How This Calculator Works
Pick the tab that matches what you already know. Find Speed takes a distance and a time and divides one by the other. Find Distance takes a speed and a time and multiplies them together. Find Time takes a speed and a distance and divides one by the other. Speed Converter is a standalone two-way tool for switching a speed value between any of 26 units, with no distance or time involved at all. Every field supports a long list of units — from everyday ones like mph and km/h to specialized ones like knots, light speed, and inches per minute — so the same calculator works for a road trip, a physics problem, or a shipping-speed spec sheet.
The Speed, Distance & Time Formula
All three ways of solving this rearrange the same underlying relationship, sometimes remembered as the "DST triangle":
- Speed = distance ÷ time
- Distance = speed × time
- Time = distance ÷ speed
For example, riding a bike at a steady 10 meters per second for 60 seconds covers 10 × 60 = 600 meters. Flip it around: covering 600 meters in 60 seconds means a speed of 600 ÷ 60 = 10 m/s. The one rule that trips people up is unit consistency — the formula only gives a correct answer when distance and time are already expressed in units that match the speed unit you want out (miles and hours for mph, meters and seconds for m/s, and so on). This calculator handles that conversion automatically, so you can freely mix, say, kilometers with minutes and still get a correct mph result.
Why So Many Speed Units?
Different fields settled on different units because the numbers work out more conveniently for what they're measuring. Meters per second (m/s) is the SI base unit used in physics and engineering. Kilometers per hour (km/h) is the everyday road-speed unit across most of the world, while the US and UK use miles per hour (mph). Maritime and aviation use knots — nautical miles per hour — because a nautical mile is tied directly to a minute of latitude, which made manual navigation arithmetic simpler centuries before GPS existed. At the extreme end, light speed [c] (299,792,458 m/s exactly, by definition) shows up in physics problems and gives a quick way to express a fraction of the universe's speed limit.
A quick reference for the five most common units, each converted to the other four:
- 1 m/s = 3.6 km/h = 2.237 mph = 1.944 knots = 3.281 ft/s
- 1 km/h = 0.278 m/s = 0.621 mph = 0.540 knots = 0.911 ft/s
- 1 mph = 0.447 m/s = 1.609 km/h = 0.869 knots = 1.467 ft/s
- 1 knot = 0.514 m/s = 1.852 km/h = 1.151 mph = 1.688 ft/s
Speed vs. Velocity
This calculator computes speed, not velocity, and the difference matters more than it might seem. Speed is a scalar — just a magnitude, like "60 mph." Velocity is a vector — a magnitude and a direction, like "60 mph due north." A car looping a race track at a constant 150 mph has a constant speed but a constantly changing velocity, since its direction is always turning. For any straight-line trip in one direction, speed and the magnitude of velocity are identical, which is why the two terms get used interchangeably in everyday conversation. The distinction only actually changes the math once direction changes are involved — for a round trip, for instance, the average velocity is zero (you end up back where you started), even though the average speed is very much not zero.
The Average Speed Trap (A Classic Mistake)
Here's a genuinely common error, including in textbook and exam questions: if you drive somewhere at 60 mph and drive back the same route at 40 mph, your average speed for the whole round trip is not (60 + 40) ÷ 2 = 50 mph. That simple average only works if you spend equal time at each speed — but on a round trip of equal distance, you spend more time at the slower speed, which pulls the true average down.
The correct method uses the harmonic mean: average speed = 2AB ÷ (A + B), where A and B are the two speeds. For 60 mph and 40 mph: 2 × 60 × 40 ÷ (60 + 40) = 4,800 ÷ 100 = 48 mph — noticeably lower than the naive 50 mph guess. The reasoning holds for any two-leg trip of equal distance: use this calculator's Find Time tab to work out how long each leg actually takes at its own speed, add the two times together, then use Find Speed with the total round-trip distance over that combined time to get the true average.
Real-World Speed Reference Points
Numbers are easier to sanity-check against something familiar. A relaxed walking pace is about 3 mph (5 km/h, 1.4 m/s). A competitive marathon finish today requires holding roughly 13 mph (21 km/h) for over two hours straight — for comparison, the current men's marathon world record, set by Sabastian Sawe at the 2026 London Marathon in 1:59:30, works out to almost exactly that pace. Highway driving in the US tops out at 65–70 mph in most states. Sound travels at about 767 mph (1,235 km/h) through sea-level air at 20°C, and a typical jet airliner cruises at a meaningful fraction of that, around 550 mph. For context, a moderate earthquake's seismic waves travel around 3–8 km/s depending on wave type — far faster than any everyday vehicle, but far slower than light.
Frequently Asked Questions
What's the formula for speed, distance, and time?
Speed = distance ÷ time, distance = speed × time, and time = distance ÷ speed. All three are the same relationship rearranged, and this calculator's three tabs each apply one version depending on which value is missing.
How do I convert mph to km/h?
Multiply by 1.609344 — for example, 60 mph × 1.609344 = 96.56 km/h. The reverse conversion (km/h to mph) divides by the same number, or equivalently multiplies by 0.621371.
What's a knot, and why isn't it just another word for mph?
A knot is one nautical mile per hour, and a nautical mile (1,852 meters exactly) is a different length than a standard mile (1,609.344 meters), so knots and mph don't convert with a clean round number — 1 knot equals about 1.151 mph, not exactly 1.15 or any simpler figure.
Why is my average speed for a round trip not just the average of the two speeds?
Because you spend more time traveling at the slower speed over the same distance, which pulls the true average down. The correct formula is the harmonic mean, 2AB ÷ (A + B) — for a 60 mph leg and a 40 mph leg, that's 48 mph, not the 50 mph a simple average would suggest.
Is speed the same thing as velocity?
Not quite — speed is just a magnitude (how fast), while velocity also includes direction (how fast, and which way). They're numerically identical for motion in a single straight line, which is why the terms are often used loosely as synonyms, but they diverge for anything involving a turn, a loop, or a round trip.
How fast is the speed of light, and why is it listed as a speed unit?
Light in a vacuum travels at exactly 299,792,458 meters per second — an exact figure by definition, since the meter itself is now defined in terms of the speed of light rather than the other way around. It's included as a unit ("light speed [c]") because physics problems often express other speeds as a fraction of it, like "0.1c."
Does this calculator account for acceleration, or only constant speed?
Only constant (or average) speed — the distance ÷ time formula gives you the average speed over the whole trip, not the instantaneous speed at any single moment. A trip that includes stop-and-go traffic, acceleration, or braking still has a perfectly well-defined average speed; this calculator just can't tell you what the speedometer read at any specific second along the way.
What's the difference between a light year and a light speed unit?
A light year is a unit of distance — how far light travels in one year (about 9.46 trillion km) — while light speed [c] is a unit of speed itself. They're easy to conflate because both reference the speed of light, but one measures how far, and the other measures how fast.
This calculator provides general estimates for informational purposes only. Always double-check unit selections for anything safety-critical, such as aviation, maritime navigation, or vehicle speed compliance.