Speed / Distance / Time Calculator

Two values in, the third one out. Set distance and time to get speed. Set speed and time to get distance. Set speed and distance to get time. Pick units freely on each field — miles or kilometers, hours or minutes, mph or m/s.

The formula is d = v × t, but the unit handling is where most spreadsheets fall over. Mixing 60 mph with 30 minutes gives 30 miles only if you remember to convert minutes to hours first. The calculator does this automatically — type 30 minutes and 60 mph, read off 30 miles, no manual unit math.

For average vs instantaneous speed, this is the average-speed formula. If you went 60 mph for half the trip and 30 mph for the other half, the average isn't 45 — it's the harmonic mean of the two. Run the calculator twice over the two segments to get the right total.

The core formula is dimensional. Distance has units of length. Time has units of duration. Speed (their ratio) has units of length per duration. The calculator's job is unit consistency: m/s and meters and seconds match; mph and miles and hours match; mixing creates errors unless explicit conversions happen. Internally it converts everything to SI (meters and seconds) for math, then back to your chosen output units. That's how you can input speed in mph, time in minutes, and read distance in kilometers without doing any conversion yourself.

Worked example: planning a 380-mile drive. At a steady 65 mph the calculator returns 5 hours, 51 minutes — driving time only, no stops. Now plan a 100km bike ride at 25 km/h average: 4 hours flat. Mix units: 100 km at 65 mph = 100 km × (1 mi / 1.609 km) / 65 mph = 0.96 hours = 57 minutes 36 seconds. The calculator handles all three transparently. For a runner: 5 km at 4:30/km pace = 22:30 total. Pace is the inverse view of speed (time per distance), which the calculator can output directly when you toggle the display.

The averaging trap that causes wrong answers: speed averaged over time vs speed averaged over distance gives different numbers. If you cycle 10 km uphill at 10 km/h (1 hour) and 10 km downhill at 30 km/h (20 min), total distance is 20 km, total time is 80 min, average speed = 15 km/h. NOT (10 + 30)/2 = 20 km/h. The harmonic mean weights by time-spent, which is right because you spent more time on the slow segment. Whenever you have multi-segment trips, run the calculator on each segment separately, then add the times and divide by total distance for the true average.