Speed Converter

Free Online Speed Converter — mph, km/h, m/s, Knots, Mach and More

Speed is simultaneously one of the most intuitive physical quantities in everyday experience and one of the most fragmented in terms of unit conventions. The world is divided between miles per hour (dominant in the US and UK) and kilometers per hour (used in most other countries). Maritime and aviation professionals worldwide use knots. Scientists and engineers use meters per second. Aeronautical engineers reference Mach numbers for high-speed flight. And physicists working with extreme phenomena reference the speed of light. Our free online speed converter handles all of these unit systems in one place, delivering instant conversions for any velocity-related measurement need.

Kilometers per Hour and Miles per Hour — The Everyday Speed Divide

The split between km/h and mph in everyday speed measurement is one of the most visible manifestations of the metric-imperial divide in daily life. Every country that has adopted the metric system uses km/h for road speed limits, vehicle speedometers, weather forecasting wind speeds, and athletic performance metrics. The United States uses mph for all of these contexts. The United Kingdom is in a hybrid position — road speed limits and speedometers use mph, but scientific and technical measurements use km/h.

For drivers crossing the US-Canada border, the speed limit change from mph to km/h is significant: a US highway speed limit of 65 mph corresponds to approximately 105 km/h, while a Canadian limit of 100 km/h corresponds to approximately 62 mph. The numbers look dramatically different even though the actual speed is nearly the same. Getting this conversion wrong has cost many cross-border travelers speed camera fines and confused navigation directions.

Athletic performance metrics provide another common mph-to-km/h conversion context. Baseball pitch speeds are broadcast in mph in the US and km/h internationally. A fastball pitcher throwing at 95 mph is throwing at approximately 153 km/h. Tennis serve speeds at major tournaments are announced in mph in North American coverage and km/h in European broadcasts. A 130 mph serve — considered quite fast — equals approximately 209 km/h. Running speeds in fitness contexts switch between systems constantly: a 6-minute-per-mile running pace equals approximately 10 mph, which equals 16 km/h.

Meters per Second — The Scientific Velocity Unit

Meters per second (m/s) is the SI unit of velocity and the standard for scientific and technical velocity calculations. When physicists apply Newton's second law (F = ma), kinematic equations (v = u + at, s = ut + ½at²), or energy equations (KE = ½mv²), velocities must be in meters per second for the equations to yield results in SI base units without conversion factors.

Meters per second provides useful practical reference points. Walking pace is approximately 1.4 m/s. Casual running is around 3-4 m/s. World-class sprinters peak at approximately 10-12 m/s over short distances (100m). The speed of sound at sea level is approximately 343 m/s. Earth's orbital velocity around the Sun is approximately 29,800 m/s (29.8 km/s). The escape velocity from Earth's surface is approximately 11,200 m/s (11.2 km/s).

Converting km/h to m/s (divide by 3.6) and m/s to km/h (multiply by 3.6) are calculations that physics and engineering students perform constantly. The factor of 3.6 comes from the unit conversion: 1 km/h × (1,000 m/km) ÷ (3,600 s/hr) = 1/3.6 m/s. Remembering this factor enables quick mental conversions between the scientific m/s standard and the more intuitive km/h for sanity-checking calculations.

Knots — Speed at Sea and in the Air

One knot is defined as one nautical mile per hour. Since a nautical mile equals exactly 1,852 meters, one knot equals 1.852 km/h or approximately 1.15 mph. The knot is the universal speed unit for maritime navigation and international aviation, standardized under international treaty to ensure consistent communication between ships, aircraft, and control centers across different countries.

The origin of the knot as a speed unit is historical and evocative: before mechanical logs, sailors determined ship speed by throwing a wooden chip (the "chip log") attached to a knotted rope overboard and counting how many knots passed through their hands in a fixed time interval (typically 28 seconds, measured with a sandglass). The spacing of the knots was calibrated so that the count directly gave the speed in nautical miles per hour. This direct measurement method explains why the unit name "knot" means both the physical knot in a rope and the unit of nautical speed.

Typical ship speeds range from approximately 12-15 knots for large container vessels and bulk carriers, to 20-25 knots for passenger ferries, to over 30 knots for naval vessels and high-speed craft. Commercial airline cruising speeds are typically 450-500 knots (roughly Mach 0.8). Military fighter jets cruise at 500-700 knots and achieve supersonic speeds above Mach 1 (approximately 661 knots at cruise altitude).

Mach Number — Speed Relative to Sound

The Mach number expresses speed as a multiple of the local speed of sound. Mach 1 is exactly the speed of sound in the medium and conditions being considered. Mach 2 is twice the speed of sound. This unit is particularly useful in aerodynamics because many critical aerodynamic behaviors — wave drag, shock wave formation, sonic boom generation, and heat generation from air compression — are primarily functions of Mach number rather than absolute speed.

The speed of sound varies significantly with temperature (and, to a lesser extent, with humidity and pressure). At sea level in standard conditions (15°C), the speed of sound is approximately 340 m/s, 1,224 km/h, or 761 mph. At typical commercial aircraft cruising altitude (approximately 10,000 m, where temperature is around -57°C), the speed of sound drops to approximately 295 m/s, 1,062 km/h, or 660 mph. This is why jet aircraft Mach numbers change with altitude even when their airspeed remains constant.

The subsonic regime extends from 0 to approximately Mach 0.8. The transonic regime (Mach 0.8-1.2) involves mixed subsonic and supersonic airflow around the aircraft and is where shock wave drag becomes significant. The supersonic regime extends from Mach 1.2 to approximately Mach 5. Above Mach 5 is the hypersonic regime, relevant for ballistic missiles, re-entering spacecraft, and experimental hypersonic aircraft. The Space Shuttle re-entered the atmosphere at approximately Mach 25, where aerodynamic heating generates temperatures that require specialized thermal protection systems.