This speed calculator solves the speed–distance–time triangle in any direction: find speed from distance and time, distance from speed and time, or time from distance and speed — with mph, km/h, m/s and knots handled for you.
The Speed Formula
speed = distance ÷ time — and its two rearrangements, distance = speed × time and time = distance ÷ speed. One triangle, three questions; this calculator answers all of them. It is part of our Physics Calculators collection.
Worked Examples (Verified)
- 100 km in 1.5 hours → speed = 100 ÷ 1.5 = 66.67 km/h (41.4 mph).
- A marathon (26.2 miles) in 2 hours → 13.1 mph — elite territory; a 4-hour marathon averages 6.55 mph. Runners usually think in the inverse — minutes per mile — which is exactly what our Pace Calculator computes.
- At 60 mph for 2.5 hours → 150 miles of distance.
Speed Conversions Worth Knowing
| Convert | Multiply by |
|---|---|
| mph → km/h | 1.609 |
| km/h → mph | 0.621 |
| m/s → km/h | 3.6 |
| km/h → m/s | 0.278 |
| knots → km/h | 1.852 |
| m/s → mph | 2.237 |
Reference points: walking ≈ 5 km/h, city cycling ≈ 15–20 km/h, highway driving ≈ 100–120 km/h, a cruising airliner ≈ 900 km/h, sound ≈ 1,235 km/h (Mach 1), light = 1.08 billion km/h.
Average vs. Instantaneous Speed
This calculator computes average speed over a whole trip. Real journeys vary — traffic, hills, stops — so an average of 66.7 km/h usually means stretches at 90+ and stretches at 20. When speed itself changes steadily, that change per second is acceleration — see the Acceleration Calculator. And speed with a direction is velocity: driving 60 mph north and 60 mph south are the same speed but opposite velocities, which is why round trips can have zero average velocity but a healthy average speed.
Related Tools
- Acceleration Calculator — changing speed
- Pace Calculator — running pace and race times
- Physics Calculators — the full collection
Frequently Asked Questions
How do I calculate speed?
Divide distance by time: 100 km ÷ 1.5 h = 66.7 km/h. Keep the units matched — kilometers and hours give km/h.
How do I find travel time?
Time = distance ÷ speed. 300 km at 75 km/h takes exactly 4 hours — before real-world stops.
What is the difference between speed and velocity?
Speed is a magnitude only; velocity includes direction. Both average 60 mph on a straight highway drive north.
How do I convert mph to km/h?
Multiply by 1.609. 60 mph ≈ 96.6 km/h — the results card above shows mph automatically.
What is a knot?
One nautical mile per hour = 1.852 km/h, used in shipping and aviation because nautical miles tie to Earth’s geometry (1 minute of latitude).
Is this speed calculator free?
Yes — free, instant, no sign-up.