Miles per hour (mph) sparks confusion for many. Is it part of the metric system, or does it belong to the imperial legacy? In this article, we’ll clear up the misunderstanding, show how mph fits into measurement systems, and explain where and why it’s still used in 2025.
You will also learn how to convert mph to metric units, where mph is used around the world, and the implications for travel and science. All this you’ll find in this article.
What Does “Metric” and “Imperial” Mean?
The metric system, officially called the International System of Units (SI), is based on powers of ten: meters, liters, grams, and related prefixes (kilo-, centi-, milli-). The imperial system, sometimes called the British Imperial system or the U.S. customary system, evolved historically from British standards and includes units like miles, feet, gallons, pounds, and more.
In short: metric = decimal and universal; imperial = historical and regionally specific.
Is MPH Metric or Imperial?
MPH is an imperial (or U.S. customary) unit of speed, not metric. It expresses how many statute miles are traveled in one hour. The abbreviation “mph” comes from “miles per hour.” Since the mile is an imperial unit for distance, mph belongs to the same system.
The metric equivalent unit is kilometers per hour (km/h), which aligns with the metric system’s use of meters and kilometers. MPH is not recognized in the metric (SI) framework.
Historical Roots and Adoption
The imperial measurement system was formalized in the British Empire in the early 19th century to unify various local units. As the British Empire grew, many colonies adopted the imperial standard for trade, governance, and infrastructure. The United States, having inherited British standards before independence, continued using its version—U.S. customary units—which largely mirrors imperial units for many measures, including the mile and mph.
Meanwhile, scientific communities and many nations in Europe embraced the metric system over the 19th and 20th centuries. The ease of scaling by factors of ten gave metric a clear advantage in engineering, commerce, and education.
By the mid-20th century, many countries gradually converted to the metric system. Today, only a few still use mph for road speed limits and daily speed measurement.
Where Is MPH Still Used?
Despite global metric adoption, mph remains in official use in several places:
- United States: Speed limits, car speedometers, and road signage use mph.
- United Kingdom: While much of the UK uses metric for other measures, road speeds use mph.
- Other countries/territories: Some former British colonies and smaller locales keep mph as their legal road-speed unit (e.g. some Caribbean islands, parts of Liberia, etc.)
About 17 countries out of nearly 200 still use mph. That’s fewer than 10 percent of nations. The rest use kilometers per hour or other metric speed units.
Converting Between MPH and Metric Units
To go between mph and metric speed:
- 1 mph = 1.60934 km/h
- 1 km/h = 0.621371 mph
So, if you drive at 60 mph, you are going about 96.56 km/h. Conversely, a speed of 100 km/h is around 62.14 mph.
These conversion factors are exact to several decimal places and widely accepted in engineering and transportation.
Why Some Countries Favor MPH
There are practical and cultural reasons why mph remains:
- Legacy infrastructure: Road signage, speed limits, driver habits, and vehicle dials already use mph, making conversion expensive or confusing.
- Cultural identity: In places like the U.S. and UK, people are accustomed to mph for decades.
- Regulation inertia: Law, standards, and regulation systems already set in mph are slow to change.
- Partial metrication: Some countries adopt metric for many things but keep mph for roads as a compromise.
Impacts for Travelers and Science
For travelers, encountering mph in a metric world can cause misinterpretation of speed limits. If you forget to convert, you might drive too fast (or too slow) in a foreign country.
In science, mph is rarely used—metrics like meters per second (m/s) or km/h dominate. The metric system offers consistency across distance, volume, weight, and temperature, which is vital in scientific work.
MPH vs Metric in Everyday Use
Here’s how people typically use mph and metric:
- Americans overwhelmingly prefer imperial units for distance, speed, and height measurement. A YouGov poll showed nearly 88 percent of Americans use feet and inches for height. A modest shift toward metric occurs among younger generations, but mph remains dominant for speed.
- In the UK, many people mix. They may measure weight or liquids in metric but use mph for speed. This hybrid approach reflects gradual change rather than abrupt overhaul.
Pros and Cons of MPH Compared to Metric Speed Units
Advantages of mph in its domain:
- Familiarity for users in mph-using countries.
- Continuity: no cost of redoing signs, rewriting laws, or re-educating the public.
- Sufficient precision in daily driving contexts.
Disadvantages:
- It lacks the easy scaling of metric units (decimal system).
- It complicates engineering, data exchange, and international standards.
- It creates friction for visitors, trade, and scientific communication.
How Global Trends Show Metric Dominance
The overwhelming majority of countries, especially in Europe, Asia, Africa, and Latin America, use km/h for road speeds. Even in many formerly British territories, metric has replaced mph for transportation. The metric system has become the international norm for science, industry, and education.
When road conversion happens, countries usually switch speedometers, signage, legislation, and public campaign simultaneously. Conversion is rare today because required investments and public adaptation barriers are high.
Why the Confusion Persists
Many people assume mph might be metric because it’s “per hour,” sounding similar to metric units like km/h. But the key difference lies in the base unit: mile vs kilometer. Mile is an imperial measure, so mph cannot be metric.
Occasionally, writers will call mph “non-SI” or “non-metric.” That’s accurate—mph lies outside the metric (SI) system.
Summary: MPH Belongs to Imperial, Not Metric
So to answer simply: mph is an imperial (or U.S. customary) unit, not metric. It measures speed in miles per hour, where the “mile” is an imperial distance unit. The metric system uses its own speed units like km/h or m/s.
Despite metric’s global prevalence, mph persists in a handful of countries tied by history and infrastructure. But when it comes to science, industry, and international standards, metric units reign supreme.
If you travel to a country using mph, remember to convert. If you deal with data or science, stick with metric. Knowing both systems is smart in a connected world.