Speed = Distance ÷ Time
Last reviewed: May 2026
Speed, distance, and time are related by one of the most fundamental formulas in physics: S = D/T. Given any two values, you can solve for the third.[1] This calculator handles all three scenarios and converts between speed units automatically. For running-specific pace calculations, use the Pace Calculator. For scientific speed conversions, try the Speed Converter.
| Object/Event | mph | km/h | m/s |
|---|---|---|---|
| Walking | 3.1 | 5.0 | 1.4 |
| Usain Bolt (peak) | 27.8 | 44.7 | 12.4 |
| Highway driving | 65 | 105 | 29 |
| Commercial jet | 575 | 925 | 257 |
| Speed of sound | 767 | 1,235 | 343 |
| Earth orbital speed | 66,627 | 107,218 | 29,783 |
The three forms of the speed equation are: Speed = Distance ÷ Time, Distance = Speed × Time, and Time = Distance ÷ Speed. These are the same formula rearranged to solve for different unknowns. The key to using them correctly is ensuring your units are consistent — if distance is in miles and time is in hours, speed will be in miles per hour. If distance is in meters and time is in seconds, speed will be in meters per second. Mixing units (kilometers for distance, hours for time, expecting m/s) is the most common calculation error. This calculator handles unit conversion automatically, but understanding the underlying relationship helps you estimate answers mentally and catch obvious errors.
| Unit | mph | km/h | m/s | knots |
|---|---|---|---|---|
| 1 mph | 1 | 1.609 | 0.447 | 0.869 |
| 1 km/h | 0.621 | 1 | 0.278 | 0.540 |
| 1 m/s | 2.237 | 3.600 | 1 | 1.944 |
| 1 knot | 1.151 | 1.852 | 0.514 | 1 |
| 1 ft/s | 0.682 | 1.097 | 0.305 | 0.592 |
| Object/Event | Speed (mph) | Speed (km/h) | Speed (m/s) |
|---|---|---|---|
| Human walking | 3.1 | 5.0 | 1.4 |
| Usain Bolt (peak sprint) | 27.8 | 44.7 | 12.4 |
| Cycling (Tour de France avg) | 25 | 40 | 11.1 |
| Speed limit (US highway) | 65–75 | 105–121 | 29–34 |
| Cheetah (top speed) | 70 | 112 | 31.3 |
| Formula 1 car (top speed) | 230 | 370 | 103 |
| Commercial airplane | 575 | 925 | 257 |
| Speed of sound (sea level) | 767 | 1,235 | 343 |
| SR-71 Blackbird | 2,193 | 3,529 | 980 |
| Space station (ISS) | 17,150 | 27,600 | 7,667 |
Average speed is the total distance traveled divided by total elapsed time — it smooths out all the stops, starts, accelerations, and decelerations along the way. Instantaneous speed is how fast you are moving at a single moment (what your speedometer reads). These can differ dramatically: a 30-mile commute that takes 1 hour has an average speed of 30 mph, but you might have been driving 60 mph on the highway and 0 mph at red lights. GPS navigation apps estimate arrival times using average speeds that account for typical traffic patterns — not the speed limit. For trip planning, average speed is always more useful than top speed: a road trip covering 300 miles at a realistic average of 55 mph (accounting for rest stops, traffic, and speed changes) takes 5 hours 27 minutes, not the 4 hours that a constant 75 mph would suggest.
Aerodynamic drag increases with the square of speed, which directly impacts fuel economy. Most vehicles achieve peak fuel efficiency between 35–50 mph (55–80 km/h). At 55 mph, a typical sedan might get 35 MPG. At 70 mph, that drops to about 28 MPG. At 85 mph, efficiency might fall to 22 MPG. The rule of thumb is that every 5 mph above 50 mph costs approximately 3–7% in fuel economy, depending on the vehicle's aerodynamic profile. This relationship means that driving 70 mph instead of 60 mph saves only 8.5 minutes per 100-mile trip but costs an additional 15–25% in fuel. For long highway trips, maintaining a moderate speed often saves enough fuel to offset the extra time, especially at current gas prices.
When two objects move toward each other, their closing speed is the sum of their individual speeds. Two cars approaching each other at 60 mph each have a closing speed of 120 mph — the effective impact speed in a head-on collision. When moving in the same direction, relative speed is the difference: a car going 70 mph overtaking a truck going 55 mph has a relative speed of 15 mph. This concept is critical for calculating safe following distances, passing time, and collision dynamics. On a two-lane highway, a car traveling at 60 mph overtaking a slower vehicle at 45 mph with an oncoming vehicle at 60 mph faces a closing rate of 120 mph with the oncoming traffic — the passing window is shockingly short.
The speed of sound in air at sea level and 20°C is approximately 343 m/s (767 mph, 1,235 km/h). Mach numbers express speed as a ratio of the speed of sound: Mach 1 = speed of sound, Mach 2 = twice the speed of sound. The speed of sound varies with temperature and medium — it increases in warmer air (about 0.6 m/s per degree Celsius) and travels much faster in water (approximately 1,480 m/s) and solids (5,120 m/s in steel). Supersonic flight above Mach 1 creates a shock wave experienced on the ground as a sonic boom. For detailed calculations of sound speed in various conditions, see our Speed of Sound Calculator.
→ Use average speed for trip planning. Assume 50–60 mph average for highway trips (accounting for stops and traffic), not the speed limit. Add 15–20% to your estimated time for a realistic arrival window.
→ Quick km/h to mph conversion. Multiply km/h by 0.6 for a fast estimate: 100 km/h ≈ 60 mph (actual: 62.1 mph). Close enough for speedometer reading or speed limit interpretation.
→ Remember the fuel economy sweet spot. Driving 55–60 mph on highways typically maximizes fuel efficiency. Every 5 mph above 60 costs roughly 5% more fuel per mile.
See also: Average Speed · Speed of Sound · Distance Calculator · Kinetic Energy
The unit of speed used depends entirely on context and convention. Road traffic uses mph (US, UK) or km/h (most of the world). Aviation uses knots (nautical miles per hour) because navigation is based on latitude and longitude — one nautical mile equals one minute of latitude, making knot-based calculations directly compatible with charts and GPS coordinates. Maritime navigation uses knots for the same reason. Science and engineering use meters per second (m/s) as the SI standard. Running and cycling performance is often expressed as pace (minutes per mile or minutes per kilometer) rather than speed, because pace is more intuitive for tracking effort — going from 9:00/mile to 8:30/mile is a clearly meaningful 30-second improvement, while the equivalent speed change from 6.67 to 7.06 mph feels less intuitive. For pace calculations, use our Pace Calculator.
| Country | Highway Limit | Urban Limit | Unit |
|---|---|---|---|
| United States | 55–85 mph | 25–35 mph | mph |
| Germany (Autobahn) | No limit (advisory 130) | 50 km/h | km/h |
| United Kingdom | 70 mph | 30 mph | mph |
| Japan | 100–120 km/h | 30–60 km/h | km/h |
| Australia | 100–130 km/h | 50 km/h | km/h |
| India | 100–120 km/h | 50 km/h | km/h |
→ Check unit consistency. Distance in miles with time in hours gives mph.[1]
→ Average speed ≠ average of speeds. If you drove 30 mph for 1 hour and 60 mph for 1 hour, average speed is 45 mph (not the harmonic mean).
→ For running pace, use min/mile. The Pace Calculator handles minutes per mile and per km.
→ Convert with simple ratios. 1 mph = 1.467 ft/s = 0.447 m/s = 1.609 km/h.
See also: Speed Converter · Pace · Distance · Momentum