Short answer
Speed tells you how fast something moves. Velocity tells you how fast and in what direction. At a particular instant, speed is the magnitude of velocity. A car moving at 30 mph east has a speed of 30 mph and a velocity of 30 mph east. 1
For a whole trip, there is another distinction: average speed uses total distance traveled; average velocity uses displacement, the change from starting position to ending position. A round trip can therefore have a positive average speed and an average velocity of zero. 1
On this page
At a glance
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| Question | Speed | Velocity |
|---|---|---|
| Type of quantity | Scalar: magnitude without direction. | Vector: magnitude and direction. |
| Example at one instant | 30 mph. | 30 mph east. |
| SI unit | Meters per second. | Meters per second, with direction specified. |
| Trip average | Total distance divided by elapsed time. | Displacement divided by elapsed time. |
| Can a direction change affect it? | Speed may remain unchanged. | Yes; direction is part of velocity. |
| Can its one-dimensional value be negative? | Speed is nonnegative. | A negative component can mark the chosen negative direction. |
Sources: definitions, units, and averages 1; direction changes and acceleration 2.
What each thing is
Speed
Speed describes the rate at which distance is covered. Instantaneous speed concerns a particular moment, such as the reading on a car’s speedometer. Average speed concerns an interval and is calculated from the total distance traveled divided by the total elapsed time. 1
Velocity
Velocity describes the rate of change of position, including direction. In motion along a straight line, a plus or minus sign can encode direction after a coordinate direction has been chosen. In two or three dimensions, more information is needed to specify the vector. 1
Average velocity uses the displacement between the endpoints. It does not tell you the whole route, whether the traveler stopped, or how speed changed along the way. 1
Key differences
Direction can change while speed stays constant. A car turning a corner at a steady speed has a changing velocity. Because acceleration measures change in velocity, the car is accelerating even though the speedometer reading stays steady. 2
Distance and displacement are different inputs. Distance accumulates along the route. Displacement compares the final position with the initial one. Going away and returning adds distance while canceling the net displacement. 1
The instantaneous and average relationships are not interchangeable. At each instant, speed is the magnitude of velocity. Over an interval, average speed is generally not the magnitude of average velocity. Taking a magnitude and then averaging need not give the same result as averaging a directional quantity first. 1
How to tell them apart
Check the information supplied. A number with units such as mph or m/s gives a speed unless direction is otherwise provided or understood. A signed value such as −3 m/s can describe one-dimensional velocity when the negative direction has been defined. 1
For a calculation, identify the interval and the numerator. Total route length gives average speed; end-to-start displacement gives average velocity. Include the whole elapsed interval, including stops, when that interval is the trip being averaged. The choice of interval must be explicit. 1
Where they overlap
Both describe motion and use distance-per-time units. Knowing instantaneous velocity determines instantaneous speed by taking its magnitude. If motion continues along one straight direction without reversal, average speed also equals the magnitude of average velocity for that interval. That special case helps explain why the words can seem interchangeable. 1
Edge cases
Negative velocity does not mean negative speed. A velocity component of −5 m/s indicates motion in the chosen negative direction; the associated speed for motion along that line is 5 m/s. 1
Zero average velocity does not mean no movement. A traveler returning to the starting position has zero net displacement even after a long journey. 1
Constant speed does not mean zero acceleration. A changing direction is enough to change velocity. Conversely, an acceleration pointing opposite the direction of motion reduces speed. 2
Why the distinction exists
Predicting motion requires direction. Two objects moving equally fast in opposite directions do not have the same velocity, and a turn changes motion even when the rate of covering distance stays steady. Separating a scalar speed from a vector velocity makes those distinctions explicit in mechanics. 1 2
Common misconceptions
- “Velocity is just a more technical word for speed.” It also specifies direction. 1
- “A constant speedometer reading proves no acceleration.” Turning changes velocity. 2
- “Average speed is the magnitude of average velocity.” A round trip disproves the general claim. 1
- “A negative velocity means slowing down.” Its sign identifies direction; slowing depends on how velocity changes. 2
Examples
Imagine walking 2 miles east and then 2 miles west, returning to the start in 1 hour. Total distance is 4 miles, so average speed is 4 mph. Displacement is zero, so average velocity is 0 mph. This is an illustrative calculation using the definitions. 1
For a second example, imagine a vehicle turning at a steady 20 mph. Its speed stays 20 mph, while its velocity changes direction through the turn. It therefore has acceleration during that turn. 2
Sources
Sources checked October 3, 2026.
- OpenStax, College Physics 2e — Time, Velocity, and Speed. Instantaneous quantities, averages, distance, and displacement.
- OpenStax, College Physics 2e — Acceleration. Changes in speed or direction; coordinate signs.