Solve for speed, distance, or time when you know the other two. Works with any unit combination.
The relationship between speed, distance, and time is one of the most fundamental equations in physics and everyday math. It has three forms depending on which variable you're solving for: Speed = Distance / Time, Distance = Speed x Time, and Time = Distance / Speed. If you know any two of the three, you can calculate the third.
This calculator handles unit conversions automatically. Enter values in any supported unit (mph, km/h, m/s, knots, miles, kilometers, meters, hours, minutes, seconds) and the result shows in the most common unit with automatic conversions to all other units displayed below.
Road trips: If you need to drive 450 miles and average 65 mph, how long will it take? Time = 450 / 65 = 6.9 hours, or about 6 hours and 55 minutes. Add 30-60 minutes for gas stops and rest breaks for a realistic estimate of 7.5-8 hours total.
Running and fitness: If you ran 5 kilometers in 28 minutes, what was your pace? Speed = 5 / (28/60) = 10.7 km/h, or about 6.65 mph. That's roughly a 9:00/mile pace. Runners often think in minutes per mile or minutes per kilometer rather than mph, but the underlying math is the same formula rearranged. For a more detailed running analysis, see our Running Pace Calculator.
Aviation and maritime: Speed is measured in knots (nautical miles per hour) for both ships and aircraft. 1 knot = 1.15 mph = 1.852 km/h. A commercial aircraft cruising at 500 knots covers about 575 mph or 926 km/h.
Physics and engineering: Scientific calculations typically use meters per second (m/s) as the base unit. The speed of sound at sea level is approximately 343 m/s (767 mph). The speed of light is 299,792,458 m/s (about 186,282 miles per second).
Some handy conversion factors: 1 mph = 1.609 km/h = 0.447 m/s = 0.869 knots = 1.467 ft/s. To quickly estimate km/h from mph, multiply by 1.6. To go the other direction, multiply km/h by 0.6. For example, 100 km/h is roughly 60 mph (exact: 62.1 mph). A speed limit of 120 km/h is about 75 mph.
For very small speeds (like walking), m/s or ft/s is more intuitive. Average walking speed is about 1.4 m/s (3.1 mph). For very large speeds (aircraft, spacecraft), km/h or knots is standard. The International Space Station orbits at about 27,600 km/h (17,150 mph).
Having a mental library of common speeds helps sanity-check calculations. Average walking speed is 3.1 mph (5 km/h). Jogging pace is 5-6 mph (8-10 km/h). City speed limits are typically 25-35 mph (40-55 km/h). Highway speed limits are 55-75 mph (90-120 km/h). A commercial airplane cruises at about 575 mph (925 km/h). The speed of sound at sea level is 767 mph (1,235 km/h). The International Space Station orbits at 17,150 mph (27,600 km/h).
For runners: a 10-minute mile pace is 6 mph. A 8-minute mile is 7.5 mph. A 6-minute mile (competitive runner) is 10 mph. A sub-4-minute mile (elite) is 15+ mph. For more detailed running calculations including splits and pacing, see our Running Pace Calculator.
If you drive 60 mph for the first half of a trip and 30 mph for the second half, your average speed is not 45 mph. It depends on whether "half" means half the distance or half the time. If you drove 60 miles at 60 mph (1 hour) and then 60 miles at 30 mph (2 hours), you covered 120 miles in 3 hours for an average of 40 mph. This is called the harmonic mean, and it's always lower than the arithmetic mean (simple average) when speeds vary.
This matters for trip planning. If you expect to average 60 mph but hit traffic for part of the drive, your actual average speed drops more than you'd intuitively expect. A 300-mile trip at a true average of 55 mph takes 5 hours and 27 minutes. At 50 mph, it takes 6 hours. That 5 mph difference adds 33 minutes. Always pad your estimated arrival time by 10-15% to account for speed variations, stops, and traffic.
Maritime and aviation use nautical miles (1 nautical mile = 1.151 statute miles = 1.852 km) because they're based on the geometry of the Earth. One nautical mile equals one minute of latitude, which makes navigation calculations simpler. Speed in knots means nautical miles per hour. A ship traveling at 20 knots covers about 23 mph or 37 km/h.
In scientific contexts, speed is measured in meters per second (m/s). To convert from mph to m/s, multiply by 0.447. To convert from km/h to m/s, divide by 3.6. The speed of light (the universal speed limit) is exactly 299,792,458 m/s or about 670,616,629 mph. Nothing with mass can reach this speed, only approach it.