Speed Converter
Convert between km/h, mph, m/s, knots, Mach and more. See your result against real-world reference speeds, or flip to the pace calculator to turn a running pace into a predicted race finish time.
Enter a running or cycling pace to see the equivalent speed and predicted finish times.
How to Convert a Speed
Features
A Knot Is Not a Kilometre an Hour
Ten units of speed exist in everyday use, and a handful of them are close enough in size to genuinely mislead someone reading quickly. A knot is 1.852 km/h — read a weather bulletin's "25 knots" as 25 km/h instead and a fairly stiff breeze suddenly looks like nothing worth mentioning, an error significant enough to matter to anyone actually sailing in it. Road users cross a similar trap at borders: a speedometer reading "100" means completely different things in a country that measures in mph versus one that measures in km/h — the same needle position, a 60% difference in actual velocity. None of these units are hard to convert once you know which one you're looking at; the risk is entirely in assuming two differently-labelled numbers are close enough to treat as the same.
Why Mach 1 Isn't a Fixed Number
Every other unit on this page means exactly one speed, always. Mach doesn't — it's a ratio to the local speed of sound, and the speed of sound depends on air temperature, not altitude directly. At sea level under standard conditions, Mach 1 is about 340.29 m/s (1,225 km/h). Climb to cruising altitude, where the air is roughly −56°C, and the speed of sound drops to around 295 m/s — so an aircraft holding Mach 1 up there is physically moving slower than an aircraft holding Mach 1 at sea level, despite the identical Mach reading. This converter uses the sea-level standard value as its fixed reference point, which is the conventional baseline for comparing supersonic performance figures, but it's worth remembering that "Mach 2" on a spec sheet is really shorthand for "twice the local speed of sound, wherever that aircraft happens to be."
A Cockpit, a Finish Line, and a Rental Car
A student pilot reads "250 knots" in a weather briefing and needs it in km/h to cross-check against a chart printed in metric — converted, that's about 463 km/h, a five-second lookup instead of a mental-math detour before a flight. A runner training at 5:30 per kilometre wants to know what that pace predicts for an upcoming half marathon — the Pace & Race Time Calculator below turns that straight into a 1:56:03 estimate, along with the equivalent pace in minutes per mile for a training plan written the other way. And a traveller picking up a rental car abroad glances at a speedometer that suddenly reads in unfamiliar units, converting a home-country 70 mph speed limit into the 113 km/h they now need to actually watch for on signs. A baseball fan reading that a pitcher threw a fastball at 102 mph converts it to 164 km/h to compare against a cricket bowler's speed gun reading from a match on the other side of the world, settling a genuinely common argument about which sport's fastest deliveries are actually faster. For the distance side of any of these — a flight plan leg, a race course, a road trip — the Length Converter is the natural next stop.
Built for More Than One Number
Most speed converters stop at a single result. This one adds two things that turn an abstract conversion into something you can actually use. The real-world reference table, visible next to the main result, lists eight recognisable speeds — from a brisk walk up to the International Space Station's orbital velocity — converted live into your chosen target unit, with whichever one sits closest to your current result highlighted automatically so a number like "340 km/h" instantly registers as roughly Formula 1 top-speed territory rather than just a figure on a screen. Separately, the Pace & Race Time Calculator flips the whole problem around: instead of starting from a speed, it starts from a pace — minutes and seconds per kilometre or mile — and works out the equivalent speed plus predicted finish times for a 5K, 10K, half marathon, and full marathon, which is the exact question most runners actually have rather than a raw km/h figure. Above the all-units table, three filter chips — Everyday, Marine & Aviation, Scientific & Extreme — narrow the full list down to just the family that matters for whatever's being converted, so a road-trip speed check isn't sharing a table with the speed of light.
Where "Fast" Gets Misread
Speed and velocity get used interchangeably in conversation, but they aren't the same thing: speed is a single number, velocity is that number plus a direction. Every unit in this converter measures speed only, which is the right and sufficient choice for a speedometer, an airspeed indicator, or a race pace — direction never actually enters the arithmetic for any of them. Geography causes a subtler mix-up: only around 17 of the world's roughly 195 countries post road speeds in mph, essentially the US and UK plus a scattering of smaller nations and territories with close historical ties to one or the other, including Myanmar, Liberia, and several Caribbean islands. Every other country runs on km/h, which is precisely why this converter treats the two as equally first-class rather than assuming either one is the default.
Where the Word "Knot" Actually Came From
It's not a metaphor. Sailors on 17th-century ships measured speed with a device called a chip log — a wooden panel tied to a rope with knots spaced at regular intervals, thrown overboard and let run out for a fixed time measured by a sandglass. Counting how many knots passed through a sailor's hand in that interval gave a direct reading of the ship's speed, and the name simply stuck once the unit was formalised centuries later against the nautical mile, long after the chip log itself had been retired in favour of mechanical and electronic logs. It's a rare case of a technical unit whose name is a literal description of how it used to be measured rather than an abstraction — and it explains why "knots per hour" is a genuine mistake, not just informal phrasing: the hourly rate is already built into the name.
What This Speedometer Won't Show
This tool converts a speed value between units — it does not calculate distance from speed and time, or time from speed and distance, beyond what the Pace & Race Time Calculator already covers for running. It also has no way to account for wind or current: an aircraft's airspeed in knots and its actual groundspeed can differ meaningfully with a strong headwind or tailwind, and the same is true of a boat crossing a current, neither of which this converter attempts to model. As always, nothing typed here is sent anywhere — the arithmetic runs as plain JavaScript inside the browser tab, with no server call and nothing logged, which matters for flight-test data or unpublished vehicle performance figures as much as for a training log. For anything with safety or certification weight behind it — aviation, marine navigation, vehicle homologation — treat the output as a fast reference and confirm it against certified instruments, and the tool keeps working identically with no internet connection at all once the page has loaded.