Speed Conversion Calculator
Compare driving, running and physics speeds in the same units
Speed conversion FAQ
What is the exact relationship between mph and km/h?
By definition, one international mile is exactly 1,609.344 metres, and one hour is 3,600 seconds. That gives 1 mph = 1.609344 km/h and roughly 0.44704 m/s. Those constants are what the calculator uses whenever you convert road speeds between US style mph and metric km/h.
What does “knot” mean and where is it used?
A knot (kn) is one nautical mile per hour, with the nautical mile defined as exactly 1,852 metres. That works out to 1 kn ≈ 1.852 km/h ≈ 0.51444 m/s. Knots are standard in aviation, shipping and marine weather reports, which is why boat and aircraft speeds often appear in kn instead of mph or km/h.
What is Mach and how is it related to the speed of sound?
Mach 1 is the local speed of sound. Near room temperature in air, that’s about 343 m/s, roughly 1,235 km/h or 767 mph. The calculator treats 1 Mach as that typical value so you can quickly compare everyday speeds to “Mach numbers” you see in aeronautics examples.
What does “c” mean in this calculator?
Here, c stands for the speed of light in vacuum, defined as exactly 299,792,458 m/s. Converting to or from c gives you a feel for how ordinary speeds compare with this fundamental limit in physics, even though real-world travel speeds are tiny fractions of c.
When should I think in m/s instead of km/h or mph?
Metres per second (m/s) is the SI base unit for speed and fits naturally into physics formulas like v = d / t. It’s ideal for classroom problems, simulations and engineering work. For driving, running or cycling, km/h or mph often “feel” more intuitive, so this tool makes it easy to switch between them.
Is the speed of sound always 343 m/s?
Not exactly. The famous 343 m/s figure is for dry air at about 20 °C near sea level. The true speed of sound changes with temperature and medium (air, water, steel and so on). This calculator uses the 343 m/s value as a practical reference so you can compare everyday speeds with a typical “speed of sound in air”.
From road limits to extreme physics in one place
This speed conversion calculator is built to handle everything from speed limits and treadmill settings to back-of-the-envelope physics. Instead of memorising half a dozen numbers, you pick a starting unit like mph or km/h, choose a target like m/s, knots, or even Mach or c, enter a value and the tool shows a clear line such as “65 mph = 104.61 km/h”.
1. Pick the speed units that match your context
The dropdowns cover the units you actually see:
- Miles per hour (mph) for US road signs and many car dashboards.
- Kilometres per hour (km/h) for most of the world’s road limits and treadmills.
- Metres per second (m/s) and centimetres per second (cm/s) for physics and lab-style work.
- Feet per second (ft/s) for ballistics, sports timing and some engineering uses.
- Knots (kn) for boats, ships, aircraft and marine weather.
- Kilometres per second (km/s) for orbital speeds, meteors and astronomy examples.
- Mach and speed of sound for aerodynamics and shockwave discussions.
- Speed of light (c) for relativity-style comparisons and sci-fi scale speeds.
US-style units (mph) appear at the top so you can quickly translate everyday numbers into the metric or specialised units used in notes, reports and problem sets.
2. Base-unit method: everything through metres per second
Internally, every conversion runs through metres per second (m/s), the SI base unit for speed. Each unit has a fixed factor which tells the calculator how many m/s it equals:
- 1 mph ≈ 0.44704 m/s (because 1 mi = 1,609.344 m and 1 h = 3,600 s).
- 1 km/h ≈ 0.27778 m/s (1,000 m / 3,600 s).
- 1 kn ≈ 0.51444 m/s (1 nautical mile = 1,852 m per hour).
- 1 ft/s = 0.3048 m/s (since 1 ft = 0.3048 m).
- 1 cm/s = 0.01 m/s, 1 km/s = 1,000 m/s.
- 1 Mach ≈ 343 m/s at about 20 °C in air, matching a typical speed of sound.
- 1 c = 299,792,458 m/s by definition.
The calculator first converts your input to m/s, then divides by the target unit’s factor to produce the new value. This keeps everything consistent with the modern definitions used in science and engineering.
3. Speed conversion factors at a glance
If you like to do quick mental checks, here are some of the core conversion factors the calculator uses. Multiply the starting unit by the factor to get the resulting unit.
| Starting Unit | Resulting Unit | Conversion Factor |
|---|---|---|
| Everyday and travel speeds | ||
| miles per hour (mph) | kilometres per hour (km/h) | 1.609344 |
| miles per hour (mph) | metres per second (m/s) | 0.44704 |
| kilometres per hour (km/h) | metres per second (m/s) | 0.27778 |
| metres per second (m/s) | kilometres per hour (km/h) | 3.6 |
| feet per second (ft/s) | metres per second (m/s) | 0.3048 |
| Knots and small-scale units | ||
| knots (kn) | kilometres per hour (km/h) | 1.852 |
| knots (kn) | metres per second (m/s) | 0.51444 |
| centimetres per second (cm/s) | metres per second (m/s) | 0.01 |
| Sound and light benchmarks | ||
| Mach 1 (air, ~20 °C) | metres per second (m/s) | 343 |
| speed of sound (air, ~20 °C) | miles per hour (mph) | ≈ 767 |
| speed of light (c) | metres per second (m/s) | 299,792,458 |
4. Reading and using the result
The result card stays deliberately simple. If you enter 10 and convert from km/h to m/s, you’ll see something like “10 km/h = 2.7778 m/s”. For driving or fitness pace, you might round that to 2.78 m/s; for physics homework, you can keep more digits. A similar pattern works if you compare a cruising speed to Mach, or a satellite’s orbital speed to km/s.
Because every value flows through m/s and uses standard definitions for mph, knots, sound speed and the speed of light, your conversions line up with the conventions used in textbooks, engineering references and physics problems.
References and further reading on speed units
These references explain how the core speed units and constants used here are defined:
- Mile per hour — covers mph as a unit of speed and shows its relationship to metres per second and kilometres per hour.
- Knot (unit) — defines the knot as one nautical mile per hour and lists standard conversions to km/h, mph and m/s.
- Speed of sound — explains why the speed of sound in air is about 343 m/s at 20 °C and how it depends on medium and temperature.
- Speed of light — describes the constant c, defined as 299,792,458 m/s in vacuum, and its role as a fundamental speed limit.
For navigation, certification or engineering sign-off, always double-check critical limits and tolerances against your local standards and official documentation before finalising values.