Speed Converter

How to use this calculator#

1. 1
   Enter the speed in the unit it was quoted inA dashboard, a road sign and a weather forecast can all use different units in the same country. Marine and aviation forecasts are in knots, road signs in mph or km/h, and physics problems in m/s.
2. 2
   Pick the target unit and read the resultAll six units update from the same base of metres per second, so the arithmetic is a single ratio and no offset is involved — unlike temperature, speed scales share a zero.
3. 3
   Round conservatively against a legal limitA 70 mph limit is 112.65 km/h. Rounding down to 112 km/h keeps you legal; rounding up to 113 does not. Do the reverse for a minimum speed.
4. 4
   Convert pace separatelyMinutes per kilometre or per mile is the reciprocal of speed, not a multiple of it. Convert the pace to a speed first, convert the speed, then invert again — never apply a speed factor directly to a pace.

The formula#

Converting km/h, mph and m/s#

Every speed conversion reduces to a length factor divided by a time factor. Because a mile is exactly 1.609344 km, 1 mph equals 1.609344 km/h and 1 km/h equals 0.621371 mph. Metres per second is the SI unit, and the relationships are exact there too: 1 m/s is precisely 3.6 km/h, and 1 mph is precisely 0.44704 m/s.

Common road speeds converted: 30 mph = 48.3 km/h, 50 mph = 80.5 km/h, 60 mph = 96.6 km/h and 70 mph = 112.7 km/h. In the other direction, 50 km/h = 31.1 mph, 100 km/h = 62.1 mph and 130 km/h = 80.8 mph. A workable mental shortcut is to take a bit more than a third off a km/h figure to approximate mph.

Knots, Mach and specialist units#

A knot is one nautical mile per hour, or exactly 1.852 km/h, and it remains standard in aviation and shipping because a nautical mile equals one minute of latitude — so a speed in knots maps straight onto a navigation chart. An airliner cruising at 500 knots is travelling 926 km/h or 575 mph, and marine wind forecasts are quoted in knots for the same navigational reason.

Mach is a ratio rather than a fixed speed: it compares an object's speed to the local speed of sound, which drops as air gets colder. At sea level in standard conditions sound travels at about 340.29 m/s, or 1,225 km/h, but at an 11,000 m cruising altitude it falls to roughly 295 m/s — so the same Mach number corresponds to a slower ground speed up there.

Pace, speed and rounding safely#

Runners and cyclists usually think in pace — minutes per kilometre or per mile — which is the reciprocal of speed rather than a multiple of it. A 12 km/h speed is a 5:00 per km pace, and a 4:00 per km pace is 15 km/h or 9.32 mph. Halving your speed doubles your pace instead of halving it, so convert to speed first and only then invert.

Rounding matters most around legal limits. A 70 mph limit is 112.65 km/h, which speedometers usually show as 113 km/h; rounding down to 112 is safe while rounding up to 115 is not. The familiar 100 km/h limit is 62.14 mph, so a 62 mph reading complies but 63 does not. Always round conservatively when converting any kind of enforced limit.

Worked examples#

Reference tables#

Common mistakes#

• Applying a speed factor to a pacePace is time per distance, so it scales the opposite way to speed. A 5:00/km pace becomes 5:00 × 1.609344 = 8:03 per mile. Apply the speed factor 0.621371 instead and you get 3:06 per mile, a world-record pace out of a club run. Averaging goes wrong for the same reason: run 4:00/km and 6:00/km for equal amounts of time and your overall pace is 4:48/km, not the 5:00 the two numbers suggest.
• Rounding a converted limit upward100 km/h is 62.14 mph, so 62 mph complies and 63 does not. Using the sloppy 'multiply by 0.6' shortcut gives 60 mph, which is legal but slow; using 'multiply by 0.65' gives 65 mph, which is a ticket. Always round toward the safe side of an enforced limit.
• Treating Mach as a fixed speedThe speed of sound depends on air temperature, not on pressure or altitude directly. Mach 1 is 1,225 km/h at sea level but about 1,062 km/h at cruising altitude, a 13% difference. A Mach figure without a stated atmosphere is incomplete.
• Confusing knots with 'knots per hour'A knot already means one nautical mile per hour, so 'knots per hour' would be an acceleration. It is not just pedantry: in a navigation log the unit error compounds every leg of a passage plan.

Frequently asked questions#

Key terms#

Metre per second (m/s)

The SI unit of speed, exactly 3.6 km/h. Used in physics, engineering, wind measurement and athletics timing because it combines cleanly with SI units of acceleration and force.

Knot

One nautical mile per hour, exactly 1.852 km/h. Standard in aviation and at sea because a nautical mile is one minute of latitude, so knots translate directly into chart position.

Mach number

The ratio of an object's speed to the local speed of sound. Dimensionless, so it has no fixed value in km/h — it changes with air temperature and therefore with altitude and weather.

Pace

Time taken per unit distance, such as minutes per kilometre. The reciprocal of speed: as speed rises, pace falls. Runners and swimmers use pace; vehicles use speed.

Ground speed versus airspeed

Airspeed is measured relative to the air an aircraft is moving through; ground speed is measured relative to the earth. A tailwind makes ground speed higher than airspeed, which is why flight times differ in each direction.

Sources#