Convert km/h to kn

Kilometres per hour is the speed limit almost everywhere in the world. Dividing by 3.6 gives metres per second, because there are 3,600 seconds in an hour and 1,000 metres in a kilometre.

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.

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 kilometre per hour 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 speed of 30 km/h is nothing on a road — it is a residential limit, slower than a competent cyclist. At sea it is 16.2 knots of wind, which is the upper end of a fresh breeze and enough that a cruising yacht is thinking about reducing sail. That mismatch is the real content of this conversion: the arithmetic is easy and the recalibration is not.

The same applies to vessel speeds from the other end. A boat doing 20 km/h is doing 10.8 knots, which is fast for a displacement hull and unremarkable for anything planing. A tidal stream of 5 km/h is 2.7 knots and will dominate the passage of a small boat entirely. Nothing about the road-speed number signals how significant these are, because roads are two orders of magnitude more forgiving of speed than water is.

The true factor is 0.539957, so multiplying by 0.54 overstates by eight parts in a hundred thousand. At 100 km/h that is four thousandths of a knot. There is no realistic marine or aviation use in which the shortcut and the exact value would produce different decisions, which makes this the friendliest conversion in the whole speed category.

If 0.54 is awkward to multiply by, halve the figure and add eight per cent: 60 km/h gives 30 plus 2.4, so 32.4 knots, which is right. The anchors worth memorising are 10 km/h as 5.4 knots, 50 as 27, and 100 as 54, and everything else can be built from those three by addition rather than by multiplication.

A chartplotter can usually be set to display kilometres per hour, and doing so is a trap rather than a convenience. Every other number you will be working with stays in knots — the forecast, the tidal atlas, the pilot book, the port's traffic instructions, the speed limits inside a harbour — and a plotter reading 11 km/h next to a tidal diamond reading 2 knots is two units on the same problem.

The same is true in the air, more strictly, because the instruments are marked in knots and cannot be switched at all. A metric-country pilot flies indicated airspeed in knots, altitude in feet and fuel in litres, and converting the airspeed to something familiar would break the relationship between the number and the limit markings on the dial. The conversion belongs to the planning stage, not to the moment of use.

The wind speeds that change a decision on the water sit in a narrow band, and it is worth learning them in knots directly rather than reconverting each time. Ten knots is 18.5 km/h and is a pleasant sailing breeze. Sixteen to twenty knots — 30 to 37 km/h — is where many cruising boats take a first reef. Thirty-four knots, 63 km/h, is a gale.

Reading those in kilometres per hour makes them sound survivable, because 63 km/h is a speed most people have driven through a town at. That is exactly the misjudgement worth heading off. Boats and crews differ enormously and no table replaces experience of a particular hull, but the general shape holds: the interesting range for wind is roughly 5 to 40 knots, and the metric equivalents spread the same information across 9 to 74 with no extra resolution.

A current or tidal stream is subtracted from or added to a boat's speed, not applied as a percentage, so the units have to match before anything is done with them. A stream of 3.7 km/h is 2 knots. Against a boat making 6 knots that leaves 4 over the ground, which is a third of the progress gone — and it is the difference between two absolute numbers, which the conversion preserves and the intuition does not.

Working the whole calculation in knots is the only way to keep it legible, because chart distances are nautical and the tidal data is published in knots. Converting your speed into the unit the tide is already in, once, at the planning stage, means the passage arithmetic is a subtraction and a division with no factors in it at all.

A boat sold in a metric country can carry specifications in either unit or in both, and the choice usually follows the market rather than the vessel. Maximum speed on a European powerboat brochure often appears in kilometres per hour because that is what a road-going buyer understands, while the same builder's offshore models are quoted in knots. Engine data, hull speed calculations and any figure taken from a design study will be in knots or metres per second.

When you compare two boats, convert both into knots and compare there, because that is the unit the rest of the boating world will use for the whole of the vessel's life. A top speed advertised as 74 km/h is 40 knots; one advertised as 38 knots is 70 km/h. Written side by side in the same unit, the difference between them stops being a matter of how the brochure was written.

Sea kayaks, canoes and paddleboards live in the part of the scale where a road-speed intuition is least useful. A comfortable touring pace of 8 km/h is 4.3 knots. A fast sea kayak at 11 km/h is 5.9. Those look like nothing written in kilometres per hour and they are close to the limit of what a person can sustain for hours.

The tidal numbers are the reason to convert them. A stream running at 2 knots is 3.7 km/h, which against a 4.3-knot paddler leaves 2.3 knots over the ground — barely half the progress, from a current that a motor vessel would not notice. Tidal atlases and pilot books are in knots, so converting your own cruising speed into knots once, at the planning stage, is what makes the subtraction possible without a second conversion in the middle of a crossing.

This conversion is a transitional one. Somebody who works on the water for a season stops doing it, because the knot figures acquire their own meaning and translating them back into road speed loses information rather than adding it. The right goal is not a faster shortcut but a small set of anchors — a reefing wind, a hull speed, a tidal rate, a harbour limit — held directly in knots.

Until then, the number to keep is 0.54, and the direction to remember is downwards: the knot figure is always a little over half the kilometres-per-hour one. An answer larger than what you typed means the conversion ran the wrong way, and on a page where both units describe the same activities in overlapping ranges, direction is the only error that will not announce itself.