Enter any two values — speed, distance, or time — and instantly calculate the third.
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Speed = Distance ÷ Time. Distance = Speed × Time. Time = Distance ÷ Speed.
Speed of sound in air ≈ 343 m/s = 1,235 km/h = 767 mph (at 20°C sea level).
Speed of light = 299,792 km/s = 186,282 miles/s. Light travels around Earth ~7.5 times per second.
A 10 min/mile pace = 6 mph = 9.66 km/h. A 5-minute mile = 12 mph = 19.3 km/h.
Speed = Distance ÷ Time. This is the fundamental relationship between the three variables. If you know any two, you can calculate the third: Distance = Speed × Time. Time = Distance ÷ Speed. Example: A car travels 150 miles in 2.5 hours. Speed = 150 ÷ 2.5 = 60 mph. Or: A runner maintains 8 mph for 45 minutes. Distance = 8 × (45/60) = 6 miles. This formula assumes constant (average) speed — actual speed varies, but average speed = total distance ÷ total time.
Multiply km/h by 0.621371 to get mph. Or divide by 1.60934. Common conversions: 30 km/h = 18.6 mph. 50 km/h = 31.1 mph. 80 km/h = 49.7 mph. 100 km/h = 62.1 mph. 120 km/h = 74.6 mph. 130 km/h = 80.8 mph. Quick estimate: multiply km/h by 0.6 (accurate to within 3%). Reverse (mph to km/h): multiply by 1.609. 60 mph = 96.6 km/h. 70 mph = 112.7 km/h.
The speed of sound in dry air at 20°C (68°F) at sea level is approximately 343 meters per second (m/s), or 1,235 km/h (767 mph). The speed varies with temperature and medium: At 0°C: 331 m/s. At 20°C: 343 m/s. In water: ~1,480 m/s. In steel: ~5,960 m/s. Mach 1 = 1× the local speed of sound. Supersonic means traveling faster than Mach 1. The Concorde cruised at Mach 2 (about 2,179 km/h / 1,354 mph).
A knot is a unit of speed equal to 1 nautical mile per hour = 1.852 km/h = 1.15078 mph. Knots are used in maritime and aviation navigation. Typical speeds: Sailing boat: 6–10 knots. Container ship: 20–24 knots. Commercial aircraft cruise: 450–500 knots (520–575 mph / 835–925 km/h). Military jet: 600–1,000+ knots. The term 'knot' derives from the historical practice of measuring ship speed by counting knots on a rope thrown overboard.
Pace (min/mile or min/km) = 60 ÷ Speed (mph or km/h). Example: Running at 6 mph → pace = 60 ÷ 6 = 10 min/mile. Running at 10 km/h → pace = 60 ÷ 10 = 6:00 min/km. Common running speeds: 5K race (average recreational runner): ~6:00–7:00 min/km (8.6–10 km/h). 5K race (competitive): under 4:00 min/km (15+ km/h). Marathon average for 4-hour finish: 5:41 min/km (10.5 km/h). World marathon record pace (Kelvin Kiptum, 2:00:35): 2:51 min/km (21 km/h).
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