Speed Distance Time Calculator

What this tool is used for

• Working out arrival time from a distance and an average speed.
• Finding the average speed needed to cover a distance in a time.
• Converting a journey between units while solving it.
• Checking a route planner's estimate against the arithmetic.
• Working out how much a speed increase actually saves.

Frequently Asked Questions

What are the three formulas?

speed = distance ÷ time, distance = speed × time, time = distance ÷ speed. The units must agree: mixing km with hours gives km/h, but mixing km with minutes silently gives km/min, which is 60× off.

How do I convert a decimal time to minutes?

Multiply the fractional part by 60, not by 100. 2.5 hours is 2 h 30 min, but 2.75 hours is 2 h 45 min — not 2 h 75 min. Treating decimal hours as if they were minutes is the most common error in journey planning.

Why can I not average two speeds directly?

Because you spend more time at the slower speed. Driving out at 60 and back at 30 averages 40, not 45 — it needs the harmonic mean, 2ab/(a+b). Arithmetic averaging of speeds over equal DISTANCES is always wrong.

What is pace and how does it relate to speed?

Pace is the inverse of speed — time per unit distance rather than distance per unit time. Runners use min/km or min/mile, so a 5 min/km pace is 12 km/h. Because they are reciprocals, halving your pace doubles your speed.

Is this valid for accelerating motion?

No — these formulas describe constant speed. For acceleration you need the SUVAT equations, such as s = ut + ½at². Applying constant-speed maths to a journey with stops gives an average, not an instantaneous, speed.

How do I combine speeds correctly for a round trip?

With the harmonic mean, because equal distances take unequal times. Out at 60 and back at 30 averages 40, not 45 — the slower leg occupies more of the trip, so it carries more weight in the result.

What is relative speed?

The rate at which a gap changes. Approaching head-on, speeds add; travelling the same way, they subtract. It is the quantity that answers overtaking and closing questions, and using absolute speeds instead is the usual mistake.

Common errors and gotchas

• Mixing units, such as a distance in kilometres with a speed in miles per hour.
• Averaging two speeds directly, which is wrong — the average of speeds over equal distances is the harmonic mean.
• Ignoring stops, which dominate the difference between calculated and real journey times.
• Confusing average speed with the speed limit, which a real journey rarely sustains.
• Reading a decimal hour as hours and minutes, where 1.5 is ninety minutes.