Speed Distance Time Calculator — solve for whichever one you are missing

A speed distance time calculator exists because one relationship — distance equals speed multiplied by time — has to be rearranged two different ways depending on which number is missing, and because the three numbers almost never arrive in matching units. You have kilometres and a speed in miles per hour. You have a duration written as 1:47:30 and a distance in nautical miles. The arithmetic is trivial; the unit conversion is where the errors come from. This tool handles both at once and shows the working in units you can check.

Arb Digital builds the free tool library at arbsbuy.com, and this one sits in the everyday category alongside the converters and planners people reach for without thinking. It differs from the live speed converter in one clear way: that tool changes a speed from one unit into another and nothing more, while this one solves the triangle. If all you need is 60 mph expressed in km/h, use the converter. If you need to know how long 340 miles takes at 60 mph, you are in the right place.

What This Speed Distance Time Calculator Does

Choose which of the three quantities you want, enter the other two, and the calculator returns the missing one. Distance accepts kilometres, miles, metres, feet and nautical miles. Speed accepts kilometres per hour, miles per hour, metres per second and knots. Time is entered as three separate boxes for hours, minutes and seconds, so you can copy a duration straight off a watch or a timetable without converting it first.

Everything is converted internally to metres and seconds, the calculation is done once in those base units, and the answer is converted back into whatever unit you selected. That single-pipeline approach is why you can mix a distance in nautical miles with a speed in miles per hour and still get a sensible time — the calculator never tries to divide one unit by an incompatible one.

The four supporting figures go beyond the bare answer. Pace inverts the speed so you see minutes per kilometre or minutes per mile rather than distance per hour, which is how runners, cyclists and swimmers actually think. Elapsed time adds your rest stops to the moving time. Average including rest divides the full distance by that longer elapsed time, which is the number that decides what time you arrive. Distance per hour restates the speed as ground covered in sixty minutes, useful for sketching a multi-day route.

How to Use It

1. Pick what you are solving for. The selected quantity's input box is ignored and its value is replaced by the answer, so you do not have to clear it first.
2. Enter the two you know, with their units. The unit dropdowns are independent — kilometres with miles per hour is a perfectly valid combination and the conversion is handled for you.
3. Type the duration into the three time boxes. Hours, minutes and seconds left to right. Leave any of them at zero. Values above 59 in the minutes or seconds box are still added correctly, so 0h 95m is treated as an hour and thirty-five minutes.
4. Add your rest stops in minutes. This never changes the solved speed, which is always a moving speed. It changes elapsed time and the average-including-rest figure only.
5. Read the hero for the answer and the grid for pace, elapsed time and the honest door-to-door average. Click Calculate after any change, or just type — the fields recalculate as you go.

The Formula / How It's Calculated

One relationship generates all three formulas. Distance equals speed multiplied by time. Rearranged, speed = distance ÷ time and time = distance ÷ speed. The only discipline required is that the units agree before you divide, which is what the internal conversion to metres and seconds guarantees.

Take the default values. A distance of 120 kilometres is 120,000 metres. A moving time of one hour and thirty minutes is 3,600 + 1,800 = 5,400 seconds. Speed is therefore 120,000 ÷ 5,400 = 22.222 metres per second. To display that in kilometres per hour, divide by the metres-per-second value of one km/h, which is 0.27778, giving exactly 80 km/h. Pace is the inverse: sixty minutes divided by 80 gives 0.75 minutes per kilometre, which the tool prints as 0:45 per km.

Now the rest stops. Fifteen minutes is 900 seconds, so elapsed time is 5,400 + 900 = 6,300 seconds, or one hour and forty-five minutes. Average speed including rest is 120,000 ÷ 6,300 = 19.048 m/s, which is 68.57 km/h. The moving speed and the trip average differ by more than eleven kilometres per hour, and that gap is the single most common reason an estimated arrival time turns out to be wrong.

Average Speed Is Not the Average of Two Speeds

This is the trap that catches almost everyone, including people comfortable with arithmetic. Drive fifty kilometres at 100 km/h, then fifty kilometres at 50 km/h. The obvious answer for the trip average is 75 km/h. It is wrong. The first half takes 0.5 hours, the second takes 1.0 hours, so 100 kilometres took 1.5 hours and the average is 66.7 km/h.

The reason is that speed is a rate, and rates over equal distances combine as a harmonic mean rather than an arithmetic one. You spend more time at the slower speed, so the slower speed carries more weight. The correct expression for two equal legs is 2ab ÷ (a + b), which for 100 and 50 gives 20,000 ÷ 150 = 66.7. Averaging the two speeds directly only works when you spend equal time at each, which on a real journey almost never happens.

The practical consequence is that slow sections punish a schedule far more than fast sections rescue it. Losing ten minutes in a town takes ten minutes back at highway speed only if the highway section is long enough to contain that recovery, and it usually is not. To check any two-leg journey properly, solve each leg separately here, add the times, then divide total distance by total time.

Why the Journey Always Takes Longer Than the Maths Says

A route planner says 340 kilometres at an average of 85 km/h, so four hours. The trip takes four hours and fifty minutes and nobody can point to where the time went. It went into three places, none of which appears in the speed calculation.

The first is acceleration and deceleration. A vehicle at a standstill is not travelling at its cruise speed, and every junction, roundabout and set of lights costs a slice of time that the constant-speed formula assumes away. Twenty sets of lights, thirty seconds each, is ten minutes before anyone has stopped for coffee.

The second is that posted speed is a ceiling, not an average. Traffic density, weather, roadworks and the vehicle in front all pull the achieved speed below the limit for long stretches. The third is rest, which is why this calculator has a dedicated field for it. Fuel stops, food, bathroom breaks and simple fatigue add up, and on a long day they are the dominant term. Enter them honestly and the average-including-rest figure gives you an arrival time you can actually plan around. For the fuel side of the same journey, the fuel cost calculator and the road trip cost calculator take over where this one stops.

Pace Versus Speed, and Why Athletes Use the Inverse

Speed is distance per unit time. Pace is time per unit distance. They carry identical information, and yet runners, rowers and swimmers universally use pace while drivers and pilots universally use speed. The reason is what each one makes easy.

If you are racing a fixed distance, pace multiplies straight into a finish time: 5:00 per kilometre over ten kilometres is fifty minutes, done in your head at the roadside. Speed would require you to divide, which nobody wants to do while breathing hard. If you are covering an open-ended distance and want to know how far you will get, speed multiplies straight into distance and pace would need inverting.

The grid on this page shows pace in minutes and seconds per kilometre or per mile, matching the unit you chose for distance, so you can read the number in the form your sport uses. For race-specific projections across distances, the live running pace calculator and the marathon time predictor apply the fatigue models that a pure speed calculation deliberately leaves out — this tool assumes a constant rate and does not pretend otherwise.

Choosing Units Without Getting Caught Out

Three unit traps show up repeatedly. The first is the nautical mile, which is 1,852 metres rather than the statute mile's 1,609.344. A knot is one nautical mile per hour, so 20 knots is 37.04 km/h, not 32.19. Marine and aviation figures use nautical units almost exclusively, and mixing them with statute miles silently inflates or deflates a distance by fifteen percent.

The second is metres per second, which is the SI coherent unit for speed and the one physics problems are set in. The NIST guide to SI units sets out why the base units are metre and second and why derived units such as m/s follow from them, and the BIPM page on the International System of Units gives the defining constants behind those definitions, including the fixed speed of light. Converting m/s to km/h means multiplying by 3.6, and the reverse means dividing by 3.6 — a factor people invert under pressure.

The third is decimal time. A duration of "2.5 hours" is two hours thirty minutes, but "2:50" on a stopwatch is two hours fifty minutes, which is 2.833 hours. Feeding 2.50 into a field expecting hours-and-minutes, or the reverse, produces an error of twenty minutes with no warning. That is exactly the conversion the time to decimal converter handles, and the time duration calculator will give you the gap between two clock times if you need to derive the duration first.

Physics Problems Versus Real Journeys

This calculator assumes constant speed. That assumption is exactly right for a textbook problem and only approximately right for anything that happens outdoors, so it is worth being clear about where the line falls.

For a problem that says "a train travels at a constant 90 km/h for 2 hours 40 minutes", the answer here is exact: 240 kilometres, no caveats. For a cyclist who says they averaged 28 km/h over ninety minutes, the answer is also exact, because an average speed already absorbs every variation within it. The assumption only becomes a limitation when you try to predict forwards from an instantaneous or peak speed, because no journey is spent entirely at its peak.

Where acceleration itself is the subject rather than a nuisance — a vehicle changing speed at a known rate, an object under gravity — the constant-speed relationship no longer applies and you need the kinematic equations instead. The acceleration calculator and the free fall calculator cover that ground. Use this page for average and constant-rate work, which is the overwhelming majority of practical cases.

Common Mistakes to Avoid

• Averaging two speeds directly — over equal distances the correct combination is 2ab ÷ (a + b), and the naive average always overstates the result.
• Treating decimal hours as hours and minutes — 1.45 hours is one hour twenty-seven minutes, not one hour forty-five.
• Mixing nautical and statute miles — a nautical mile is 1,852 metres against 1,609.344, a fifteen percent gap that changes every downstream figure.
• Leaving rest stops out of an arrival estimate — moving speed answers a different question from the one "what time will I get there" is asking.
• Planning at the speed limit — the limit is a ceiling that traffic, weather and junctions pull you below for most of any real route.

Related Free Tools From Arb Digital

Use the speed converter when you only need a unit change, the running pace calculator for race pacing, the time duration calculator to turn two clock times into a duration, the fuel cost calculator for what the distance costs to drive, the great circle distance calculator for point-to-point distance across the globe, and the unit converter for everything else. The full free online tools hub lists the rest.

Frequently Asked Questions

This calculator assumes a constant rate of travel and is provided for planning and educational use. It does not account for traffic, terrain, weather or vehicle behaviour, and its output should not be relied on for navigation or safety-critical decisions.