Mile per Hour
Symbol: mphUnited States, United Kingdom
O que é um/uma Mile per Hour (mph)?
Formal Definition
The mile per hour (symbol: mph, or mi/h) is a unit of speed equal to one statute mile (5,280 feet or 1,609.344 meters) traveled in one hour. In SI units, 1 mph equals exactly 0.44704 m/s or 1.609344 km/h. The mph is the standard unit of speed for road transport in the United States, United Kingdom, and several other countries.
One mph equals approximately 1.609 km/h, 0.447 m/s, 0.869 knots, or 1.467 feet per second. These conversions are essential for international travelers and professionals working across measurement systems.
Usage Context
The mph is deeply embedded in American and British culture. Speed limits, speedometers, weather reports, sports statistics, and everyday conversation in the US and UK use mph. The unit is intuitive in these countries because people grow up with reference points: 3 mph is a walk, 30 mph is a neighborhood drive, 65 mph is highway cruising, and 100 mph is very fast. This built-in calibration makes mph resistant to replacement by km/h, despite international standardization efforts.
Etymology
Origin of the Mile
The word "mile" derives from the Latin "milia passuum" (a thousand paces), where a Roman pace equaled two steps (about 1.48 meters). The Roman mile was thus approximately 1,480 meters. Over centuries, the mile's length varied across countries and regions. The English statute mile was standardized at 5,280 feet by an Act of Parliament in 1593, and this definition persists to this day.
The Compound Unit
The phrase "miles per hour" became standard in the 19th century with the growth of railroads and later automobiles. Before motorized transport, speed was rarely quantified precisely. Horse speeds were described qualitatively (walk, trot, canter, gallop) rather than numerically. The need for precise speed measurement — for railway timetables, speed limits, and vehicle performance — made mph a common everyday term.
Precise Definition
Definition
One mile per hour is defined as one international mile (1,609.344 meters exactly) per hour (3,600 seconds exactly): 1 mph = 1,609.344 m / 3,600 s = 0.44704 m/s exactly. This definition is based on the international yard and pound agreement of 1959, which defined the yard as exactly 0.9144 meters.
Legal Standards
In the United States, the Federal Highway Administration uses mph for all speed-related regulations. Vehicle speedometers must display mph as the primary unit (km/h may appear as a secondary scale). Speed limit signs display mph values. The National Weather Service reports wind speeds in mph for public forecasts.
In the United Kingdom, road speed limits are legally defined in mph under the Road Traffic Regulation Act 1984. Despite the UK's general metrication, road distances remain in miles and speed limits in mph. UK speedometers must display mph, typically with km/h as a secondary scale.
International Aviation
While aviation primarily uses knots, ground speed in the US is sometimes communicated to pilots in mph, particularly for light aircraft and helicopter operations. The FAA's regulations reference both knots and mph in different contexts.
História
Pre-Automotive Era
Before automobiles, speed in mph was primarily relevant to horse-drawn transport and railways. The first passenger railway, the Liverpool and Manchester Railway (1830), advertised speeds of 24-30 mph. The first speed limit in the UK was the Locomotive Act of 1861, which limited powered vehicles to 10 mph on open roads and 5 mph in towns. The infamous Red Flag Act of 1865 reduced the limit to 4 mph (2 mph in towns) and required a person carrying a red flag to walk ahead of each vehicle.
The Automobile Revolution
The first automobile speed limit in the US was enacted in Connecticut in 1901: 12 mph in cities and 15 mph in the countryside. As cars became faster and more common, speed limits increased. The US National Maximum Speed Law of 1974 set a nationwide 55 mph limit to conserve fuel during the oil crisis. This was raised to 65 mph in 1987 and eventually repealed in 1995, allowing states to set their own limits (now typically 65-85 mph).
Speed Records
The history of land speed records has been primarily British and American — and therefore measured in mph. The first official land speed record was 39.24 mph (63.15 km/h) by Gaston de Chasseloup-Laubat in 1898. The 100 mph barrier was broken in 1904. The 200 mph barrier fell in 1927. The 400 mph barrier was broken in 1935. The current record of 763 mph (1,228 km/h) was set by ThrustSSC in 1997.
UK Metrication Debate
The UK's ongoing use of mph for road transport is one of the most visible exceptions to its otherwise broad metrication. Multiple proposals to convert road signs to km/h have been considered and rejected — most recently in 2006 when the Department for Transport estimated the cost at £690 million. As of 2024, there are no active plans to convert UK road signs to km/h.
Uso atual
Road Transport
The mph is the legally mandated speed unit for roads in the United States, United Kingdom, and several Caribbean and Pacific island nations. Common US speed limits: school zones 15-25 mph, residential 25-35 mph, urban arterials 35-45 mph, rural highways 55-65 mph, interstate highways 65-75 mph, and some Texas highways 80-85 mph. Common UK limits: residential 20-30 mph, single carriageway 60 mph, dual carriageway 70 mph, motorway 70 mph.
Weather Reporting
The US National Weather Service reports wind speeds in mph for public forecasts. The Enhanced Fujita (EF) tornado scale defines intensity by estimated wind speeds in mph: EF0 (65-85 mph), EF1 (86-110 mph), EF2 (111-135 mph), EF3 (136-165 mph), EF4 (166-200 mph), EF5 (over 200 mph). Hurricane categories also reference mph: Category 1 (74-95 mph) through Category 5 (157+ mph).
Baseball
Baseball pitch speed is measured exclusively in mph in the United States. A fastball of 95+ mph is considered exceptional. The MLB record is 105.1 mph (169.1 km/h) by Aroldis Chapman. Stadium radar guns and television broadcasts display pitch speed in mph, making it one of the most familiar speed measurements in American culture.
Law Enforcement
Speed enforcement in the US and UK uses mph. Radar guns, lidar devices, and speed cameras measure vehicle speed in mph. Speeding tickets cite the offending speed and the speed limit, both in mph. The legal consequences of speeding are typically tiered by how many mph over the limit the driver was traveling.
Everyday Use
Driving in the US and UK
For approximately 400 million people in the US and UK, mph is the primary speed reference. Drivers check speedometers in mph, observe speed limit signs in mph, and discuss driving speeds in mph. GPS navigation systems in these countries default to mph. The phrase "doing sixty" universally means 60 mph to American and British drivers.
Running and Fitness
In the US, treadmill speeds are displayed in mph. A walking pace is 3-4 mph. A moderate jog is 5-6 mph. Running at 8 mph covers a mile in 7.5 minutes. A sub-4-minute mile requires maintaining over 15 mph. Marathon world record pace is approximately 13.1 mph (21.1 km/h).
Wind and Storms
Americans describe wind in mph. A gentle breeze is 5-10 mph. A strong wind is 30-40 mph. Tropical storm force begins at 39 mph. Hurricane force begins at 74 mph. Understanding these numbers helps Americans prepare for severe weather. UK residents similarly use mph for wind speeds in weather forecasts.
Sports
Beyond baseball, mph dominates American sports reporting. NFL quarterback throws: 55-62 mph. Hockey slap shots: 90-110 mph. NASCAR racing speeds: 180-200+ mph at superspeedways. Golf ball off the tee: 150-180 mph (ball speed). These familiar reference points anchor the mph scale in American culture.
In Science & Industry
Crash Testing
US automotive crash testing specifies impact speeds in mph. The Insurance Institute for Highway Safety (IIHS) frontal crash test uses 40 mph (64 km/h). Side impact tests use 31 mph. The National Highway Traffic Safety Administration (NHTSA) frontal crash test uses 35 mph. These standardized speeds enable consistent safety comparisons.
Wind Engineering
In the US, building codes specify design wind speeds in mph. ASCE 7 (Minimum Design Loads for Buildings) defines ultimate wind speed maps in mph. Typical values: 115-180 mph depending on location and risk category. Coastal and hurricane-prone areas have the highest design wind speeds.
Atmospheric Science
The Saffir-Simpson Hurricane Wind Scale uses mph: Category 1 (74-95 mph), Category 2 (96-110 mph), Category 3 (111-129 mph), Category 4 (130-156 mph), Category 5 (157+ mph). Tornado intensity on the Enhanced Fujita Scale also references mph. These scales were developed in the US and retain mph in their primary definitions.
Ballistics
Firearm muzzle velocities in US contexts are often reported in feet per second (fps) but sometimes in mph for public communication. A 9mm handgun bullet at approximately 820 mph. A .223 rifle bullet at approximately 2,200 mph. The speed of sound at sea level is approximately 767 mph (Mach 1), providing a familiar reference point.
Interesting Facts
The highest speed limit in the United States is 85 mph (137 km/h) on a 41-mile stretch of Texas State Highway 130 near Austin. The lowest general speed limit is 25 mph in most residential areas.
The UK's motorway speed limit has been 70 mph since 1965, when it was introduced as a 'temporary measure' following a series of accidents in fog on the M1. It has never been raised despite decades of debate.
The fastest animal on Earth, the peregrine falcon, reaches speeds over 240 mph (386 km/h) in a hunting dive (stoop). This is faster than a Formula 1 car's top speed.
The speed of sound at sea level is approximately 767 mph. Chuck Yeager first broke the sound barrier on October 14, 1947, in the Bell X-1 at approximately 807 mph (Mach 1.06) — just 40 mph above the speed of sound.
NASCAR's Talladega Superspeedway has seen qualifying speeds of over 212 mph (341 km/h). Restrictor plates were introduced in 1988 after Bobby Allison's car became airborne at approximately 210 mph.
The world's fastest elevator, in the Shanghai Tower, travels at 46 mph (74 km/h). It ascends 121 floors in 55 seconds — faster than most vehicles travel on city streets.