Miles Per Hour — Definition, Formula & Examples
Miles per hour (mph) is a unit of speed that tells you how many miles something travels in one hour. It is the most common speed unit used in everyday life and math word problems in the United States.
Miles per hour is a rate expressing the ratio of distance in miles to elapsed time in hours. As a compound unit, it is written as or and belongs to the imperial system of measurement.
Key Formula
Where:
- = Rate of travel in miles per hour (mph)
- = Distance traveled in miles
- = Time elapsed in hours
How It Works
Speed connects three quantities: distance, rate, and time. If you know any two, you can find the third. When speed is given in miles per hour, distance must be in miles and time must be in hours for the units to work out. If time is given in minutes, convert it to hours by dividing by 60 before using the formula.
Worked Example
Problem: A car drives 150 miles in 3 hours. What is its average speed in miles per hour?
Identify values: Distance is 150 miles and time is 3 hours.
Apply the formula: Divide distance by time to find speed.
Answer: The car's average speed is 50 miles per hour.
Why It Matters
Distance-rate-time problems appear constantly in middle school math and on standardized tests. Understanding mph also matters for real tasks like estimating travel time on a road trip or comparing speed limits.
Common Mistakes
Mistake: Using minutes instead of hours in the formula without converting.
Correction: Always convert minutes to hours first by dividing by 60. For example, 90 minutes is hours.