Speed Converter

km/h vs mph: Why the World Is Still Divided

The metric system was formalised in France in the late 18th century, and the International System of Units (SI) adopted kilometres per hour as the standard unit for road speed. Yet adoption was never universal. Today, the United States, Liberia, and Myanmar remain the only countries that still use miles per hour on public road signs, while the rest of the world has moved to km/h.

In Southeast Asia, the transition to metric was completed decades ago. Malaysia and Singapore adopted the metric system fully in the 1970s, and road speed limits across the region are expressed in km/h. Singapore's expressways — the Pan-Island Expressway (PIE), the Central Expressway (CTE), the Ayer Rajah Expressway (AYE) — all post limits in km/h, typically 90 km/h for urban expressways. Malaysians colloquially refer to highway speeds as "100 kilo" meaning 100 km/h. Australia completed its metric conversion in 1974, and all Australian road signs display km/h.

The United Kingdom presents a curious hybrid: metric units are used for almost everything else, but British road signs still display miles and miles per hour — a legacy of infrastructure costs that would make full conversion prohibitively expensive. The Singapore–Malaysia Second Link (Tuas–Gelang Patah) uses km/h limits on the Singapore side and km/h on the Malaysian side as well — unlike the older Johor Causeway, where both sides have historically posted slightly different speed limits reflecting each country's independent enforcement practices.

Knots and Naval Speed — A Standard That Never Changed

A knot is defined as one nautical mile per hour, and a nautical mile is exactly 1,852 metres — corresponding to one arc-minute of latitude on the Earth's surface. This elegant connection to the geometry of the globe made knots invaluable to navigators long before GPS existed: a ship knowing its speed in knots could directly calculate its position on a nautical chart without any unit conversion.

Aviation adopted knots from maritime tradition, and the relationship has proven so useful that neither industry has ever seriously attempted to convert to km/h. Singapore's Changi Airport — one of the world's busiest — operates entirely on the knot standard for air traffic control. When a Singapore Airlines Airbus A380 lands at Changi, its approach speed of roughly 140 knots (about 259 km/h) is communicated in knots between the flight crew and ATC. The same is true at Seletar Airport and Paya Lebar Air Base.

Singapore's position as the world's second-busiest container port by tonnage — with the Port of Singapore Authority (PSA) handling over 37 million TEUs annually — means that vessel speed is a constant operational concern. Container ships entering the Malacca and Singapore Straits operate under strict speed guidelines measured in knots, and maritime pilots boarding vessels at the Eastern and Western Anchorages communicate entirely in nautical units. The Vessel Traffic Information System (VTIS) operated by the Maritime and Port Authority of Singapore (MPA) tracks all ships in knots.

Sound Barrier and Mach Numbers — From Theory to Paya Lebar

Ernst Mach (1838–1916) was an Austrian physicist and philosopher who studied the behaviour of projectiles and gas at high velocities. The Mach number — the ratio of an object's speed to the local speed of sound — was named in his honour. Mach 1, the speed of sound, is approximately 1,235 km/h (343 m/s) at sea level at 20°C, but it varies significantly with altitude and temperature: at the cruising altitude of a commercial jet (around 35,000 feet), where temperatures fall to around −57°C, the speed of sound drops to roughly 1,062 km/h (295 m/s).

The sound barrier was first broken on 14 October 1947 by USAF test pilot Chuck Yeager in the Bell X-1 aircraft over the Mojave Desert, reaching Mach 1.06. Since then, supersonic flight has become routine for military aircraft. The Republic of Singapore Air Force (RSAF) operates the F-15SG Eagle, capable of speeds exceeding Mach 2.5, and has operated the F-16C/D Fighting Falcon. The RSAF's next-generation fighter, the F-35B Lightning II (selected by Singapore), can sustain supersonic flight at around Mach 1.6. Military supersonic operations in Singapore are conducted over the sea or during overseas training deployments, as supersonic flight over populated land is prohibited due to the sonic boom — the shockwave that produces a sharp thunderclap audible on the ground.

The Concorde, the retired Anglo-French supersonic airliner, cruised at Mach 2.04 (approximately 2,179 km/h), cutting the transatlantic crossing time to under four hours. Concorde never flew commercial routes to Singapore, but its legacy helped frame the debate about whether Singapore Airlines might consider supersonic options for its ultra-long-haul routes in future decades — a conversation that Boom Supersonic and others are now renewing with new-generation aircraft designs.