Speed, Distance and Time Calculator

How to use it

Choose Speed, Distance or Time in the first menu. The field you are solving for disappears and the other two stay on screen.

Type your two known values. Each one has its own unit menu: km, miles or metres for distance, hours, minutes or seconds for time, and km/h, mph or m/s for speed.

The answer updates live. Nothing needs pressing.

You can mix units freely. Distance in miles, time in minutes, speed in km/h is fine. Everything is converted to metres and seconds first, the sum is done in those units, then the answer is converted back for display.

Every result is shown twice. Solve for speed and you get km/h on the big number with mph and m/s underneath. Solve for distance and you get km, then miles and metres. Solve for time and you get decimal hours, then minutes and seconds.

The one formula, written three ways

• Speed = distance ÷ time
• Distance = speed × time
• Time = distance ÷ speed

These are the same equation rearranged. Know any two of the three and the third follows. The only thing that trips people up is units, which is why the conversion happens behind the scenes here rather than in your head.

Your speedometer is legally allowed to overstate

Here is why the average speed this tool gives you almost never matches what the needle said.

Under UN Regulation No 39, the international rule that governs speedometers in the UK and across Europe, paragraph 5.4 sets the relationship between the speed shown on the dial (V1) and the true speed (V2) as:

0 ≤ V1 - V2 ≤ 0.1 × V2 + 4 km/h

Read that in plain English and it says two things. A speedometer may never read low. And it may read high by up to ten per cent of your actual speed plus another 4 km/h.

Run the numbers at motorway pace. A dial showing 70 mph can legally sit above a true speed of 61 mph. A car genuinely doing 70 mph is allowed to display nearly 80. Both cars are fully compliant.

The UK route into this is regulation 35 of the Road Vehicles (Construction and Use) Regulations 1986, which lets a vehicle comply either with its own paragraph (1) or with Community Directive 97/39 or ECE Regulation 39.

So when you take the distance off your odometer, divide by the elapsed clock time and get a number lower than you expected, the calculator is not wrong. Manufacturers deliberately build in that upward bias, partly to stay on the safe side of the never-read-low rule and partly because tyre wear shrinks the rolling radius over the life of a tyre, which would otherwise push the reading the other way.

Sources:UN Regulation No 39, paragraph 5.4andregulation 35, Road Vehicles (Construction and Use) Regulations 1986.

Average speed is not the average of your speeds

This one catches people out constantly.

Drive 60 km at 30 km/h, then another 60 km at 60 km/h. The obvious answer for your average is 45 km/h. The real answer is 40.

Check it. The slow leg takes 2 hours, the quick leg takes 1 hour. That is 120 km in 3 hours, so 40 km/h.

The slow leg dominates because you spend twice as long inside it. Averaging two speeds only works when you held each one for the same amount of time, not the same distance. For equal distances the correct operation is the harmonic mean, and it always lands below the arithmetic average.

Which is the practical case against flooring it between traffic lights. Halving the time you spend on a short fast stretch moves your door-to-door average far less than most drivers assume, because the slow sections keep the clock running.

Total distance divided by total time is the only average speed worth quoting. Feed those two totals into this calculator and you get the honest figure.

What the split table shows

Under the result you get a table of split times: how long it takes to reach each point along the trip at that constant speed.

The distance steps snap to tidy 1, 2 or 5 intervals rather than awkward fractions, capped at twelve rows so a 2 mile run and a 2,000 mile drive both stay readable. A final row is added for the finish line if the last tidy step falls short of it.

Times in that table are formatted properly, so 1:23:45 for anything past the hour and 4:37for anything under it. That is deliberately different from the big Time result above, which reads out in decimal hours. 1.5 h there means 1 hour 30 minutes, not 1 hour 50.

For runners, a 4 hour marathon over the standard 42.195 km works out at 5:41 per km, or 9:09 per mile. Set the calculator to find Time, enter the distance and your target pace as a speed, and the table gives you every checkpoint.

One quirk worth knowing. When you solve for distance, the distance unit menu is hidden, so the table borrows its unit from the speed you entered: mph gives you miles, m/s gives you metres, km/h gives you kilometres.

Worked example

You cover 150 km and the trip takes 2 hours. Speed = 150 ÷ 2 =75 km/h, which the tool also reports as 46.6 mph and 20.83 m/s.

Turn it round. Hold 75 km/h for 3 hours and distance = 75 × 3 = 225 km.

Turn it round again. 225 km at 75 km/h and time = 225 ÷ 75 = 3 hours.

Frequently asked questions