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Speed, Distance and Time: Formulas, Conversions and Exam Tricks
The three formulas, unit conversions, why average speed isn't the simple average, and how to solve train and relative speed problems.
By Team Vind · Updated · 5 min read
Speed, distance and time are tied together by one simple relationship. Know any two and you can find the third, whether you're planning a road trip or solving a time-and-distance question in an SSC, bank or railway exam.
The three formulas
Speed = distance ÷ time · Distance = speed × time · Time = distance ÷ speed
A trick to remember them: draw a triangle with D on top and S and T below. Cover the one you want; what's left shows the formula (D over S × T).
- Speed: Delhi to Jaipur, 280 km in 5 hours 36 minutes (5.6 h): 280 ÷ 5.6 = 50 km/h.
- Distance: 2 hours 30 minutes at 64 km/h: 64 × 2.5 = 160 km.
- Time: 350 km at 70 km/h: 350 ÷ 70 = 5 hours.
Try it: Speed, Distance & Time Calculator
Travel Time
Converting km/h and m/s
m/s = km/h × 5 ÷ 18 · km/h = m/s × 18 ÷ 5
| km/h | m/s |
|---|---|
| 18 | 5 |
| 36 | 10 |
| 54 | 15 |
| 72 | 20 |
| 90 | 25 |
| 108 | 30 |
Always convert to matching units before using the formula: kilometres with hours, or metres with seconds. Convert minutes to hours by dividing by 60 (45 minutes = 0.75 hours).
Average speed: the classic trap
You drive to a town at 40 km/h and back at 60 km/h. The average speed is not 50 km/h, because you spend longer at the slower speed. Average speed is total distance ÷ total time, and for two equal distances that gives:
Average speed = 2ab ÷ (a + b) = 2 × 40 × 60 ÷ 100 = 48 km/h
Trains and relative speed
- A train passing a pole or person covers its own length.
- Passing a platform or bridge, it covers its own length plus the platform's.
- Two objects moving in opposite directions: add the speeds. In the same direction: subtract them.
Example: a 300 m train at 72 km/h crosses a 200 m platform. Speed = 72 × 5 ÷ 18 = 20 m/s. Distance = 300 + 200 = 500 m. Time = 500 ÷ 20 = 25 seconds.
Running or walking? The pace calculator works in minutes per kilometre.
Frequently asked questions
How do I convert 72 km/h to m/s?
Multiply by 5/18: 72 × 5 ÷ 18 = 20 m/s.
What is the formula for average speed?
Total distance ÷ total time. For two equal distances at speeds a and b, it's 2ab ÷ (a + b).
How long does it take to travel 150 km at 60 km/h?
150 ÷ 60 = 2.5 hours, or 2 hours 30 minutes.
How do I find the time for a train to cross a platform?
Add the train's length and the platform's length, convert the speed to m/s, and divide the total length by the speed.
Sources
Last reviewed: 10 October 2026