Speed, distance, depth and draft, pressure and temperature, converted as you type. Built around the units actually used at sea — knots, nautical miles, cables and fathoms alongside the metric and imperial ones, and kg/cm² next to bar for older engine room gauges. Everything runs in your browser.
Type into any box and the others update as you type. Nothing is sent anywhere — it all runs in your browser.
Speed
Base unit: metres per second.
Distance
A cable is one tenth of a nautical mile.
Depth & draft
A fathom is six feet.
Pressure
kg/cm² still appears on older engine room gauges.
Temperature
Base unit: degrees Celsius.
Wind speed to Beaufort force
Wave heights are the probable mean in the open sea, not a maximum. Fetch, depth and swell change what you actually get.
The units that cause the most trouble
Nautical mile against statute mile. A nautical mile is exactly 1,852 metres and corresponds to one minute of latitude, which is why distance at sea is measured in it — you can take a range straight off the latitude scale in the margin of the chart. A statute mile is 1,609.344 metres. One nautical mile is about 1.151 statute miles, and one knot is one nautical mile per hour.
Cables. One tenth of a nautical mile, 185.2 metres. Used for short ranges: a distance off, a passing distance, a scope of anchor cable. Ten cables to the mile.
Fathoms. Six feet, exactly 1.8288 metres. Large parts of the world were surveyed in fathoms and older chart editions still carry soundings in fathoms and feet, so this stays a live conversion rather than a historical one.
kg/cm² against bar. The one that quietly causes errors. One kg/cm² is 98,066.5 pascals, so a bar is 1.01972 kg/cm². Under two per cent apart — small enough to be waved away as rounding, large enough to matter on a relief valve setting or a test pressure.
Beaufort scale reference
Wind speeds are the standard bands in knots at ten metres above sea level. The wave heights are the probable mean in the open sea with a fully developed sea — that is, the wind has blown long enough over enough open water. In a confined sea, close to a lee shore, with a young sea or where swell runs against wind or current, what you actually get will differ, sometimes considerably.
| Force | Knots | Description | Wave height | Sea state |
|---|---|---|---|---|
| 0 | under 1 | Calm | — | Sea like a mirror. |
| 1 | 1–3 | Light air | 0.1 m | Ripples, no foam crests. |
| 2 | 4–6 | Light breeze | 0.2 m | Small wavelets, glassy crests. |
| 3 | 7–10 | Gentle breeze | 0.6 m | Large wavelets, scattered white horses. |
| 4 | 11–16 | Moderate breeze | 1 m | Small waves, fairly frequent white horses. |
| 5 | 17–21 | Fresh breeze | 2 m | Moderate waves, many white horses. |
| 6 | 22–27 | Strong breeze | 3 m | Large waves, extensive foam crests, spray. |
| 7 | 28–33 | Near gale | 4 m | Sea heaps up, foam blown in streaks. |
| 8 | 34–40 | Gale | 5.5 m | Moderately high waves, crests break into spindrift. |
| 9 | 41–47 | Strong gale | 7 m | High waves, dense foam, spray affects visibility. |
| 10 | 48–55 | Storm | 9 m | Very high waves, sea takes on a white appearance. |
| 11 | 56–63 | Violent storm | 11.5 m | Exceptionally high waves, visibility affected. |
| 12 | 64 + | Hurricane force | 14 m + | Air filled with foam and spray, sea completely white. |
Quick reference: draft marks
Draft marks are cut in either metres or feet depending on where and when the ship was built. Metre marks are 10 cm high and 10 cm apart, so the bottom of a figure gives the draft directly. Foot marks are 6 inches high and 6 inches apart. If you are reading feet and reporting metres, the converter above is the fastest route; one foot is 0.3048 metres exactly, so a draft of 30 feet is 9.144 metres.
Then put the numbers to work
Once you have a speed and a distance in the units you need, the voyage and ETA calculator turns them into an arrival time, and the fuel consumption calculator turns the passage into tonnes of bunkers and carbon dioxide.
Marine unit converter FAQs
What is the difference between a nautical mile and a statute mile?
A nautical mile is exactly 1,852 metres and corresponds to one minute of latitude, which is why it is the unit used at sea — a distance on the chart can be measured straight off the latitude scale in the margin. A statute mile is 1,609.344 metres. One nautical mile is therefore about 1.151 statute miles, and a speed of one knot is one nautical mile per hour.
How many metres is a fathom?
A fathom is six feet, which is exactly 1.8288 metres. Charts in many parts of the world were surveyed in fathoms and some older editions still carry soundings in fathoms and feet, so converting between fathoms and metres remains a live task rather than a historical curiosity.
Why do engine room gauges read in kg/cm²?
It is a metric-technical unit that predates the SI system and stayed in service on machinery built in yards that used it. One kg/cm² is 98,066.5 pascals, so it is close to but not the same as a bar: 1 bar is 1.01972 kg/cm². The difference is under two per cent, which is exactly why it causes trouble — it is small enough to be dismissed as a rounding error and large enough to matter on a setting.
How many knots is a cable?
A cable is a distance, not a speed. It is one tenth of a nautical mile, or 185.2 metres, and it is the unit normally used for short ranges such as a distance off, an anchor cable scope or a passing distance. Ten cables make one nautical mile.
What wind speed is Beaufort force 8?
Force 8 is a gale, 34 to 40 knots, with a probable mean wave height of about 5.5 metres in the open sea. The wave heights on the Beaufort scale assume the wind has been blowing long enough over enough open water to raise a fully developed sea, so in a confined sea, close to a coast, or with a young sea the actual height will be lower — and where swell runs against wind or current it can be considerably worse.
Part of the marine engineering calculators on this site — sea service, bunkers and emissions, and passage planning.