Start the chronograph as an object crosses the beginning of a known distance, then stop it as the object crosses the end. The tachymeter scale, read where it meets the chronograph seconds hand, gives an approximate average speed per hour—kilometers per hour for a one-kilometer interval, or miles per hour for a one-mile interval.
What a tachymeter measures
A tachymeter converts elapsed time over a known distance into an hourly rate. It works together with the chronograph: the chronograph measures time, while the scale translates that elapsed time into a speed reading. TAG Heuer’s tachymeter guide and Hamilton’s instruction manual describe the one-kilometer and one-mile method.
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The scale may be printed on the bezel or on a ring around the dial; its position varies by watch model, as Citizen’s guidance notes. Look for numbered markings commonly labeled as a tachymeter, and identify the chronograph seconds hand before timing.
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- Choose a known distance. Use a clearly marked one kilometer or one mile for the direct reading.
- Start the chronograph at the start marker. Press the start control as the moving object crosses the beginning of the measured distance.
- Stop at the end marker. Press the stop control as it crosses the end of that same distance.
- Read the scale. Find the tachymeter value aligned with the chronograph seconds hand. For one kilometer, read the result as kilometers per hour; for one mile, read it as miles per hour.
- Reset before the next measurement. Follow your watch’s instructions for stopping and resetting; controls and chronograph layouts vary by model.
For example, Casio’s official tachymeter explanation gives 120 km/h when one kilometer takes 30 seconds. Seiko’s instruction page gives 80 km/h for one kilometer in 45 seconds. These illustrate how the hand’s elapsed-time position corresponds to a speed on the scale.
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What if the distance is not one kilometer or one mile?
The reading is proportional to the distance covered. Casio explains that for two kilometers, double the indicated value; for 500 meters, divide the indicated value by two. This adjustment is appropriate only when the timed interval and scale reading otherwise suit the measurement.
For instance, if a watch indicates 120 after timing 500 meters, the adjusted average speed is 60 km/h. Be sure the distance unit stays consistent: a result based on meters is expressed in kilometers per hour after applying the scale’s proportion.
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Know the timing and accuracy limits
Typical scales cover up to 60 seconds
Hamilton’s 2025 manual edition specifies that its described tachymeter measurement applies to elapsed times of no more than 60 seconds. A typical scale is therefore not a direct solution for timing a distance interval that takes longer than one minute; check the manual for your specific watch before relying on its markings.
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The tachymeter gives an average over the measured distance, not a continuously updated speed reading. It depends on starting and stopping at the distance boundaries. A late button press or uncertain marker changes the elapsed time and therefore the result, so treat it as an estimate rather than precision instrumentation.
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Using a tachymeter for repeated tasks
The same scale can convert the time for one completed task into an hourly production rate. Time one repetition with the chronograph, then read the tachymeter value as repetitions per hour if each repetition takes about the same time. Seiko’s example is one product completed in 20 seconds, corresponding to 180 products per hour on the scale. This is a rate extrapolation, not a count of actual output over a full hour.
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