The Speed-Distance-Time formula is one of the most common math/physics problems — used in travel planning, race calculations, school exams, and competitive tests like CAT, RRB, and SSC.
This guide covers the formula, common unit conversions, and worked examples.
Calculate Speed/Distance/Time — Free
Solve any SDT problem with auto unit conversion.
The Core Formula
Speed = Distance / Time
Distance = Speed × Time
Time = Distance / SpeedMnemonic
Use the SDT triangle: Distance on top, Speed × Time at bottom. Cover the unknown to see the formula.
Unit Conversions
| From | To |
|---|---|
| 1 km/h | 0.278 m/s |
| 1 m/s | 3.6 km/h |
| 1 mile/h (mph) | 1.609 km/h |
| 1 km | 1000 m / 0.621 mi |
| 1 hour | 60 min / 3600 sec |
m/s to km/h: multiply by 3.6 (or 18/5). km/h to m/s: divide by 3.6 (or multiply by 5/18).
Worked Examples
Example 1: Travel Time
Distance: 240 km. Speed: 60 km/h. Time?
Time = 240 / 60 = 4 hoursExample 2: Average Speed
Drove 100 km in 2 hours. Speed?
Speed = 100 / 2 = 50 km/hExample 3: Two-Speed Trip
Drove 60 km at 60 km/h, then 60 km at 30 km/h. Average speed?
Time1 = 60/60 = 1 hour
Time2 = 60/30 = 2 hours
Total time = 3 hours
Total distance = 120 km
Average = 120/3 = 40 km/h (NOT 45 km/h!)Common SDT Question Types
- Trains crossing platforms — train length + platform length
- Trains passing each other — relative speed (sum or difference)
- Boats and streams — upstream = boat speed - current
- Average speed — total distance / total time
- Catching up — relative speed × time = distance gap
How to Use the Tool (Step by Step)
- 1
Pick Unknown
Speed, distance, or time?
- 2
Enter Two Known Values
With correct units.
- 3
Convert Units If Needed
Tool auto-converts.
- 4
See Result
Tool calculates the unknown.
- 5
Try Different Units
Switch km/h to m/s, etc.
Frequently Asked Questions
How to convert km/h to m/s quickly?+−
Multiply by 5/18. 90 km/h × 5/18 = 25 m/s.
Why average speed isn't simple average?+−
Time at slower speed is longer, so it weights more. Use total distance / total time, not (s1+s2)/2.
How to solve relative speed?+−
Same direction: subtract speeds. Opposite direction: add speeds. Then use SDT formula.
Can SDT handle non-uniform motion?+−
Basic formula assumes constant speed. For changing speed, use total distance / total time for average.
What is velocity vs speed?+−
Speed is scalar (just magnitude). Velocity is vector (magnitude + direction). For most problems, treated same.
Does the formula work for any units?+−
Yes, as long as units are consistent. Distance in km, time in hours = speed in km/h.
Calculate Speed/Distance/Time — Free
Solve any SDT problem with auto unit conversion.
Open SDT Calculator ->Related Guides
How to Add and Subtract Time
Calculate time differences, add hours/minutes, convert between units, and compute durations across days.
How to Calculate Fuel Cost for a Trip
Calculate trip fuel cost, compare petrol vs diesel vs CNG vs EV running costs with current Indian prices.
How to Calculate the Mileage of a Car or Bike (km/l) and Fuel Cost Per Km
Calculate vehicle mileage (km/l), fuel cost per kilometre, trip fuel requirements, and compare fuel costs between petrol, diesel, CNG, and electric vehicles in India.