How do I convert km/h to mph quickly in my head?

Multiply by 0.6 for a quick estimate (actual factor: 0.6214). Or divide by 1.6. Example: 100 km/h × 0.6 = 60 mph (actual: 62.1 mph — close enough for estimation). For the reverse (mph to km/h), multiply by 1.6: 60 mph × 1.6 = 96 km/h (actual: 96.56 km/h).

Why do weather forecasts sometimes use different speed units?

In the US, wind speed is reported in mph for public forecasts. Most of the world uses km/h. Marine and aviation weather forecasts use knots worldwide. Some scientific reports use m/s. All four are in active use in different forecasting and operational contexts, which is why weather apps often let you choose your preferred unit.

Mach 1 is the speed of sound, which varies with air temperature and altitude — it is not a fixed speed. At sea level at 20°C, Mach 1 ≈ 343 m/s = 1,235 km/h = 767 mph = 667 kn. At cruising altitude (~35,000 ft, −56°C), Mach 1 ≈ 295 m/s = 1,062 km/h = 660 mph = 573 kn. Mach is a ratio (speed relative to local speed of sound), not a unit. Commercial airliners cruise at about Mach 0.85.

Speed Units Explained: mph, km/h, m/s, and Knots

Four speed units dominate different domains worldwide: metres per second (physics), kilometres per hour (most road systems), miles per hour (US and UK roads), and knots (aviation and maritime). Each exists for practical reasons tied to the measurement systems of its field. This guide explains each unit, where it's used, and how to convert between all four.

Published March 20, 2026

Key takeaways

m/s is the SI unit of speed. 1 m/s = 3.6 km/h = 2.237 mph.
km/h is the everyday unit in most of the world for road speeds, weather, and sports.
mph is the road standard in the US, UK, and a handful of other countries.
Knots (nautical miles per hour) are the standard in aviation and maritime navigation worldwide.

Metres per second (m/s): the SI unit

Metres per second is the SI derived unit of speed, combining the SI base units of length (metre) and time (second). It appears directly in physics equations: kinetic energy (KE = ½mv²), Newton's second law (F = ma), and wave mechanics all use m/s.

Familiar m/s reference speeds:

Human walking pace: ~1.4 m/s (5 km/h)
Sprint running (100 m world record pace): ~10.4 m/s
Highway driving: ~28 m/s (100 km/h) to ~33 m/s (120 km/h)
Commercial airliner cruising: ~230–260 m/s (830–940 km/h)
Speed of sound at 20°C sea level: 343 m/s
Speed of light (exact): 299,792,458 m/s

Kilometres per hour (km/h)

One km/h = 1,000 m ÷ 3,600 s = 1/3.6 m/s ≈ 0.2778 m/s. Kilometres per hour is the standard for road speed limits, weather wind speeds, and sports performance in approximately 170 countries.

Reference speeds in km/h:

European urban speed limit: 50 km/h (31 mph)
European motorway limit: 130 km/h (81 mph)
High-speed rail (TGV, Shinkansen): 300–320 km/h
Category 5 hurricane: wind speed >252 km/h (>157 mph)
Sound at sea level: 1,235 km/h

Weather reports across most of the world use km/h for wind speed (the US uses mph, and marine/aviation forecasts use knots globally).

Miles per hour (mph)

One mile = 1,609.344 metres exactly (the international definition since 1959). Therefore: 1 mph = 1,609.344 m ÷ 3,600 s ≈ 0.44704 m/s = 1.60934 km/h.

Countries using mph for road speeds: the United States, United Kingdom, and a small number of others (Myanmar, Antigua and Barbuda, etc.). The UK officially uses SI but retains mph for road speed limits and distances.

Reference speeds in mph:

US residential speed limit: 25–35 mph (40–56 km/h)
US Interstate speed limit: 55–80 mph (89–129 km/h) depending on state
UK motorway limit: 70 mph = 112.7 km/h
F1 racing car top speed: ~230 mph (370 km/h)
Cheetah (fastest land animal): ~75 mph (120 km/h)

Knots (nautical miles per hour)

Formula

1 knot = 1 nautical mile per hour.
1 nautical mile = 1,852 metres exactly.
Therefore: 1 knot = 1,852 ÷ 3,600 m/s ≈ 0.51444 m/s = 1.852 km/h = 1.15078 mph.

The nautical mile is defined as 1 arc-minute of latitude along any meridian. This makes knots naturally convenient for navigation: flying at 1 knot covers 1 arc-minute of latitude per hour, simplifying position calculations on charts.

Knots are the universal standard for:
• Aviation: aircraft airspeed, wind speed, and speed-over-ground are all in knots. Air traffic control worldwide uses knots.
• Maritime: ship speed, current speed, and wind speed in marine weather forecasts.
• Meteorology: tropical storm and hurricane wind speeds are reported in both mph/km/h for public and in knots for marine/aviation forecasts.

Note: aircraft altitude is almost universally measured in feet (not metres) in international aviation — a quirk where an imperial unit survived in an otherwise metricated field.

Conversion reference

Formula

Full conversion table:

1 m/s  = 3.6000 km/h = 2.2369 mph = 1.9438 kn
1 km/h = 0.2778 m/s  = 0.6214 mph = 0.5400 kn
1 mph  = 0.4470 m/s  = 1.6093 km/h = 0.8690 kn
1 kn   = 0.5144 m/s  = 1.8520 km/h = 1.1508 mph

Common practical conversions:
• 100 km/h = 62.14 mph  (highway speed comparison)
• 60 mph   = 96.56 km/h (common US speed limit)
• 30 mph   = 48.28 km/h (urban speed limit comparison)
• 500 kn   ≈ 926 km/h  ≈ 575 mph (typical jet airliner speed)
• 15 kn    ≈ 27.8 km/h ≈ 17.3 mph (typical container ship speed)

Quick mental approximation: km/h × 0.62 ≈ mph (exact: × 0.6214). mph × 1.6 ≈ km/h (exact: × 1.60934).

Why aviation uses knots

Note

Aviation standardized on knots before metrication became widespread, and the unit persisted for practical reasons. A nautical mile equals 1 arc-minute of latitude — so at a speed of 1 knot, a pilot covers exactly 1 arc-minute of latitude per hour. This makes chart navigation and position fixing mathematically clean.

All ICAO (International Civil Aviation Organization) standards use knots for speed. When pilots receive ATC (air traffic control) instructions anywhere in the world — New York, London, Tokyo, Sydney — speeds are in knots. Transitioning aviation to km/h would require replacing or recalibrating millions of instruments, charts, and procedures globally.

Altitude, however, is the exception: most countries use feet for aircraft altitude (the US, UK, and most of the world) while a few countries (Russia, China, and some others) have historically used metres for altitude.