Convert kn to mph

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.

Miles per hour is the road speed of the United States and the United Kingdom, which is a rare pairing — the UK uses miles for road speed and metric for nearly everything else.

The factor is 1.150779, and almost nobody carries that around. Rounded to 1.15 it is off by 0.07 % — which stays invisible on small numbers and turns into a whole unit somewhere around 10,000 kn.

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 mile 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.

The mile per hour has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 0.868976242 kn to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is not the whole of it, though: this page stops at nine significant digits and the ratio carries on past them.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

The factor is 1.150779, which means adding fifteen per cent is right to seven parts in ten thousand. Twenty knots becomes 23 mph, forty becomes 46, sixty becomes 69, a hundred becomes 115. At a hundred knots the shortcut is off by seven hundredths of a mile an hour, which is smaller than the rounding in any bulletin that will ever quote it.

That makes this one of the few conversions on the site where the mental version and the exact version are interchangeable for practical purposes. There is no reason to reach for a calculator to read a marine forecast, and no benefit in carrying more digits than the forecast itself does — wind speeds are issued to the nearest knot at best and usually in five-knot bands.

The Saffir-Simpson category boundaries are 64, 83, 96, 113 and 137 knots. Published in miles per hour they are 74, 96, 111, 130 and 157 — and the second set is what almost every American reader has seen. The 74 mph figure that defines a hurricane is 64 knots rounded up by a third of a mile an hour.

The reason for the double life is that the National Hurricane Center works in knots and publishes for a public that does not. Advisories, forecast tracks and the wind fields behind them are computed in knots because that is the unit aviation reconnaissance and marine operations use; the public bulletins and the graphics carry miles per hour. Both numbers are official, and knowing which one is the original explains why one set is round and the other is not.

A sustained wind is an average, and the length of the average is a convention rather than a physical fact. The National Hurricane Center uses a one-minute mean. Most of the rest of the world, following World Meteorological Organization practice, uses a ten-minute mean, which for the same storm comes out roughly twelve per cent lower because the short average catches more of the peaks.

This matters more than the unit conversion it is usually mistaken for. A typhoon described as 105 knots by an American agency and 90 knots by a regional centre is not a disagreement about the weather; it is two different definitions of what "sustained" means. Converting both to miles per hour gives 121 and 104 and preserves the gap exactly. If two sources differ by around ten to fifteen per cent, the averaging period is the first thing to check and the unit is the second.

Marine and tropical forecasts quote a sustained wind and a gust, and the gust is typically around a third higher over open water — more over land, where terrain and buildings churn the airflow. A bulletin reading "50 knots gusting 70" is 58 mph gusting 81, and both figures should be converted rather than one being derived from the other.

The distinction matters for anyone acting on the number. Damage thresholds and category assignments key off the sustained wind, while the thing that actually removes a roof or capsizes a small boat is the gust. Reporting only the converted sustained figure to somebody making a decision understates the peak they will experience by roughly the gust factor, which is a larger error than any conversion on this page could introduce.

The unit conversion is easy and the intuition is not, because the mile-per-hour numbers are small by road standards and large by every other standard. Twenty knots is 23 mph, which is nothing in a car and is a fresh breeze that has most cruising sailors thinking about reefing. Thirty-four knots — 39 mph — is a gale. Forty-eight knots, 55 mph, is a storm.

Vessel speeds run the same way. A cruising yacht makes 6 knots, which is 6.9 mph, slower than a jogger. A container ship at 20 knots is doing 23 mph, and a fast ferry at 35 knots is doing 40. Modern foiling race boats exceed 50 knots, or 58 mph, which is the point at which a boat is genuinely moving at road speed and is treated as remarkable for exactly that reason.

American marine forecasts are issued in knots throughout, including the advisory levels that most recreational boaters actually act on. A small craft advisory typically covers sustained winds in the region of 22 to 33 knots, which is 25 to 38 mph; a gale warning starts at 34 knots, or 39 mph; a storm warning at 48 knots, 55 mph. Exact advisory criteria vary by region and the local forecast office states them.

The mile-per-hour translations are worth keeping alongside the knot figures rather than instead of them, because the forecast will always be in knots and the intuition will always be in miles per hour. Writing both on the same line of a passage plan removes the conversion from the moment when it is most likely to be done badly, which is the moment the weather has already changed.

Surface and upper-air charts encode wind as a barb rather than a number, and the barb counts knots. A short half-barb is 5 knots, a full barb is 10, and a solid triangular pennant is 50; the symbols are added together, so a pennant plus two full barbs is 70 knots. The shaft points in the direction the wind is coming from and a bare circle means calm.

That means every chart of this kind has to be converted before it means anything to a reader who thinks in miles per hour. A pennant with two barbs is 81 mph. Three full barbs is 30 knots, or 35 mph. The counting is in fives and tens because knots are what the observation network reports, and no American chart of this type is drawn in miles per hour — the unit is built into the notation rather than printed alongside it.

The mile-per-hour figure is always the larger one, by about a seventh. If your result came out below the knot value, the conversion ran backwards, and it is the easiest mistake to make on a page where the two numbers are close enough to look interchangeable.

That closeness is also the reason the error is expensive in one direction only. Reading a 64-knot warning as 64 mph understates the wind by fifteen per cent and puts a hurricane-force forecast below the hurricane threshold. Reading an mph figure as knots overstates it by the same fraction, which is merely cautious. When in doubt about which unit a source used, the safe assumption for a marine or aviation product is knots.