Speed Calculator
Calculate speed, distance, or time
How It Works
Overview
A speed calculator solves the classic distance-rate-time relationship: pick any two values and it computes the third. Want to know how fast you averaged on a road trip? Enter the miles and the elapsed hours. Want your ETA? Enter speed and distance. Want how far you'll go in a fixed time? Enter speed and time. The calculator handles unit conversions between mph, km/h, m/s, miles, kilometers, and meters under the hood.
Technically this is average speed — total distance divided by total elapsed time — not instantaneous speed (what a speedometer shows at a moment) and not velocity (which carries direction). For everyday trip planning, training pace, or physics homework, average speed is exactly what you want.
The Formula
All three forms are the same equation rearranged. The "magic triangle" mnemonic: cover the variable you want, and the remaining two show whether to multiply or divide.
- Speed — covered, leaves distance over time → divide
- Distance — covered, leaves speed and time side by side → multiply
- Time — covered, leaves distance over speed → divide
Make sure the units cancel properly: miles ÷ hours gives mph; meters ÷ seconds gives m/s. Mixing units (miles ÷ minutes) gives a number you'll need to convert.
Worked Example
You drive 180 miles and the trip takes 3 hours and 15 minutes. What was your average speed?
- Convert time to hours: 3 + 15/60 = 3.25 hours
- Speed = 180 ÷ 3.25 ≈ 55.38 mph
Now suppose you want to drive 400 miles at an average of 65 mph — how long will it take?
- Time = 400 ÷ 65 ≈ 6.154 hours
- = 6 hours, 9 minutes, and about 14 seconds
Add roughly 15% for fuel, food, and rest stops on a long drive, and you're looking at about 7 hours door to door.
When to Use This
- Road-trip planning — estimate ETA from distance and a realistic average speed (highway drives average 50–60 mph including stops).
- Running and cycling — convert race times to pace per mile or per km, or convert pace back to total time for a target distance.
- Physics problems — kinematics in 1D for constant-speed motion.
- Logistics — delivery windows, dispatch ETAs, or warehouse picker travel time.
- Sailing and aviation — convert knots and nautical miles to or from km/h, m/s, and km.
Common Mistakes to Avoid
- Mixing time formats. "1 hour 30 minutes" is 1.5 hours, not 1.30. Convert minutes to a fraction of 60.
- Forgetting to convert units. If distance is in km and you want mph, the calculator handles it — but doing it by hand requires the 0.621 factor.
- Averaging two speeds directly. Driving 60 mph for 1 hour, then 30 mph for 1 hour averages 45 mph. But for 60 mph one way and 30 mph back over the same distance, the round-trip average is 40 mph (harmonic mean), not 45.
- Using average speed when instantaneous matters. If a problem asks "how fast at the moment of impact," that's instantaneous — different calculation.
- Negative speeds. Speed is non-negative. If you got a negative result, you probably swapped two variables; velocity can be signed but speed cannot.
Frequently Asked Questions
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