Fathom: how many metres?
A fathom (fm) is 6 feet, exactly 1.8288 m. It is the unit of soundings on older British and American charts, the unit of the lead line, and the unit anchor chain is counted in: one shackle of chain is 15 fathoms.
Quick try:
The cable here is the international one, 0.1 NM (185.2 m); the US Navy counted 219.5 m and the Royal Navy 185.3 m. A shackle of anchor chain is 15 fathoms (27.43 m).
Fathoms converted
| fm | m | ft |
|---|---|---|
| 1 | 1.8288 | 6 |
| 2 | 3.6576 | 12 |
| 5 | 9.144 | 30 |
| 10 | 18.288 | 60 |
| 15 | 27.432 | 90 |
| 20 | 36.576 | 120 |
| 50 | 91.44 | 300 |
| 100 | 182.88 | 600 |
Values computed from the exact definitions of the units (NIST SP 811 and the international agreements), rounded to 7 significant digits. The first column is fathoms.
Where the fathom comes from
A fathom is the span of a grown man with his arms stretched out: that is how much line you could haul in one pull while sounding by hand. That made it the natural unit of depth. The call "mark twain" from a leadsman on the Mississippi meant two fathoms, deep enough for a steamboat, and it is where the writer took his pen name.
Anchor chain keeps the fathom alive on every ship. One shackle is 15 fathoms, that is 27.43 m, so "three shackles in the water" is about 82 m of chain. The Royal Navy used a shackle of 12.5 fathoms (22.86 m) until 1949, which is why older books give different figures.
Frequent questions
How many metres is a fathom?
Exactly 1.8288 m, that is 6 feet. Ten fathoms are 18.288 m, and 100 fathoms are 182.88 m.
How many fathoms is 100 metres of depth?
About 54.7 fathoms. The other way round, 100 fathoms are 182.88 m.
How long is a shackle of anchor chain?
Today a shackle is 15 fathoms, that is 27.43 m; American usage calls the same length a shot. Before 1949 the Royal Navy counted a shackle of 12.5 fathoms, which is why older books give 22.86 m.
Do charts still show depths in fathoms?
New editions are metric, but older British charts and part of the American ones are in fathoms or feet and they are still in use. Check the chart title before you read a single sounding.