How this instrument works
Sailors named this unit after actual knots. Weighted wooden chips went over the stern on lines tied at intervals of roughly 47 feet 3 inches, the 28-second sand glass was turned, and one hand counted knots slipping through his fingers until sand ran out. Eight knots counted meant eight nautical miles in an hour, and that counting method quietly became this unit of speed.
Both ends of our conversion are legal definitions rather than measurements. The First International Extraordinary Hydrographic Conference at Monaco fixed one nautical mile at exactly 1852 metres in 1929; America adopted that figure in 1954 and Britain in 1970, retiring an Admiralty mile of 6080 feet. One statute mile became exactly 1609.344 metres under the 1959 international yard and pound agreement. Divide one by the other — 1852 ÷ 1609.344 — and out comes 14468750⁄12573, an ordinary fraction whose decimal form 1.150779448023… runs forever yet carries no uncertainty whatever.
Knots survive because charts are drawn in latitude: one minute of arc measured up the side scale of any chart is one nautical mile, so distance and speed read straight off paper with no arithmetic between them. Aviation and shipping keep knots worldwide under ICAO and IMO practice, and tropical cyclone advisories are issued in knots before being restated for broadcast. Miles per hour, by contrast, rules road signs across America and Britain, plus a short list of smaller jurisdictions — which is how 64-knot storms reach television audiences as 74 mph hurricanes.
- Type your speed into Knots (kn). The preloaded 20 kn — a fast reach under sail — gives you somewhere to start.
- Read Miles per hour (mph) directly beneath it. Results recompute on each keystroke, so nothing needs pressing.
- Fractional readings are welcome: steps of 0.001 kn cover log entries such as 7.5 kn without forcing a round number.
- Working backwards from mph? Multiply your figure by 0.868976242, or divide by 1.15077944802 — identical operations.
- Reading a forecast, remember that a 34 kn gale warning and a 39 mph wind describe one and the same air.
Worked example — logging 20 knots under sail
A delivery skipper watches her log settle at 20 kn on a broad reach and wants one number her shore crew can picture. Typing 20 into Knots (kn) returns 23.0155889605 in Miles per hour (mph). Call it 23.02 mph — about what cyclists hold on flat tarmac, and surprisingly quick for forty tonnes of hull.
Check it by hand and no stored factor is needed. Twenty nautical miles is 20 × 1852 = 37,040 metres; divide by 1609.344 metres per statute mile and 23.0155889… statute miles is what she covers in that hour. Same digits, reached from raw definitions rather than from a constant — which is precisely what a converter ought to let you verify.
Questions
Is 1.15077944802 an exact conversion factor?
Every digit shown is exact; expansion simply stops there. Both units rest on definitions — a nautical mile of exactly 1852 metres, a statute mile of exactly 1609.344 metres — so their ratio equals 14468750⁄12573, an ordinary fraction whose decimal form 1.150779448023… never terminates and never wobbles. Nothing here was measured, so nothing carries uncertainty. Only display gets rounded.
What is wrong with saying 'knots per hour'?
A knot already contains an hour. It means one nautical mile per hour, so 'knots per hour' would describe nautical miles per hour per hour — acceleration, not speed. Ten knots is simply ten knots. That slip turns up in novels and in press releases alike, and sailing instructors tend to correct it within a student's first afternoon afloat.
Can I just multiply by 1.15 in my head?
For mental arithmetic, yes. Adding 15% lands within 0.07% of the true figure, finer than most instruments resolve: at 20 kn that shortcut gives 23.00 mph against 23.02 mph properly computed. Gaps only start to bite at aviation speeds. An airliner cruising at 470 kn is 540.87 mph, while 15% gives 540.50 — nearly four tenths of a mile per hour adrift, which matters if you are reconciling flight-plan figures.
Does imperial versus US customary change this answer?
Not any more. Britain and America share an international statute mile of 1609.344 metres, fixed by a 1959 agreement, so mph means one thing on either side of an ocean. History was messier at sea: Britain measured sea distance in Admiralty nautical miles of 6080 feet until 1970, America in units of 6080.20 feet until 1954. Either shifts this conversion by roughly 0.06% — irrelevant today, worth knowing when reading an old logbook.
Why does 64 knots appear as 74 mph in hurricane scales?
Because thresholds were rounded within each unit rather than converted digit for digit. Strictly, 64 kn is 73.65 mph. Saffir–Simpson category one nevertheless begins at 74 mph, and tropical-storm force sits at 34 kn against 39 mph when 39.13 mph is truer. Forecasters prefer round numbers in every scale they publish, so a bulletin checked against this page will disagree by a fraction of a mile per hour at category boundaries. That is presentation, not error.
How do knots relate to kilometres per hour?
Multiply by exactly 1.852, which falls straight out of a nautical mile's 1852-metre definition with no rounding at all. That tidiness explains why most national marine forecasts pair knots with km/h and leave mph aside. Chain both conversions as a check: 1.852 × 0.621371192237 — that second figure being one kilometre expressed in statute miles — returns 1.150779448023, matching every digit shown here.