Time, distance and speed calculator

Hours and minutes: the most common mistake

The calculator expects time in decimal hours. 1 h 20 min is not 1.2 h but 1 + 20 ÷ 60 = 1.3333 h. Enter 100 km and 1.2 h and you get 83.33 km/h; with the correct 1.3333 h the answer is 75 km/h. That is a gap of more than 8 km/h from one mistyped time.

Convert minutes to hours by dividing by 60. For the result, do the reverse: multiply the decimal part by 60. For example, 1.4444 h is 1 hour and 0.4444 × 60 = 26.7 minutes, so roughly 1 h 27 min.

• 15 min = 0.25 h
• 20 min = 0.3333 h
• 30 min = 0.5 h
• 40 min = 0.6667 h
• 45 min = 0.75 h

Examples from Estonian roads

Tallinn to Tartu is roughly 185 km. If the drive took 2 h 15 min, or 2.25 h, the average speed was 185 ÷ 2.25 = 82.22 km/h. That includes slowing down through villages and any stops, so the average is lower than the speedometer usually showed.

Tallinn to Pärnu is roughly 130 km. At an average of 90 km/h it takes 1.4444 h, about 1 h 27 min; at an average of 110 km/h it takes 1.1818 h, about 1 h 11 min. Driving faster saves about 16 minutes on this route. Both times assume a constant speed the whole way, which real traffic rarely allows.

The average speed trap

A classic problem: a car drives 60 km at 60 km/h and the next 60 km at 120 km/h. What is the average speed? The tempting answer is 90 km/h, but it is wrong.

The correct solution works through time. The first leg takes 60 ÷ 60 = 1 h, the second 60 ÷ 120 = 0.5 h. That is 120 km in 1.5 hours, so the average speed is 120 ÷ 1.5 = 80 km/h. More time is spent on the slower leg, so it weighs more in the average.

A simple average of two speeds only works when each speed is held for the same amount of time, not the same distance.

School problems and other units

The calculator works in kilometres, hours and km/h, while physics problems often use metres and seconds. A runner covers 100 m in 12.5 seconds: compute 100 ÷ 12.5 = 8 m/s directly and convert 8 × 3.6 = 28.8 km/h.

A cyclist rides for 40 minutes at 15 km/h. The time is 40 ÷ 60 = 0.6667 h and the distance 15 × 0.6667 = 10.0005 km. The exact answer is 10 km: the small gap comes from rounding the time, so use as many decimals as possible for fractional minutes or compute 15 × 40 ÷ 60.

A pedestrian walks at 5 km/h and has 3.5 km to go. The calculator shows 0.7 h, which is 0.7 × 60 = 42 minutes.

What the calculator does not account for

The formula v = s ÷ t describes uniform motion or gives an average speed. It does not see acceleration, braking or stops separately. To estimate how long a drive will really take, add rest breaks and traffic conditions.

If the time is zero when finding speed, or the speed is zero when finding time, the calculator shows 0. Such a result has no physical meaning, since you cannot divide by zero, so make sure the value you divide by is positive.