The speed–distance–time relationship is one of the most used calculations in everyday life: estimating journey times, working out average speeds from GPS data, planning fuel stops, solving physics problems, or just answering “are we there yet?” with a real number. This calculator solves for any one of the three when you supply the other two, showing step-by-step working and presenting the result in your choice of unit.
The formula and how to use it
The three quantities are linked by a single identity — usually written as the DST triangle:
S = D ÷ T · D = S × T · T = D ÷ S
Pick which quantity you want to calculate, enter the two known values (with their units), and the answer appears instantly. A unit selector next to the result lets you view it in any supported unit without re-entering anything.
Supported units
| Quantity | Units |
|---|---|
| Speed | m/s, km/h, mph, knots, ft/s, Mach |
| Distance | m, km, mi, ft, yd, nautical miles, light-years |
| Time | seconds, minutes, hours, days, weeks |
All inputs are converted to metres and seconds internally before the formula is applied, so mixing kilometres with hours or miles with minutes works correctly.
Worked example — road journey
A motorway journey of 312 miles at an average speed of 65 mph. How long does it take?
Step 1 — identify the formula. Solving for time: T = D ÷ S.
Step 2 — plug in. T = 312 ÷ 65 = 4.8 hours.
Step 3 — convert. 0.8 hours = 48 minutes, so the journey takes 4 hours 48 minutes. The calculator shows this breakdown automatically next to the decimal result.
| Scenario | Speed | Distance | Time |
|---|---|---|---|
| Motorway journey | 65 mph | 312 mi | 4h 48m |
| City cycling | 15 km/h | 12 km | 48 min |
| 100 m sprint (10 s) | 10 m/s | 100 m | 10 s |
| Sound crossing a room (10 m) | 343 m/s | 10 m | 29 ms |
| Light to the Moon | 299,792,458 m/s | 384,400 km | ~1.28 s |
How mixed units are handled (formula note)
Because speed carries two units (distance per time unit), naive arithmetic with mixed units gives wrong answers. The calculator avoids this by converting every input to SI base units first:
- Speed in km/h is multiplied by 1/3.6 to get m/s.
- Distance in miles is multiplied by 1609.344 to get metres.
- Time in minutes is multiplied by 60 to get seconds.
The formula is then applied in metres and seconds, and the SI result is divided by the output unit’s conversion factor. The step-by-step working panel shows both the raw inputs and the converted SI values so you can verify each stage.
Quick reference — unit conversion factors
| Speed | metres per second |
|---|---|
| 1 km/h | 0.2778 m/s |
| 1 mph | 0.4470 m/s |
| 1 knot | 0.5144 m/s |
| 1 ft/s | 0.3048 m/s |
| Mach 1 | 343 m/s (at 20 °C) |
For physics coursework the tool is useful for constant-velocity problems. For kinematics with acceleration, see the Acceleration Calculator — it solves v = u + at and related equations.