Convert m/s to kn

Metres per second is the SI unit and the one physics is done in, which is why it turns up in wind readings and in almost nothing a driver would look at.

A knot is one nautical mile per hour, and a nautical mile is exactly 1,852 metres — originally one minute of latitude, so a knot relates speed directly to position on a chart. Its name comes from counting knots in a rope paid out behind a ship.

The factor is 1.943844, and almost nobody carries that around. Rounded to 1.94 it is off by 0.2 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 m/s.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

A knot is one nautical mile per hour, and the nautical mile is exactly 1,852 metres — fixed by definition in 1929, not measured. It was chosen to be one minute of latitude, which is what makes it useful at sea: sixty nautical miles is one degree up the chart, on any chart.

So the conversion to metre per second is exact rather than approximate, but the round number is on the nautical side. Expect the metric figure to be the untidy one here, and note that the knot is already a speed — "knots per hour" is a quantity nobody means.

A single storm is routinely described in four different units in the same week. A European national weather service publishes the wind in metres per second, because that is what its instruments read and its models produce. The marine and aviation bulletins for the same area give knots. General European media convert to kilometres per hour, because that is what the audience drives in. American and British outlets use miles per hour.

None of them is wrong, and the numbers are wildly different: a wind of 25 m/s is 48.6 knots, 90 km/h and 55.9 mph. A reader comparing coverage across two of those sources without noticing the unit will conclude that the forecasts disagree, when in fact the only thing that changed was the audience. This page handles the first of those translations — the one from the measuring unit to the unit the people at sea and in the air work in.

Marine warnings are keyed to specific wind speeds, and those speeds are round numbers in knots and nothing in particular in metres per second. A gale warning begins at 34 knots, which is 17.5 m/s. A storm warning begins at 48 knots, or 24.7 m/s. Hurricane-force is 64 knots, or 32.9 m/s.

That asymmetry is the useful diagnostic. Whenever a set of thresholds is round in one unit and untidy in the other, the round side is the original, and here the original is knots. It also means the conversion has a consequence: a model output of 17.4 m/s sits a shade under a gale and 17.6 sits a shade over, and the difference between those two figures is far smaller than the uncertainty in the forecast that produced them. Convert, then treat the threshold as a band rather than a line.

The warning levels are not arbitrary numbers that happen to be round in knots; they are Beaufort force boundaries. Force 8 begins at 34 knots, which is the gale threshold. Force 10 begins at 48 knots, the storm threshold. Force 12 begins at 64 knots, hurricane force. The scale in between runs force 4 at 11 to 16 knots, force 6 at 22 to 27, and force 7 at 28 to 33.

Underneath the table is a relation in SI: wind speed of about 0.836 times the force raised to the power of 1.5, in metres per second. The knot ranges and the metres-per-second ranges printed in reference tables are both roundings of the boundaries that relation implies, which is why neither column is perfectly tidy and why the two do not convert into each other digit for digit. If you are turning an m/s reading into a force number, take it to knots first and read the knot ranges, because those are what a marine forecast is quoting.

The factor is 1.9438, so doubling is the obvious shortcut and it runs 2.9 per cent high. 10 m/s doubles to 20 against a true 19.4; 15 doubles to 30 against 29.2; 20 doubles to 40 against 38.9. Taking three per cent off the doubled figure recovers almost all of it.

At the speeds forecasts are issued in, plain doubling is wrong by about a knot at 20 m/s and about a knot and a half at 25, and forecasts are rarely stated to better than five knots. So the shortcut is genuinely adequate for reading a bulletin against a model output. It stops being adequate the moment the number goes into a document, because a knot at the top of the range is the difference between a storm warning and a gale.

The wind group in a METAR carries its unit as a suffix, and that suffix is not always KT. Metres per second, written MPS, is a valid alternative used by Russia and a number of neighbouring states, and kilometres per hour is permitted as well. A group reading 27008MPS is a wind from 270 degrees at 8 metres per second, which is about 16 knots — twice what a reader who assumed knots would take from it.

This is the one place in aviation where the unit genuinely varies, and it varies in a format that is otherwise rigidly standardised. The direction is always in degrees true rounded to the nearest ten, the format is always the same length, and the unit is the field that changes. Anyone reading foreign weather regularly learns to look at the last three characters of the group before the number in front of them.

The conversion is not only about wind. Oceanographic models and current meters produce metres per second, while tidal atlases, pilot books and chart annotations use knots, and a navigator has to work in the second while the data arrives in the first. One metre per second is 1.94 knots, which is a serious current — enough to stop a slow displacement boat making progress against it.

The tidal case makes the size of the numbers concrete. A stream running at 0.5 m/s is a little under a knot and is a minor correction; at 1.5 m/s it is 2.9 knots and dominates the passage plan for a boat that cruises at six. Because the effect on progress is the difference between boat speed and current speed rather than a ratio, converting both into the same unit before subtracting is the only reliable order of operations.

Model output arrives with decimals that mean nothing to the reader of a bulletin. A grid point value of 14.37 m/s is 27.93 knots, and publishing 27.9 knots claims a skill that no forecast at that range possesses. Marine and aviation products are issued to the nearest knot at most, and forecast winds usually appear in five-knot steps or as a range such as 25 to 30.

The rule that survives contact with an audience is to convert at full precision and round once, at the point of publication, to the granularity the product uses. Rounding the m/s figure first and then converting introduces a second rounding for no benefit, and on a value sitting near 34 or 48 knots that extra step is exactly where a warning threshold gets crossed by arithmetic rather than by weather.