Speed, Distance, Time Formula
Calculate speed, distance, or travel time from the other two using d = s × t.
Covers unit conversions for mph, km/h, meters per second, and knots navigation.
Need to calculate, not just reference? Use the interactive version.
Open Speed Distance Time Calculator →
The Formula
d = s × t | s = d / t | t = d / s
These three forms of the same relationship let you find any one value when you know the other two. They apply to any constant-speed motion — driving, flying, sailing, running, or cycling.
Variables
| Symbol | Meaning |
|---|---|
| d | Distance traveled (km, miles, meters, etc.) |
| s | Speed (km/h, mph, m/s, etc.) |
| t | Time taken (hours, minutes, seconds, etc.) |
Example 1
A car travels at 90 km/h for 2.5 hours. How far does it go?
d = 90 × 2.5
d = 225 km (about 140 miles)
Example 2
A flight covers 3,600 km in 4 hours. What is the average speed?
s = 3,600 / 4
s = 900 km/h (about 559 mph)
When to Use It
Use the speed-distance-time formula when:
- Estimating travel time for road trips or flights
- Calculating average speed from distance and time
- Planning routes and departure times
- Converting between different speed units
Key Notes
- The formula assumes constant speed — the result is always an average; real trips with varying speeds (city traffic then highway) should be calculated segment by segment
- Units must be consistent throughout: if speed is in km/h, time must be in hours and distance comes out in km — mixing units (km/h with minutes) is the most common calculation error
- Average speed for a round trip at two different speeds is not their arithmetic mean — it is the harmonic mean: 2/(1/v₁ + 1/v₂)
Key Notes
- Three related formulas: d = s × t; s = d/t; t = d/s: A simple but universal relationship. The "DST triangle" (cover the unknown, the remaining two show the operation) is a common teaching aid. Units must be consistent — mix km/h with hours, or m/s with seconds.
- Average speed is not the average of speeds: If you drive 60 km/h for 1 hour and 30 km/h for 1 hour, your average speed is (60+30)/2 = 45 km/h. But if you drive 60 km/h for 60 km and 30 km/h for 60 km, average speed = 120 km / 3 hours = 40 km/h. Always use total distance ÷ total time.
- Relative speed: Objects moving in the same direction: relative speed = |s₁ − s₂|. Objects moving toward each other: relative speed = s₁ + s₂. Used for calculating when two moving objects meet or pass each other.
- Unit conversions: 1 m/s = 3.6 km/h; 1 mph ≈ 1.609 km/h; 1 knot ≈ 1.852 km/h. Convert before calculating — mixing unit systems is one of the most common errors in speed-distance-time problems and real-world navigation.
- Applications: Speed-distance-time calculations are used in navigation (ETA estimation), physics (kinematics), transport planning, fluid dynamics (flow velocity), sports analysis (pace per km/mile), and engineering (conveyor belt and machinery speed).