Average Speed Calculator
speed = distance / time. Pick units and read average speed in mph or km/h.
Result
General calculation reads
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How to use this calculator
- Enter distance and unit.
- Enter total elapsed time including breaks.
- Read mph and km/h.
About this calculator
Average speed is total distance divided by total time — including stops, traffic, and rest breaks. Different from "instantaneous speed" (what your speedometer reads). For a 3-hour drive covering 200 miles, average speed is 66.7 mph regardless of how fast you went at any moment.
What this calculator does
Computes average speed given distance and time (v = d / t) or one variable given the other two. Handles all common unit combinations including mph, km/h, m/s, and knots. Also supports the reverse calculations — solving for distance given speed and time, or time given speed and distance.
How it works — the formula
v = d / t (speed = distance / time)
d = v × t
t = d / vAverage speed is total distance divided by total time. It differs from instantaneous speed (which varies moment-to-moment) — average speed captures the overall rate over the full trip regardless of pauses, accelerations, or decelerations.
Worked examples
- Inputs:
- distance = 240 miles, time = 4 hours
- Output:
- Average speed = 60 mph
- Inputs:
- distance = 42.2 km, time = 3h 30m
- Output:
- Average speed = 12.06 km/h (or 7.5 mph)
- Inputs:
- speed = 55 mph, time = 3.5 hours
- Output:
- Distance = 192.5 miles
The average vs instantaneous speed distinction
Average speed is total distance divided by total time. It is a single number that captures the overall rate of travel across a full trip. Instantaneous speed, by contrast, is the speed at a specific moment — what your speedometer reads at any given instant.
These are usually different numbers even in familiar situations. Drive 60 miles in 60 minutes and your average speed is 60 mph; but your instantaneous speed likely ranged from 0 (at stop signs) to 70+ (on the highway). The average captures the totality, not the moment.
For most planning purposes — trip duration, arrival time, fuel budget — average speed is the relevant number. For engineering, safety, or physics purposes (like calculating stopping distance) instantaneous speed matters more.
Round-trip average speed — the harmonic mean trap
A common trick question: if you drive to work at 30 mph and drive home at 60 mph, what is your average speed for the round trip?
Naive answer: (30 + 60) / 2 = 45 mph. This is wrong. You spent more time at 30 mph than at 60 mph, so the true average is weighted toward the slower speed.
Correct calculation: use the harmonic mean. For two-segment trips of equal distance, average speed = 2 × v1 × v2 / (v1 + v2) = 2 × 30 × 60 / (30 + 60) = 3600 / 90 = 40 mph. Or compute it directly: if each leg is 30 miles, the first leg takes 1 hour, the second takes 0.5 hours, total time is 1.5 hours over 60 miles = 40 mph.
This surprises people because we intuitively arithmetic-average, but average speed is always distance / time — never the arithmetic mean of speeds when segment durations differ.
Unit conversions worth memorizing
Quick mental conversions between common speed units:
| From | To | Factor | Example |
|---|---|---|---|
| mph | km/h | × 1.609 | 60 mph = 96.6 km/h |
| km/h | mph | × 0.621 | 100 km/h = 62.1 mph |
| m/s | mph | × 2.237 | 30 m/s = 67 mph |
| m/s | km/h | × 3.6 | 25 m/s = 90 km/h |
| knot | mph | × 1.151 | 10 knots = 11.5 mph |
| knot | km/h | × 1.852 | 10 knots = 18.5 km/h |
| ft/s | mph | × 0.682 | 88 ft/s = 60 mph |
Real-world trip planning
Actual driving averages are systematically lower than posted speed limits. GPS data from major navigation providers shows that highway driving averages roughly 90%% of posted limit (traffic, construction, weather), city driving averages 60-75%% of posted limit (lights, congestion, stop signs).
For trip planning, use the posted-limit as a maximum and apply reduction factors: interstate trips at 90%% of limit (75 mph limit -> 67-68 mph practical), rural highways at 85%% (65 -> 55), suburban routes at 65-70%% (45 -> 30), city driving at 50-60%% (35 -> 20).
Add 5-10 minutes per hour for rest stops and fueling on long trips. Actual "windshield time" on a 12-hour road trip is typically 9-10 hours; total elapsed clock time is closer to 13-14 hours including breaks and unexpected delays.
Limitations
- Average speed assumes constant motion in one direction. For round-trips at different speeds, the average is the harmonic mean, not the arithmetic mean.
- Real-world driving includes stops, traffic slowdowns, and elevation changes — planning tools should add 10-20%% buffer to distance/speed estimates for total trip time.
- Does not account for time zones, DST transitions, or start/end times within the same day.
Average speed calculations assume ideal conditions. This calculator does not provide navigation advice — real journeys should factor traffic, road conditions, rest stops, and fueling.
Frequently asked
Should I include rest stops?+
How is this different from cruising speed?+
Why is my GPS-displayed average lower than my speedometer reads?+
How fast can a car average legally?+
Does this account for breaks?+
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