Speed, distance, and time are linked by distance = average speed × time. Rearrange the equation to solve for another quantity.
The underlying idea
If distance is in kilometers and time is in hours, speed is in km/h. To convert m/s to km/h, multiply by 3.6. Average speed uses the entire distance and entire elapsed time.
Use consistent units
If distance is in kilometers and time is in hours, speed is in km/h. To convert m/s to km/h, multiply by 3.6. Average speed uses the entire distance and entire elapsed time. Choose measurements that describe the same object, system, or period; changing the reference changes what the result means.
Worked example
A trip of 90 km at an average of 60 km/h takes 1.5 hours. That is 1 hour 30 minutes, not 1 hour 50 minutes.
Put the method into practice
- Write down the quantity you need to find.
- Identify the measurements and their units.
- Convert units before doing the calculation.
- Apply the formula and keep unrounded intermediate values.
- Check whether the result is reasonable for your situation.
Try the Speed Calculator with your own values.
Common mistakes
Mixing units, using the wrong reference value, and rounding too early can all change the result. A calculator cannot repair inaccurate measurements. For project estimates, distinguish the calculated base quantity from allowances and supplier rounding.
What to learn next
Explore this topic’s related answers below for focused examples and alternative ways to check a result.
Sources & references
- Speed and Velocity — OpenStax
