The three quantities and how they connect
Speed, distance and time are linked by a single relationship: distance equals speed multiplied by time. Rearranged, it answers three different questions. Many people remember it as a triangle with distance on top and speed and time below: cover the one you want, and the other two show how to get it (side by side means multiply, one above the other means divide).
| To find | Calculation | Example |
|---|---|---|
| Speed | distance ÷ time | 150 km ÷ 1.875 h = 80 km/h |
| Distance | speed × time | 60 mph × 2.25 h = 135 mi |
| Time | distance ÷ speed | 320 km ÷ 90 km/h = 3.556 h |
Choose the unknown under Solve for, pick kilometers (speed in km/h) or miles (speed in mph) as the Distance unit, and fill in the two fields that remain visible. Time (h:mm:ss) accepts hours, minutes and seconds (1:52:30), hours and minutes (2:15), or a plain number of minutes (25). The answer appears with the speed, distance, time (also as decimal hours) and the pace, which is the time taken per kilometer or per mile.
Formulas, including pace
The only conversion that trips people up is time: a clock reading has 60 minutes in an hour, not 100. The calculator converts h:mm:ss to decimal hours before dividing; the minute and hour are not SI units but are accepted for use with them.
Worked examples
- Convert the time: 1 h + 52 ÷ 60 h + 30 ÷ 3,600 h = 1 + 0.8667 + 0.0083 = 1.875 h.
- Speed: 150 ÷ 1.875 = 80 km/h.
- 2 hours 15 minutes is 2 + 15 ÷ 60 = 2.25 h (enter it as 2:15).
- Distance: 60 × 2.25 = 135 mi.
- Time: 320 ÷ 90 = 3.5556 h.
- Whole hours: 3. The rest, 0.5556 h × 60 = 33.33 min, is 33 minutes and 20 seconds.
- This is driving time only; add stops yourself.
- Enter 5 km and 25 (or 0:25:00) as the time.
- Speed: 5 ÷ (25 ÷ 60) = 12 km/h.
- Pace: 25 min ÷ 5 km = 5 minutes per km. A mile is 1.609344 km, so the same effort is 5 × 1.609344 = 8.05 min, about 8:03 per mile.
Average speed over several legs
When a journey has parts at different speeds, the average speed is the total distance divided by the total time. It is not the average of the speeds, unless each leg took the same amount of time.
Take a 120 km trip: 60 km at 60 km/h, then 60 km at 40 km/h. The first leg takes 1 hour and the second 1.5 hours, so the trip is 120 km in 2.5 hours, an average of 48 km/h. The mean of 60 and 40 is 50, which overstates it, because more of the time was spent at the slower speed.
Running and cycling pace
Runners usually think in pace (minutes per kilometer or per mile) rather than speed. The two are reciprocals: 60 divided by a speed in km/h gives the pace in minutes per km. This table shows common paces with the matching speed and finish times:
| Pace per km | Speed | Pace per mile | 5 km | 10 km |
|---|---|---|---|---|
| 4:00 | 15 km/h (9.321 mph) | 6:26 | 20:00 | 40:00 |
| 4:30 | 13.33 km/h (8.285 mph) | 7:15 | 22:30 | 45:00 |
| 5:00 | 12 km/h (7.456 mph) | 8:03 | 25:00 | 50:00 |
| 5:30 | 10.91 km/h (6.779 mph) | 8:51 | 27:30 | 55:00 |
| 6:00 | 10 km/h (6.214 mph) | 9:39 | 30:00 | 1:00:00 |
| 7:00 | 8.571 km/h (5.326 mph) | 11:16 | 35:00 | 1:10:00 |
For a marathon of 42.195 km, a finish time of 4:00:00 means an average of 10.55 km/h and a pace of about 5:41 per km.
Slips that give the wrong answer
- Reading 1:45 as 1.45 hours. One hour 45 minutes is 1.75 h. Dividing 150 km by 1.45 gives 103.4 km/h; the correct speed is 85.71 km/h.
- Typing 25:00 for 25 minutes. In the time field, two numbers separated by a colon are hours and minutes, so 25:00 means 25 hours. Type 25, or 0:25:00.
- Averaging speeds instead of using totals. As shown above, the average of the leg speeds is wrong whenever the legs take different times.
- Mixing kilometers and miles. A speed in mph with a distance in km gives a meaningless time. Set the distance unit first; for other speed units, the speed converter translates them.
For trips, the result is the time spent moving at the average speed you enter. Traffic, stops and speed limits are up to you to allow for.
Sources
- Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B, National Institute of Standards and Technology.
- The International System of Units (SI Brochure), 9th edition, Bureau International des Poids et Mesures.