Speed converter guide
Speed is distance travelled per unit of time. This calculator uses metres per second as the common base and applies the exact definitions of the international foot, international mile, nautical mile, hour, and speed of light in vacuum.
Mach is not included as a fixed unit because the local speed of sound changes with temperature and medium. A Mach conversion needs stated atmospheric conditions rather than one universal multiplier.
How to convert speed units
- Enter the speed: Provide a finite, non-negative value.
- Choose the source: Select the exact metric, customary, nautical, or scientific speed unit.
- Choose the target: Select the destination or use Swap to reverse the pair.
- Convert: Select Convert speed to calculate through metres per second.
Formula and variables
The source is expressed in metres per second and then scaled to the selected target.
target speed = source speed × (m/s per source unit) ÷ (m/s per target unit)- vₛ — Source speed
- A finite, non-negative speed.
- fₛ — Source factor
- Metres per second in one source unit.
- fₜ — Target factor
- Metres per second in one target unit.
Convert 100 kilometres per hour to miles per hour
Compare a 100 km/h road speed with mph.
- Amount
- 100
- From
- km/h
- To
- mph
- 100 km/h × 1000 m/km ÷ 3600 s/h
- 27.777777... m/s ÷ 0.44704 (m/s)/mph
Result: 100 km/h ≈ 62.1371192237 mph.
The values represent the same scalar speed under exact international unit definitions.
Understanding your results
Speed versus velocity
This result is a scalar magnitude. It does not carry direction, so it is not a complete velocity vector.
Exact factors and measured data
Exact unit definitions do not make a measured speed exact. Round according to the original measurement.
Assumptions
- One international mile is exactly 1609.344 m.
- One international nautical mile is exactly 1852 m.
- One hour is exactly 3600 s.
- The speed of light in vacuum is exactly 299,792,458 m/s.
Limitations
- Mach is excluded because the speed of sound is condition-dependent.
- The calculator does not convert vector direction or acceleration.
- It does not correct indicated vehicle, aircraft, or marine instrument readings.
- Extreme values can exceed floating-point range.
Common mistakes
- Using 1.6 as though it were the exact mile-to-kilometre factor.
- Confusing knots with kilometres per hour.
- Treating Mach as a fixed speed without atmospheric conditions.
- Calling a directionless result velocity.
- Reporting more significant digits than the input supports.
Practical use cases
Road and travel speeds
Compare km/h and mph using the exact international mile.
Marine and scientific speeds
Convert knots, m/s, ft/s, or fractions of the speed of light on one base.
Planning and decision guide
Preserve conditions and precision
Record whether the source is a measured, indicated, ground, air, or water-relative speed and round the converted result to defensible precision.
Frequently asked questions
How many mph is 100 km/h?
100 km/h is approximately 62.1371192237 mph.
How many kilometres per hour are in one knot?
One international knot is exactly 1.852 km/h.
Why is Mach not included?
Mach depends on the local speed of sound, which varies with medium and temperature, so it is not one fixed speed.
Is the speed of light exact?
The speed of light in vacuum is defined as exactly 299,792,458 m/s.
Can speed be negative?
Scalar speed is non-negative. A negative component belongs to a signed velocity convention.
Sources and review
- NIST Guide to the SI, Appendix B.9: Velocity and speed conversion factors — National Institute of Standards and Technology. Accessed 2026-08-30.
- SI Brochure, 9th edition — International Bureau of Weights and Measures. Accessed 2026-08-30.
Reviewed 2026-08-30.