuing navy, hydrographic office, publisher, ship operator, or national context. British and U.S. naval references from the same period did not necessarily use the same cable.
Do not assume the publication language settles the issue. An English-language document might use an international value, a British value, a U.S. value, or a translated local convention.
3. Look for a glossary or conversion note
The strongest evidence is an explicit definition in the source itself. Check:
- Front matter
- Abbreviation and symbol lists
- Tables of measures
- Footnotes
- Chart legends
- Appendices
- Instructions for computing ranges or distances
If a document states “1 cable = 100 fathoms,” preserve that convention:
1 cable = 100 fathoms = 600 feet = 182.88 metres
Do not replace it with 185.2 metres merely because the international cable is a convenient modern default.
4. Examine surrounding units
Nearby calculations can reveal the intended meaning. If a source repeatedly equates 10 cables with approximately 6,000 feet, it likely uses the 600-foot convention. If it defines a cable as 120 fathoms, that yields 720 feet and is consistent with the old U.S. naval value reported in the historical evidence.
If the source instead gives 185.2 metres or explicitly identifies a cable as one-tenth of the fixed 1,852-metre nautical mile, the international convention is likely intended.
5. Preserve the source’s precision
Suppose a log records a position as “5 cables off” and defines a cable as 600 feet. The faithful conversion is:
5 × 600 feet = 3,000 feet
3,000 feet × 0.3048 = 914.4 metres
It would be incorrect to substitute the international calculation of 926 metres without disclosing that the convention had changed. The difference is:
926 metres − 914.4 metres = 11.6 metres
Whether that changes a broad historical interpretation depends on context, but silently removing it makes the conversion less auditable.
The same principle applies to a 608-foot cable:
5 × 608 feet = 3,040 feet
3,040 feet × 0.3048 = 926.592 metres
That is close to five international cables, or 926 metres, but it is not identical.
6. Record unresolved ambiguity
Sometimes a source provides no glossary, national context, or surrounding conversion. In that case, the responsible result is not a falsely precise number.
A research note might say:
“The source records 12 cables but does not define the cable convention. At 185.2 metres per international cable, the distance would be 2,222.4 metres. At 600 feet per traditional cable, it would be 2,194.56 metres. The intended value cannot be established from the document.”
Those results come from:
12 × 185.2 metres = 2,222.4 metres
12 × 600 feet × 0.3048 = 2,194.56 metres
If the 720-foot convention is historically plausible, the possible value is wider still:
12 × 720 feet × 0.3048 = 2,633.472 metres
Recording the uncertainty preserves the evidence. Choosing a convention without disclosing the assumption hides it.
7. Use official operational material for navigation
Historical interpretation and classroom conversion are different from operational navigation. For a live voyage, pilotage decision, exclusion zone, clearance, or other safety-critical use, follow the units and conventions stated by the applicable official chart, publication, notice, or maritime service. The supplied conversion guidance itself cautions that calculators should not replace official sources for real navigation decisions (navigation-use warning).
An unverified web converter can check arithmetic, but it cannot determine what an ambiguous governing document intended. The conversion must begin with the convention specified by that document.
Frequently asked questions
Is one cable exactly one-tenth of a nautical mile?
One international cable is exactly one-tenth of an international nautical mile under the modern conversion convention used here. Therefore:
10 international cables = 1 international nautical mile
That answer does not apply universally to every historical cable. A traditional 600-foot cable is approximately 0.09875 of the modern 1,852-metre nautical mile, while a 720-foot cable is approximately 0.1185 international nautical mile. Always qualify the cable when interpreting historical material.
How many metres and feet are in an international cable?
One international cable is 185.2 metres, derived as one-tenth of the exactly 1,852-metre international nautical mile. It is approximately 607.6 feet.
The metre value is exact under the stated international conversion convention; the foot value is rounded. The same distance is approximately 202.5 yards or 101.27 six-foot fathoms.
Why do some sources say a cable is 600 feet while others say 720 feet?
They are using different conventions. The 600-foot cable is the traditional 100-fathom value:
100 × 6 feet = 600 feet
The 720-foot cable is 120 fathoms:
120 × 6 feet = 720 feet
The latter is associated with an old U.S. Navy convention in the historical sources. Neither figure should be silently substituted for the 185.2-metre international cable.
Is a cable a distance or a type of anchor rope?
It can mean either, depending on context. A cable length is a nautical unit of distance. A physical cable is a heavy rope, wire, or chain used for work such as anchoring, mooring, or towing.
The distance unit’s name came from the length of a ship’s physical cable. Context usually makes the meaning clear: “the vessel lay two cables offshore” describes distance, while “the anchor cable was paid out” describes equipment.
What is the difference between a cable, a nautical mile, and a knot?
A cable and a nautical mile measure distance. Under the modern international convention used here:
1 international cable = 0.1 international nautical mile
A knot measures speed:
1 knot = 1 nautical mile per hour
It is therefore correct to describe a separation as two international cables, or 0.2 nautical mile, and a vessel’s speed as five knots.
For modern general-purpose conversion, the dependable answer is 10 international cables per international nautical mile, with one international cable equal to 185.2 metres. In historical or operational material, always name or verify the convention: cables of 600, 608, approximately 607.6, and 720 feet represent different definitions, and choosing the wrong one can materially change the result.