Convert between mph, km/h, m/s, knots, feet per second, and Mach. All conversions calculated simultaneously.
Value to convert
From
To
Quick values
Result — all units (click to copy)
Choosing the Right Unit
Road speed
km/h is the global standard for road speed limits and vehicle speedometers, except in the US and UK, which use mph — check which your speedometer displays before comparing to a posted limit abroad.
Marine and air navigation
Knots (nautical miles per hour) are the standard for boats and aircraft — don't substitute mph or km/h when reading a nautical chart or ATC clearance.
Physics and aerospace
m/s is the SI unit used in physics calculations; Mach number and fractions of the speed of light are used specifically for aerospace and relativistic contexts, not everyday speed.
How to Use the Speed Converter
Enter your speed value — type the speed you want to convert.
Select the input unit — choose from km/h, mph, m/s, knots, or Mach number.
Read all conversions — see your speed in all units simultaneously.
Use the reference table — common speeds (walking, driving, sound, light) provide context for your conversion.
Convert Mach numbers — Mach 1 varies with altitude and temperature; the converter uses standard conditions (sea level, 20°C = 343 m/s).
🚗 Most common conversion: km/h to mph — multiply by 0.621 (or divide by 1.609). mph to km/h — multiply by 1.609. Quick check: 100 km/h ≈ 62 mph. 60 mph ≈ 97 km/h. Speed limits in the UK are in mph; continental Europe uses km/h — drivers crossing the Channel need this conversion constantly.
Understanding Speed Units
🚗 km/h and mph
Kilometres per hour (km/h): standard in ~90% of countries for road speed. Miles per hour (mph): used in US, UK, and a few others. Conversion: 1 mph = 1.60934 km/h. Common limits: 30 mph = 48 km/h (residential). 60 mph = 97 km/h (rural). 70 mph = 113 km/h (UK motorway). 100 km/h = 62 mph (European motorway). 130 km/h = 81 mph (French autoroute).
⚡ Metres per Second (m/s)
The SI unit of speed. Used in physics and engineering. Walking: ~1.4 m/s. Running: ~3–6 m/s. Car 100 km/h = 27.8 m/s. Speed of sound: 343 m/s. Conversion: 1 m/s = 3.6 km/h. Divide km/h by 3.6 to get m/s. Multiply m/s by 3.6 to get km/h. Used in physics equations: kinetic energy = ½mv² requires speed in m/s.
⚓ Knots
1 knot = 1 nautical mile per hour = 1.852 km/h = 1.151 mph. Used globally in aviation and maritime navigation — a rare example of a non-metric unit adopted universally including in metric countries. Airspeed is always reported in knots (for international consistency). Ocean current speeds, wind speeds at sea, and ship speeds all use knots.
✈️ Mach Number
Ratio of object speed to local speed of sound. Mach 1 = speed of sound (343 m/s at 20°C sea level, 295 m/s at cruise altitude where air is colder). Subsonic: < Mach 0.8. Transonic: Mach 0.8–1.2. Supersonic: > Mach 1.2. Hypersonic: > Mach 5. Commercial jets cruise at Mach 0.85. Concorde cruised at Mach 2. SR-71 Blackbird: Mach 3.3.
🌍 Speed of Light
c = 299,792,458 m/s = 1,079,252,848 km/h = 670,616,629 mph. The universal speed limit — nothing with mass can reach or exceed it. Light takes 8.3 minutes to travel from Sun to Earth (150 million km). Light takes 1.3 seconds to travel Earth-Moon distance (384,400 km). Distance in light-seconds, light-minutes, and light-years are practical units for astronomical distances.
🏃 Human Speed Reference
Walking: 5 km/h = 3.1 mph. Running: 12–15 km/h = 7.5–9.3 mph. Sprint world record: 44.72 km/h = 27.8 mph (Usain Bolt). Cycling: 20–30 km/h casual, 50+ km/h competitive. Swimming: 7–9 km/h world-class. Cheetah (fastest land animal): 112–120 km/h = 70–75 mph. Peregrine falcon (fastest animal): 389 km/h = 242 mph in a dive.
Speed in Transportation and Physics
Road speed conversions for travellers
International drivers need speed conversions constantly. UK uses mph with metric distances (confusingly, road signs show miles but fuel is sold in litres). Europe uses km/h. US uses mph. Hiring a car abroad: speedometer may be in different units to road signs. GPS navigation typically allows unit selection. Key conversions to remember: 50 km/h = 31 mph (urban limit in most of Europe). 90 km/h = 56 mph (rural roads). 130 km/h = 81 mph (motorway). 25 mph = 40 km/h. 55 mph = 89 km/h. 65 mph = 105 km/h (US interstate speed limit).
Aviation speed conventions
Aviation uses multiple speed definitions that serve different purposes. Indicated Airspeed (IAS): what the airspeed indicator shows — used for aircraft handling. True Airspeed (TAS): actual speed through the air — increases with altitude as air density decreases. Ground Speed: actual speed over the ground — TAS adjusted for wind. Mach number: ratio to local speed of sound — used at high altitude. All reported in knots internationally. A commercial airliner cruising at 550 knots true airspeed at 35,000 feet (Mach 0.85) may have a ground speed of 500–600 knots depending on winds.
Speed in physics calculations
Physics equations require consistent units. Kinetic energy: KE = ½mv² — speed must be in m/s and mass in kg for energy in Joules. Momentum: p = mv — same units. Velocity in Newton's second law: F = ma, where a = Δv/Δt in m/s². Converting between speed units before plugging into physics equations is essential — a car's 100 km/h must become 27.78 m/s for physics calculations. The SI system (m, kg, s) is universal for physics precisely because all derived units are consistent.
🚗 Speed conversion quick reference: 1 mph = 1.609 km/h. 1 km/h = 0.621 mph. 1 knot = 1.852 km/h = 1.151 mph. 1 m/s = 3.6 km/h = 2.237 mph. Mach 1 (sea level) = 343 m/s = 1,235 km/h = 767 mph. Common: 60 mph = 96.6 km/h. 100 km/h = 62.1 mph. 30 mph = 48.3 km/h. 50 km/h = 31.1 mph. Speed of sound: 343 m/s. Speed of light: 299,792,458 m/s.
Conversion Formula
Result = Value × (source unit's m/s equivalent) ÷ (target unit's m/s equivalent) — every unit converts through metres per second as the common base.
Worked example
Converting a 100 km/h car speed to mph: 100 ÷ 3.6 = 27.78 m/s, then 27.78 ÷ 0.44704 = 62.14 mph. In knots: 27.78 ÷ 0.514444 = 54.0 knots.
Note on Mach: this calculator uses 343 m/s, the speed of sound at sea level and ~20°C. The actual speed of sound — and therefore Mach 1 — changes with altitude and air temperature, dropping to roughly 295 m/s at typical cruising altitude. Mach conversions here are a sea-level approximation, not an aerospace-precision figure.
Avoid These Mistakes
Treating Mach as a fixed speed
Mach 1 is not a constant number of km/h or mph — it depends on air temperature and altitude. A Mach number only converts to a fixed speed at the specific conditions assumed.
Mixing up knots with mph
A knot (1.852 km/h) is meaningfully faster than a mile per hour (1.609 km/h) — using the wrong one in marine or aviation contexts introduces a real navigation error.
Forgetting to convert before physics formulas
Kinetic energy, momentum, and force calculations require SI units (m/s) — plugging in km/h or mph directly into a physics formula will give a wrong answer even if the formula itself is correct.
Frequently Asked Questions
How do I convert mph to km/h?
Multiply mph by 1.60934. Example: 60 mph = 60 × 1.60934 = 96.56 km/h. Quick estimate: multiply by 1.6. To go the other way: multiply km/h by 0.621371.
What is a knot in mph?
1 knot = 1 nautical mile per hour = 1.15078 mph = 1.852 km/h. Knots are used in aviation and maritime navigation. Wind speed in weather reports is often given in knots.
How fast is Mach 1?
Mach 1 is the speed of sound — approximately 343 m/s (1,235 km/h or 767 mph) at sea level at 20°C. The speed of sound varies with temperature and altitude: it's slower at higher altitudes where air is colder and thinner.
What is the speed of light?
The speed of light in a vacuum is exactly 299,792,458 metres per second (approximately 300,000 km/s or 186,000 miles/second). Nothing with mass can reach or exceed this speed.
References
🏛️ International System of Units (SI)
Metres per second is the SI derived unit of speed; the speed of light (299,792,458 m/s exactly) is itself the basis for the SI definition of the metre.
✈️ ICAO — International Civil Aviation Organization
Knots are the ICAO-standard unit for aviation speed worldwide; the standard atmosphere model ICAO publishes is the reference for how the speed of sound (and Mach 1) varies with altitude.