Convert Miles per Hour to Kilometers per Hour – Speed Converter
This converter transforms a single speed value in miles per hour (mph) into kilometres per hour (km/h). It is intended for everyday use such as travel planning, vehicle dashboard checks, and quick unit verification.
The tool applies the internationally accepted relationship between the mile and the metre so results are consistent with national and international standards. For engineering, legal, or safety-critical uses, consider device calibration, measurement uncertainty, and the rounding rules required by the applicable authority.
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Methodology
The conversion is based on the international definition of the mile as exactly 1 609.344 metres and the kilometre as exactly 1 000 metres. Therefore the mathematical factor is exact: 1 mile = 1.609344 kilometres.
Although the numerical conversion factor is exact by definition, the overall accuracy of any measured speed depends on the measurement system, sampling method, and calibration. Follow guidance from standards bodies for traceability: consult NIST for unit definitions, ISO 80000 for quantities and units, IEEE resources for instrumentation and data-quality practices, and OSHA guidance for workplace measurement and calibration where applicable.
Worked examples
1 mph → 1.609344 km/h (exact by definition)
55 mph → 55 × 1.609344 = 88.51392 km/h (display as 88.51 km/h when rounded to two decimals)
60 mph → 60 × 1.609344 = 96.56064 km/h (commonly shown as 96.56 km/h)
Key takeaways
Multiply mph by 1.609344 to get km/h. The factor is exact by definition, but final reported accuracy depends on measurement quality and rounding rules.
Use calibration procedures and follow relevant standards when converting values used for regulatory, engineering, or safety decisions.
Expert Q&A
Why is the factor 1.609344 used?
The factor comes from the international definition of the mile as exactly 1 609.344 metres and the kilometre as 1 000 metres. The factor is therefore exact and standardized.
Is the conversion factor exact or approximate?
The conversion factor 1.609344 is exact given the defined values of the mile and kilometre. Any uncertainty in a converted number arises from how the original speed was measured and from rounding.
How many decimals should I show?
For general use 2 decimal places are often adequate. For engineering, legal, or research contexts follow the precision rules specified by the governing standard or your organisation's measurement policy.
Does this tool account for instrument calibration or measurement error?
No. This converter performs a pure mathematical conversion. Account for instrument calibration, sensor latency, sampling method, and systematic errors separately when reporting measurement results.
Can I convert km/h back to mph?
Yes. To convert km/h to mph divide by 1.609344 (mph = km/h ÷ 1.609344).
Sources & citations
- NIST — Definition of the mile (United States customary units reference) — https://physics.nist.gov/cuu/Units/mile.html
- ISO 80000 — Quantities and units (overview and purchase page) — https://www.iso.org/standard/30669.html
- IEEE Standards Association — Standards and guidance for measurements and instrumentation — https://standards.ieee.org/
- OSHA — General guidance on measurement, calibration and workplace instruments — https://www.osha.gov/
- BIPM — The International System of Units (SI Brochure) — https://www.bipm.org/en/publications/si-brochure