Convert mph to knots miles per hour ↔ knots Free Online Secure
Type a value in miles per hour and read it in knots at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Convert from miles per hour
Common values — Convert mph to knots (miles per hour ↔ knots)
| mph | kn |
|---|---|
| 1 | 0.868976241901 |
| 2 | 1.7379524838 |
| 3 | 2.6069287257 |
| 5 | 4.3448812095 |
| 10 | 8.68976241901 |
| 20 | 17.379524838 |
| 50 | 43.448812095 |
| 100 | 86.8976241901 |
Where these units come from
Speed has no unit of its own: it is a length divided by a time, so every trade simply kept the length it was already using. That is why the same motion is 100 km/h on a European road sign, 62 mph on an American one and 27.8 m/s in a physics problem. The knot is the oldest of them and the most literal: sailors paid out a line with knots tied at even spacing and counted how many ran through their fingers while a sand glass emptied, so the speed was read off in knots. One knot is one nautical mile per hour, and a nautical mile is a minute of latitude — which is why ships and aircraft still use it. The speed of light is the odd one out: since 1983 it is not measured but fixed at exactly 299,792,458 m/s, because the metre is now defined from it.
Where you meet them
Kilometres and miles per hour live on speedometers and road signs, metres per second in physics, engineering and wind reports, feet per second in ballistics and in American machine shops. Knots carry shipping, aviation and marine weather, and Mach numbers appear wherever an aircraft is fast enough for the air's own stiffness to matter. The speed of light turns up less in daily life than in distances: a light year is simply how far it gets in one.
Questions people ask
Is Mach 1 a fixed speed?
No. Mach is a ratio to the speed of sound in the surrounding air, and sound travels more slowly in colder air. This page uses the standard sea-level figure, 340.294 m/s, so Mach 1 here is about 1225 km/h. At the altitude airliners cruise at, the air is some 50 degrees colder and the same Mach 1 is roughly a tenth slower. An aircraft can therefore hold a constant Mach number while its speed over the ground changes.
Why do ships and aircraft use knots rather than km/h?
Because a nautical mile is one minute of arc of latitude. A vessel making one knot covers one minute of latitude in an hour, so speed, time and position on a chart fit together without conversion. The same reasoning kept the nautical mile at exactly 1852 metres rather than rounding it to something tidier.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Vintage airplanes and flight planning: from mph to knots
The airspeed indicator of an old Piper Cub or Cessna 150 is calibrated in miles per hour, and so is the pilot's operating handbook written before the mid-1970s. Modern flight planning, air traffic control and weather reports all run in knots. A pilot of a vintage aircraft therefore translates constantly between the two. To convert mph to knots, multiply by 0.869: 75 mph is 65.2 knots, 100 mph is 86.9 knots and 140 mph is 121.7 knots. The calculator above multiplies as you type, and the sections below show how the numbers matter for cruising, landing, fuel and wind.
You will find the rule with worked examples, a short profile of each unit, a comparison, a long set of cases from flying and the slips to avoid. The arithmetic runs in your browser, and what you type is not sent anywhere. For flight, rely on the aircraft's own handbook and on official data.
How to convert mph to knots: multiply by 0.869
The rule
A statute mile is 1,609.344 metres and a nautical mile is 1,852 metres. Their ratio is 0.869. To convert mph to knots, multiply the mph figure by 0.869. So 55 mph is 47.8 knots, 120 mph is 104.3 knots and 200 mph is 173.8 knots. To go back, multiply knots by 1.1508.
Worked examples
A landing speed of 60 mph is 52.1 knots. A cruise speed of 110 mph is 95.6 knots. A maximum speed of 175 mph is 152.1 knots, and a never-exceed speed of 230 mph is 199.9 knots.
Why 0.869
The nautical mile is about 15 per cent longer than the statute mile, so the same speed has a smaller number in knots. The factor is exact, so no rounding enters the conversion.
A quick sanity test
The knots figure is always 13 per cent smaller than the mph figure. If a “convert mph to knots” calculation result is bigger than the starting number, you used 1.1508 instead.
The mile per hour
The unit on older instruments
Older general aviation aircraft have airspeed indicators in miles per hour, and their handbooks list speeds in the same unit. Some pilots of vintage aircraft still use mph in conversation, especially in the United States.
Where it appears
Vintage aircraft, early handbooks and some flying club notes.
The knot
The unit of modern aviation
The knot is the standard unit in aviation worldwide. Air traffic control, weather reports, charts and flight planning use it, and aircraft built after the 1970s have indicators and handbooks in knots. An E6B flight computer handles the arithmetic in nautical miles and knots.
Where it appears
Air traffic control, METARs, charts and modern aircraft.
Miles per hour against knots
One knot is 1.151 mph, so a speed in knots is a smaller number. A Cessna 150 cruising at 100 mph is at 87 knots, and in an hour covers 87 nautical miles, which is 100 statute miles. Charts use nautical miles, so the pilot who mixes mph with a chart distance gets the wrong time by 15 per cent.
The factor is exact, so no rounding enters. The only trap is direction: from mph to knots the number falls.
Where the conversion shows up
Cruise and stall speeds
A Piper J-3 Cub cruises at about 75 mph, which is 65 knots, and stalls at around 40 mph, or 35 knots. A Cessna 150 cruises at 100 mph, or 87 knots, and a Cessna 172 of the 1960s at 125 mph, or 109 knots. A Beechcraft Bonanza reaches 175 mph, or 152 knots. A “convert mph to knots” calculation tells how an old handbook compares with a modern one.
Airspace speed limits
In the United States, aircraft are limited to 250 knots, which is 288 mph, below 10,000 feet, and to 200 knots, which is 230 mph, below 2,500 feet within four nautical miles of a busy airport. These limits are written in knots. A pilot of an older aircraft whose indicator reads mph must convert to be sure of compliance.
Planning time and fuel
A 150 nautical mile trip at 87 knots takes 103 minutes. The same trip, expressed as 172 statute miles at 100 mph, also takes 103 minutes. The wind aloft is given in knots, and a 20-knot headwind, which is 23 mph, reduces the ground speed of a 87-knot aircraft to 67 knots, or 77 mph.
Handbooks before and after 1976
In the mid-1970s, the industry association for general aviation manufacturers standardised the pilot's operating handbook, and airspeeds moved from mph to knots. Aircraft built before then may have mph on the dial and in the manual, while newer ones show knots. Buying a used aeroplane often means working out which, and a “convert mph to knots” calculation for each speed sets everything on one scale.
Helicopters
An older helicopter might have an mph indicator, with a cruise of 100 mph, which is 87 knots. Never-exceed speeds are marked with a red line, and a typical value is 130 mph, or 113 knots. Rotorcraft carry out tasks at low speeds, such as hovering and slow flight at 20 mph, or 17 knots, where the units matter less than the control inputs.
Aerobatics and the manoeuvring speed
Manoeuvring speed is the highest speed at which full control deflection will not damage the aircraft. For a light aircraft it may be 110 mph, which is 96 knots. It falls with weight. Aerobatic aircraft show it on the indicator, and a pilot flying in a competition abroad checks that the unit matches the rules of the organisers.
Air traffic control instructions
Controllers ask for speeds in knots: "reduce speed to 160 knots". For a pilot with an mph indicator, that is 184 mph. A pilot of a slow aircraft might say "unable, my cruise is 90 knots", the equivalent of 104 mph. Clear communication depends on using the same unit as the controller, so a “convert mph to knots” calculation is made before the call.
Weather reports and winds aloft
Wind aloft forecasts give direction in tens of degrees and speed in knots. "27035" means 270 degrees at 35 knots, which is 40 mph. A pilot planning a flight at 6,000 feet with a 100 mph aircraft needs the wind in the same unit as the airspeed. Converting the forecast to mph or the airspeed to knots gives the same result.
The E6B and the plotter
The E6B flight computer, a circular slide rule used by pilots for decades, works in any consistent unit. It is easiest with nautical miles and knots to match charts. A pilot with an mph indicator may either use statute miles with the computer or convert the airspeed to knots first. The second route avoids confusion with chart distances.
Spreadsheet formulas
Place speeds in mph in column A and use =A2*0.868976 in column B. For the reverse, use =B2*1.150779. Add a column for time over a distance in nautical miles, =distance/B2*60 for minutes. Label every column with its unit and use one decimal.
Rounding and precision
The factor is exact: 100 mph is 86.898 knots. Pilots round to whole numbers, so 87 knots is fine. For a stall speed, rounding up is the safer direction. A handbook that lists 54 mph for the stall is 47 knots when rounded, and 46.9 when exact, so 47 is the figure to remember.
A mental shortcut
Take away 13 per cent. For 100 mph: 13 less gives 87 knots. For 140 mph: 18 less gives 122 knots. For 60 mph: 8 less gives 52 knots. The shortcut is accurate to within half a knot, which is plenty for cockpit use.
Landmarks to memorise
50 mph is 43 knots, 75 mph is 65 knots, 100 mph is 87 knots, 125 mph is 109 knots, 150 mph is 130 knots and 200 mph is 174 knots. These landmarks help a pilot of a vintage aircraft interpret any figure in a modern chart or report.
Reference ladder
40 mph is 34.8 knots. 55 mph is 47.8 knots. 70 mph is 60.8 knots. 90 mph is 78.2 knots. 120 mph is 104.3 knots. 160 mph is 139 knots. 230 mph is 199.9 knots. 288 mph is 250 knots. A ladder like this speeds up a quick check.
The airspeed indicator's colour arcs
The coloured arcs on an indicator mark the safe ranges. The white arc is the flap operating range, the green arc normal operation, the yellow arc caution, and a red line the never-exceed speed. On an old indicator, a white arc from 40 to 85 mph is 35 to 74 knots. A green arc from 50 to 140 mph is 43 to 122 knots. A red line at 175 mph is at 152 knots.
Checking by reversing
To check any convert mph to knots result, multiply it by 1.1508. If 100 mph became 86.9 knots, then 86.9 times 1.1508 is 100. A second test is the size: the knots number is about seven eighths of the mph number.
Relation to km/h
One knot is 1.852 km/h and one mph is 1.609 km/h. An aircraft at 100 mph is at 161 km/h and 87 knots. European pilots use km/h in gliders and knots in powered aircraft, and a vintage aircraft imported from the US needs a new indicator or a conversion plate.
Flying across borders
An American pilot flying an older aircraft in Canada or Mexico finds that the flight plan and radio use knots, although road signs are in km/h. Fuel is sold in litres in many countries, so distance, speed and fuel all change units. A simple card with conversion factors in the cockpit avoids errors during a busy approach.
A worked example
A flight of 210 nautical miles in an aircraft that cruises at 110 mph, which is 95.6 knots, with a 15-knot headwind, which is 17 mph, has a ground speed of 80.6 knots. The trip takes 156 minutes, or 2 hours and 36 minutes. A “convert mph to knots” calculation on the cruise speed, a subtraction and a division give the answer.
Pitot-static errors and calibration
An airspeed indicator reads the difference between pressure at the pitot tube and the static port. Small errors arise from the position of the ports and from the instrument itself, so the handbook often has a calibration table that turns indicated speed into calibrated speed. In an old aircraft the table is in mph. A pilot who wants to use it with modern knots data should first apply the table and then convert mph to knots, in that order, because the table was drawn up for the original unit and its steps are not the same size as one knot.
Restoring a vintage aircraft
When owners restore an old aeroplane, one decision is whether to keep the original mph indicator or fit a knots one. Purists keep the original face and add a small conversion placard. Others replace the instrument and re-mark the arcs: a green arc that began at 50 mph now begins at 43 knots, and a red line at 122 mph moves to 106 knots. Whatever the choice, the placard should state the unit in large letters so that a passenger or a future owner is not confused.
Famous speeds in mph
Historic flying is described in mph. The Wright Flyer of 1903 made about 30 mph, which is 26 knots. Lindbergh's Spirit of St. Louis cruised at about 107 mph, or 93 knots, across the Atlantic. The unlimited racers at the Reno air races pass 500 mph, which is 434 knots. Converting such figures lets a reader compare a century of aviation on the scale used by today's pilots, who hardly ever see an mph figure anywhere in the cockpit.
Descent planning on a three-degree path
A three-degree glide path is the standard for instrument approaches. The descent rate in feet per minute is roughly five times the ground speed in knots. At 87 knots, which is 100 mph, the figure is about 435 feet per minute, while the exact trigonometry gives 462. At 65 knots, which is 75 mph, it is about 325 feet per minute. The rule is built for knots, so a pilot with an mph indicator converts first, and only then reads the rate off the chart.
Maximum structural cruising speed
The top of the green arc is the maximum structural cruising speed. Above it, flight is allowed only in smooth air, since a strong gust could overload the airframe. For an older light aircraft it may be 140 mph, which is 122 knots, and the never-exceed speed 175 mph, which is 152 knots. The gap between them, 35 mph or 30 knots, is the yellow arc. Knowing the width of that arc in knots helps in turbulence reports, which are issued in the same unit.
Flying club check-outs
When a pilot joins a flying club and is checked out on an older aircraft, the instructor lists the speeds to memorise: rotation, best climb, approach and best glide. If the club owns both a modern aircraft in knots and a vintage one in mph, the same pilot has to switch between two sets of numbers within a week. A card listing both columns side by side, produced with a “convert mph to knots” calculation for each row, saves the pilot from guessing at the moment of rotation.
Calibrated and true airspeed
Indicated airspeed differs from true airspeed, especially at altitude. At 8,000 feet the true speed is about 15 per cent higher than the indicated speed. A Cessna 172 showing 100 knots at that altitude flies at roughly 115 knots true. The conversion of units comes first, and the correction for altitude second.
Best glide and engine failure
Best glide speed gives the longest distance without power. For a small trainer it may be 70 mph, which is 61 knots. After an engine failure the pilot has a few seconds to set that speed and pick a field. Nobody should be doing arithmetic then, so the figure is learned in the unit shown on the instrument. The conversion is done on the ground, long before, and written on the checklist next to the original number.
Ground speed and wind
Ground speed is true airspeed plus or minus the wind component. With a tailwind of 17 mph, which is 14.8 knots, an aircraft at 100 mph, or 86.9 knots, has a ground speed of 117 mph, or 101.7 knots. Weather reports for pilots give wind in knots, so the pilot of an old aircraft converts the mph indicator to knots and then adds the wind.
The wind triangle
The wind triangle gives the heading to fly and the resulting ground speed. A crosswind of 20 knots, which is 23 mph, from 90 degrees to the course, on an aircraft at 100 knots, requires a correction angle of about 11.5 degrees. The unit is the same for aircraft and wind in the calculation, so convert mph to knots before drawing the triangle. A “convert mph to knots” calculation first avoids a wrong heading.
Fuel burn and range
A small aircraft that burns 6 gallons an hour at 100 mph, which is 87 knots, covers 87 nautical miles per hour, or 14.5 nautical miles per gallon. With 30 gallons usable, the range is 435 nautical miles, which is 500 statute miles, less the reserve. The gallon and the nautical mile belong together in the calculation, and the pilot must not mix in statute miles.
Landing speeds
A light aircraft lands at about 1.3 times its stall speed. A stall at 45 mph, or 39 knots, gives an approach at 58 mph, or 51 knots. The runway distance needed grows with the square of the approach speed, so a 10 per cent error in speed is a 21 per cent error in the landing roll. Pilots memorise approach speeds in whichever unit the indicator uses.
Take-off distance
Take-off distance also grows with the square of speed. An aircraft that lifts off at 60 mph, or 52 knots, needs about 19 per cent more runway than one at 55 mph, or 48 knots. At high altitude airports on hot days, true speed at lift-off is higher, so the distance grows more. The numbers appear in the handbook in the aircraft's own unit.
Gliders and sailplanes
Gliders have indicators in knots, kilometres per hour or miles per hour, depending on the country and the age of the aircraft. A thermalling speed of 45 mph is 39 knots, and a cross-country cruise of 90 mph is 78 knots. A vintage American glider with an mph indicator flown in Europe, where flight plans and radio are in knots, needs a conversion card on the panel.
More values for convert mph to knots
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 mph = 0.0868976 kn
- 0.5 mph = 0.434488 kn
- 3 mph = 2.60693 kn
- 4 mph = 3.4759 kn
- 7 mph = 6.08283 kn
- 15 mph = 13.0346 kn
- 20 mph = 17.3795 kn
- 75 mph = 65.1732 kn
- 200 mph = 173.795 kn
- 500 mph = 434.488 kn
- 2,000 mph = 1,737.95 kn
- 10,000 mph = 8,689.76 kn
Common slips when you convert mph to knots
Multiplying by 1.1508
From mph to knots the number falls. If 100 mph became 115, you used the factor for the other direction.
Mixing statute and nautical miles
A speed in knots needs distances in nautical miles, and a speed in mph needs statute miles. A mismatch gives a time that is 15 per cent wrong.
Confusing indicated and true airspeed
Converting the unit does not convert the kind of airspeed. At altitude, true speed is higher than indicated.
Short answers
How do I convert mph to knots?
Multiply by 0.869.
What is 100 mph in knots?
86.9 knots.
What is 60 mph in knots?
52.1 knots.
How many mph is 1 knot?
1.151 mph.
Does my number leave my device?
No, the calculation runs in your browser.
More pages and a wrap-up
For other speed conversions see Convert mph to Mach, Convert mph to ft/s, Convert km/h to knots, Convert m/s to knots, Convert knots to m/s, Convert km to mi, Convert mi to km, Convert °C to °F, Convert °F to °C. The reverse direction is Convert knots to mph.
In short, a “convert mph to knots” calculation means multiplying by 0.869 and writing the unit beside the answer. Your number is computed locally, with no account and no upload.