Speed Converter

Convert between km/h, mph, knots, metres per second and Mach, a line at a time.

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What this tool does

Speed Converter works from a fixed table of seven units: metres per second, kilometres per hour, miles per hour, feet per second, knots, Mach and the speed of light. Six of the seven carry an exact defined factor rather than a rounded approximation — a knot is one nautical mile per hour, which is 1.852 km/h exactly, and a mile per hour is exactly 0.44704 m/s — so every conversion is one multiplication through metres per second and back out again. Mach is the exception, and the paragraph below explains what it is standing in for.

Each line of the input box is treated as its own conversion. A bare number such as 100 uses the From and To dropdowns; a line that names its own units, like 30 mph to m/s, overrides both, so a pasted column of mixed readings converts in a single pass without touching the settings. Thousands separators written as commas, full stops or apostrophes are understood, as are simple fractions, and precision is either automatic — ten significant digits with trailing zeros trimmed, or eight where the result is small or large enough to print in exponential form — or a fixed number of decimals. Results come back as a table rather than as prose, and setting the target to All units prints a single value in every unit at once.

Two limits are worth knowing before you trust a figure. Mach is fixed at 340.29 m/s, the speed of sound in air at sea level and 15 °C, and that number does not change with the conditions you have in mind; the real speed of sound falls as the air gets colder, so a Mach number from a cruising aircraft converts here to a sea-level equivalent rather than a true airspeed. The second limit is scope: this converts velocity only, so pace units such as minutes per kilometre — the reciprocal of a speed, not a speed — are absent, and a unit belonging to another quantity is refused rather than guessed at. Nothing is uploaded either way; the table and the arithmetic both live in the page, and the numbers you type stay in the tab.

How to use it

  1. Type the values. One per line. Paste a whole column if you have one — each line is converted separately and the results come back in the same order.
  2. Set From and To, or name the units inline. Both dropdowns list all seven units. Writing the units on the line itself, as in 1 knot to km/h, takes precedence over whatever is selected.
  3. Choose the precision. Automatic gives ten significant digits with trailing zeros removed, and eight once the answer is printed in exponential form. The fixed options are 0, 1, 2, 3, 4, 6 or 8 decimals, and they round the value you typed as well as the answer.
  4. Read the table, or copy it. Results arrive as a table pairing each input with its converted value; the summary beneath carries the factor for the last pair, and Copy gives you the table as CSV.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Reading a marine or aviation forecast

Wind and vessel speeds are quoted in knots. Turning 35 kn into km/h or mph puts the number back on a scale you have a feel for.

Checking work done in SI units

Calculations run in m/s and get reported in km/h, which is where a factor of 3.6 quietly goes missing. One line settles whether it did.

Converting a logged column in one go

Export a run of readings, paste them in as separate lines, and the whole column comes back converted instead of one value at a time.

Making sense of a US specification

Conveyor rates, wind loadings and muzzle velocities are quoted in ft/s or mph. The ft/s and mph rows put them beside the metric figures they have to match.

Questions

Do the speeds I type leave the tab?
No. The unit table and the multiplication that uses it are both part of the page, so the values you type are never transmitted, logged or stored. Once the page has loaded, converting makes no request at all, so pulling the network out mid-session changes nothing.
How accurate is the Mach conversion?
It uses 340.29 m/s, the speed of sound in air at sea level and 15 °C, and that figure is fixed. Sound travels more slowly in colder air — nearer 295 m/s in the stratosphere — so a Mach number converted here is a sea-level equivalent, not the true airspeed of an aircraft at altitude.
Can it convert running or cycling pace?
No. Pace is time over distance, the reciprocal of speed, and minutes per kilometre or minutes per mile are not in the table. You can convert to km/h or m/s here, but the inversion is not something this tool does.
What happens if I type a unit it does not recognise?
The line is refused, with the unit quoted back and the speed units it does know offered as suggestions. Units from other quantities are rejected on purpose; Unit Converter is the one that accepts length, mass, volume and speed in the same box.
Are the conversion factors rounded?
Six of the seven are the defined values — 0.44704 m/s for a mile per hour, 1852/3600 for a knot, 1000/3600 for a kilometre per hour, 299792458 m/s for light — so only the display rounds, and only to the precision you picked. Mach is the one entry that is a convention rather than a definition. Automatic precision falls back to exponential notation once a result reaches a quadrillion or drops below a millionth, and shows eight significant digits there instead of ten.
Why is the speed of light in a speed converter?
Because it is a speed, and it makes the far end of the table useful for physics work: 1 c is 299,792,458 m/s, so a fraction of light speed can be read off in km/h without a separate calculation.

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