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Speed Converter

A speed converter for km/h to mph, m/s, knots to mph and Mach.

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Runs entirely in your browser. What you enter stays on your device — nothing is sent to a server. Verify it yourself

Is Mach 1 always the same speed?

No. Mach is a ratio to the local speed of sound, which falls with temperature and therefore with altitude. Mach 1 is about 1,225 km/h at sea level and roughly 1,062 km/h at cruising altitude.

Cost
Free, no limits
Upload
None — runs on your device
Sign-up
Not required
Works offline
Yes, once loaded

How to convert km/h to mph

  1. Enter the value

    Type the number into the Value box; the result updates as you type.

  2. Choose the units

    Pick From and To — kilometres per hour and miles per hour to begin with, or metres per second, knots or Mach at sea level. The swap button between them flips the direction.

  3. Read every unit at once

    The All units table below the result shows the value in every speed unit.

Examples

  • 100 km/h62.14 mph · 27.78 m/s

    A motorway speed limit in three systems.

  • 10 knots18.52 km/h

    A knot is one nautical mile per hour.

Key features of the Speed Converter

  • Converts between 6 speed units
  • Converts as you type, in both directions
  • Metric and imperial in the same list
  • Runs entirely in your browser

Supported formats

Units

  • Metres per second
  • Kilometres per hour
  • Miles per hour
  • Feet per second
  • Knots
  • Mach (at sea level)

When should you use this?

  • Reading a speed limit abroad.
  • Converting a pace or a wind speed.
  • Working with marine or aviation figures given in knots.

Limitations

  • Conversions go through a base unit, so a value converted from one unit to another and back can differ in the last decimal place. That is floating-point arithmetic, not a defect, and it is far below the precision of any real measurement.
  • Results are displayed rounded for readability. If you need a specific number of significant figures, round the displayed value yourself rather than assuming the display is exact.
  • Converting a measurement cannot make it more precise than it was. A length measured to the nearest centimetre is still that precise after conversion, however many decimal places appear.
  • Pace — minutes per kilometre or mile — is the inverse of speed and is not converted here. Doubling your pace halves your speed.

Troubleshooting

The answer disagrees with another converter

Usually a different definition of the same unit name — US versus imperial, or a short versus a long ton. Check which variant each tool means before assuming one is wrong.

The last decimal place changes on a round trip

Conversion factors are not exact in binary floating point. The error is around one part in ten thousand billion, which no physical measurement can detect.

Common mistakes to avoid

  • Assuming a unit name is unambiguous. It very often is not.
  • Rounding partway through a multi-step calculation instead of at the end.

Pro tips

Tips for better results

  • US and UK units

    The same unit name can mean two amounts

    Check which system a unit belongs to before trusting it. Several unit names mean different amounts in the US and the UK.

  • Working method

    Write the unit beside every number

    Carry the unit through your working rather than converting at the end. Most unit errors happen when a number is written down without one.

  • Marine and air

    Knots are nautical miles per hour, about 15% off mph

    Knots are nautical miles per hour and are standard in marine and aviation contexts. Using mph there will put you about 15% out.

Standards and terms behind the Speed Converter

The named things this tool works with, and the documents that define them.

SI unitsSpecification
The international system of units, maintained by the BIPM, which fixes the metre, kilogram, second and kelvin. Reference: SI units
Imperial and US customary unitsSpecification
Inches, feet, pounds, gallons and Fahrenheit — still standard in the United States and common in the UK.

Also searched as: kmh to mph, knots to mph, pace to speed.

The units and where they are used

km/h is the road-speed unit almost everywhere. mph is used on roads in the US and UK. m/s is the SI unit and what physics calculations use.

Knots — nautical miles per hour — are standard in marine and air navigation. The reason is practical: a nautical mile is one minute of latitude, so a speed in knots translates directly into position change on a chart.

Mach is not a fixed speed

Mach is a ratio to the local speed of sound, which depends on air temperature — and therefore on altitude. At sea level and 15 °C, Mach 1 is about 340 m/s (1,225 km/h). At cruising altitude, around −55 °C, it drops to roughly 295 m/s (1,062 km/h).

The conversion above uses the sea-level figure and says so. An aircraft flying at Mach 0.85 at altitude is moving considerably slower than 0.85 × the sea-level value.

A quick mental conversion

To go from km/h to mph, multiply by 0.6 — good to within 4%. From mph to km/h, multiply by 1.6. And m/s to km/h is exactly ×3.6, which is worth memorising if you read physics results.

Privacy

The Speed Converter runs entirely in your browser. What you enter and the result it produces stay on your own device — nothing is sent anywhere, because nothing needs to be.

You can confirm this rather than take our word for it: open your browser’s developer tools, watch the Network tab, and use the tool. No request carries your input. It also keeps working with the connection off.

Details are in the privacy policy.

More unit converters

What next?

For the distance part of a speed calculation.

Convert distances

Speed Converter — questions

How do I convert km/h to mph?

Multiply by 0.621371. For rough mental work, multiplying by 0.6 is within about 4%.

What is a knot?

One nautical mile per hour, about 1.852 km/h. It is standard at sea and in the air because a nautical mile is one minute of latitude, so knots map directly onto chart position.

Is Mach 1 always the same speed?

No. Mach is a ratio to the local speed of sound, which falls with temperature and therefore with altitude. Mach 1 is about 1,225 km/h at sea level and about 1,062 km/h at cruising altitude. This converter uses the sea-level value.

How do I convert m/s to km/h?

Multiply by exactly 3.6. It is worth memorising — physics results come in m/s and speed limits do not.

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