# Tachymeter Calculator-Convert Elapsed Time to Speed
A tachymeter is a scale on a watch bezel or dial that converts elapsed time into speed. It is typically used with a chronograph: start the timer when the object passes a reference point, stop it after 1 km (or 1 mile), and read the speed from the scale. This calculator does the math for you.# How a Tachymeter Works
- Formula
- Speed = 3,600 ÷ elapsed time in seconds. The constant 3,600 represents seconds per hour.
- The Scale
- Tachymeter bezels are marked from 60 (at the 3 o'clock position) up to 400 or 500 (near 12 o'clock). Faster speeds align with shorter elapsed times.
- Base Distance
- Standard tachymeters are calibrated for 1 km or 1 mile. For other distances, divide the reading accordingly.
# Common Tachymeter Measurements
| Elapsed Time | Speed | Scenario |
|---|---|---|
| 10 s | 360 km/h | Bullet train / race car |
| 12 s | 300 km/h | Private jet takeoff |
| 20 s | 180 km/h | Sports car on highway |
| 30 s | 120 km/h | Car on motorway |
| 36 s | 100 km/h | Car on highway (standard reference) |
| 45 s | 80 km/h | Car on urban road |
| 60 s | 60 km/h | City driving |
| 90 s | 40 km/h | Cycling |
| 120 s | 30 km/h | Jogging |
| 240 s | 15 km/h | Running |
| 480 s | 7.5 km/h | Walking |
# Beyond the Standard Scale
If your tachymeter bezel only goes up to 400 but your elapsed time is under 9 seconds, you can still use it. Simply use a longer base distance. For example, if you travel 2 km in 18 seconds, halve the bezel reading. Conversely, for slow speeds (over 60 seconds), use a shorter base distance (e.g., 0.5 km) and multiply by 2.Tachymeter Compatibility
Not all chronographs have a tachymeter bezel. Some watches place the tachymeter scale on the dial rehaut or on the outer chapter ring. Digital watches may include a tachymeter function in the chronograph mode. The formula always works regardless of the physical scale.