Convert m/s, km/h, mph, knots, ft/s and Mach all-to-all.
1 km/h = 0.621 mph = 0.278 m/s = 0.540 knot. Mach varies with air density and temperature; the calc uses the standard 343 m/s for 20 °C dry air at sea level.
Speed is one of the most-converted quantities in everyday life because the world stubbornly uses several different units in parallel. Cars use km/h in most of the world but mph in the US and UK. Boats and aircraft use knots (nautical miles per hour). Wind speed in scientific contexts is m/s; in weather forecasts it's km/h or mph. Mach (multiples of the speed of sound) appears in aviation. Running pace is sometimes min/km, sometimes min/mi. A simple all-to-all converter that takes any input unit and shows all common outputs simultaneously removes the constant context-switching. This calc handles the six most common units (m/s, km/h, mph, knots, ft/s, Mach) and visualizes the resulting speed on a logarithmic reference scale from walking pace to the speed of sound, so the user can instantly judge "is this fast or slow?".
The canonical unit is meters per second (m/s). Conversion factors (each unit's value in m/s):
A value V in input unit U converts to V × factor(U) m/s, then divides by factor(target) for any other unit. The mph and knot factors are exact by international agreement; m/s is exact by SI definition; Mach is approximate and marked as such.
The reference numline plot uses a log scale 1 → 1 500 km/h with anchor labels at: - 5 km/h: walk. - 30 km/h: cycle. - 50 km/h: urban. - 130 km/h: highway. - 300 km/h: TGV. - 1 235 km/h: sound (Mach 1).
Enter the speed value and pick the input unit from the dropdown. The result panel shows the same speed in all six units simultaneously: km/h, mph, m/s as headlines; knots, ft/s, Mach as smaller stats. The numline visualization places a marker at the converted km/h value, log-scaled against typical reference speeds.
Highway 130 km/h (input).
A jet's typical cruise of 480 knots.
A marathon pace of 12 km/h (5:00 min/km).
Mach is altitude- and temperature-dependent. The calc uses 343 m/s (20 °C dry air at sea level). At cruise altitude (10 000 m), air is colder (−50 °C) and thinner; speed of sound drops to ~295 m/s. So a "Mach 0.85 cruise" is 295 × 0.85 = 251 m/s = 905 km/h, not 343 × 0.85 = 1 050 km/h. Aviation speed indicators correct for this; the calc's Mach value is a sea-level reference only.
Statute vs nautical mile. 1 statute mile = 1 609.344 m. 1 nautical mile = 1 852 m. The calc's "mph" is statute miles per hour; "knots" is nautical miles per hour. Don't confuse them — 100 mph ≈ 87 knots, not 100 knots.
Imperial vs US gallons (irrelevant here, but adjacent). Speed conversions don't involve volume, but the same caution applies to other unit families.
Wind chill at speed. Speed of an aircraft, runner, or cyclist relative to the ground is what the calc converts. Speed relative to the air (used for wind chill, lift calculation, drag) requires subtracting wind vector — different problem.
Pace vs speed. Runners often think in pace (min/km or min/mi), the inverse of speed. To convert: pace_min_per_km = 60 / speed_kmh. The calc handles speed; for pace conversion use the running-pace calc.
Knots in weather. Aviation METAR/TAF reports use knots. EU surface weather reports use km/h. US ground weather uses mph. A 30-knot gust is 55 km/h, not the same as 30 km/h.
Speed of light and relativistic limits. The calc accepts arbitrarily high inputs but is non-relativistic; physically meaningful speeds are < 3 × 10⁸ m/s. At 0.1 c the relativistic time-dilation factor is ~0.5 %; at 0.5 c it's 15 %.
Negative input. Speeds are signed in physics (1D motion); the calc treats negative as invalid (returns "–"). For directionality, use velocity vectors.
Walking and running pace. Typical walking 4–6 km/h, running 8–18 km/h, sprinting 25–37 km/h, cycling 18–30 km/h, road cycling 35–50 km/h. The numline reference labels help you place inputs in context.