Speed is the most everyday physics there is: how much distance you cover in how much time. Divide one by the other and you have it — speed = distance ÷ time. The same little triangle of quantities answers every related question: rearrange it to find how far you'll travel in a given time, or how long a journey will take at a known speed. The only thing to watch is units — kilometres and hours give km/h, metres and seconds give m/s — and converting between them is just a matter of the constants 3.6 and 2.237.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the definition of average speed, v = d/t.
The speed equations
The three forms are the same relationship rearranged, often remembered as the "speed–distance–time triangle." Mixed units are the usual stumbling block: this calculator converts everything to metres and seconds internally, then presents the speed in all three common units. The conversion factor 3.6 is simply 3600 seconds per hour divided by 1000 metres per kilometre; 2.23694 does the same for the mile.
Worked example — a road trip
Scenario: Driving 150 km in 2 hours — what is the average speed, and how long for 240 km at the same pace?
The trip averages 75 km/h. At that pace a 240 km leg takes 3.2 hours, or 3 hours and 12 minutes. Note that "average speed" hides the detail — you might have done 110 km/h on the motorway and crawled through town — but for planning, total distance over total time is exactly the number you need. To go faster you either add speed or, on the same road, accept that distance and time rise together.
Frequently Asked Questions
speed = distance ÷ time. 150 km in 2 h = 75 km/h. Match units or let the calculator convert.
d = speed × time; t = distance ÷ speed. 75 km/h for 2 h = 150 km; 150 km at 75 km/h = 2 h.
×3.6 for km/h, ×2.23694 for mph. 20 m/s = 72 km/h = 44.7 mph.
Speed is scalar (just fast); velocity adds direction. Circling at steady speed = changing velocity.
Total distance ÷ total time. 100 km then 50 km in 2 h = 75 km/h, not the mean of two speeds.