What Is a Tachymeter? How to Read and Use a Watch Tachymeter
A tachymeter is a scale on a watch dial or bezel that converts elapsed seconds into a rate per hour. Used with a chronograph (a stopwatch function), it most often shows average speed over a known distance. For example, if it takes 30 seconds to travel 1 kilometer, the tachymeter reads 120—meaning 120 kilometers per hour.
Despite its racing association, a tachymeter can measure any repeated unit per hour: kilometers, miles, items produced, or another known unit. The key requirement is that you time exactly one unit and read the scale when the chronograph stops.
What is a tachymeter on a watch?
A tachymeter is a graduated scale, usually printed around the outer edge of a watch dial or on its bezel. It is calibrated from the relationship:
rate per hour = 3,600 ÷ elapsed seconds for one unit
That is why the numbers become smaller as the seconds hand moves around the dial. A reading of 120 corresponds to 30 seconds (3,600 ÷ 30 = 120); a reading of 60 corresponds to 60 seconds. The scale does not measure speed by itself. You need a known distance—or another known unit—and a way to measure elapsed time.
Tachymeters are commonly paired with a chronograph watch, because the chronograph’s central seconds hand can be started, stopped, and reset. Not every chronograph includes a tachymeter, and not every tachymeter is on a rotating bezel.
How to use a tachymeter to measure speed
To measure average speed, time travel across one known unit of distance. If your scale is being used with kilometers, use one kilometer; if you want miles per hour, use one mile. The number aligned with the chronograph seconds hand is the average rate in units per hour.
- Choose a measured distance of exactly 1 kilometer or 1 mile.
- Reset the chronograph to zero.
- Start the chronograph at the first marker.
- Stop it when you reach the second marker.
- Read the number on the tachymeter scale where the chronograph seconds hand stops.
Example: calculate average speed
Suppose a car travels 1 kilometer in 20 seconds. Stop the chronograph at the 1-kilometer marker. The seconds hand points to 180 on a standard tachymeter scale, so the average speed is 180 kilometers per hour.
The same timing method works for other repeated activities. If one item is completed every 45 seconds, the scale reads 80, meaning a rate of 80 items per hour.
How to use a tachymeter to estimate distance
If you already know your steady speed, a tachymeter can show when you have covered one unit of distance. Start the chronograph, then wait until the seconds hand reaches the number that matches your speed on the scale.
Example: estimate one kilometer of travel
At a constant 90 kilometers per hour, start the chronograph at a known point. When the seconds hand reaches the 90 mark on the tachymeter, 40 seconds have elapsed and you have traveled 1 kilometer. This follows the same formula: 90 km/h × 40 seconds = 1 kilometer.
For distances longer than one unit, use the regular distance formula: distance = speed × time. A tachymeter is quickest for one-unit measurements rather than a complete trip calculation.
Where is a tachymeter located on a watch?
A tachymeter scale may be printed in one of three places:
- Fixed external bezel: the scale sits around the outside of the crystal and does not rotate.
- Dial flange or chapter ring: the scale is printed just inside the crystal, around the dial’s perimeter.
- Inner spiral scale: some watches use a spiral, sometimes called a snail scale, to extend readings beyond the usual 60-second range.
The location does not change the underlying calculation. Check the watch’s manual if its markings differ from a conventional seconds-based scale.
Tachymeter limits and accuracy
A standard tachymeter is most useful when one unit takes less than about a minute. Many conventional scales begin near 60 units per hour, so they cannot directly display lower rates without a spiral scale or a different calculation. The exact range varies by watch.
The result is an average rate over the timed interval, not an instantaneous speed reading. Accuracy also depends on the precision of the known distance, the chronograph, and when you start and stop it. Do not use a wristwatch tachymeter as a substitute for speed-measuring equipment when accuracy or safety matters.
Tachymeter vs. chronograph: what is the difference?
A chronograph is the stopwatch function that measures elapsed time. A tachymeter is the scale that converts that elapsed time into a rate per hour. They are often used together, but they are separate features: a chronograph can exist without a tachymeter, and a tachymeter needs some way to time the event.
Frequently asked questions about tachymeters
What is a tachymeter used for?
A tachymeter converts elapsed time into a rate per hour. On a chronograph watch, it is most often used to find average speed over one known mile or kilometer, but it can also show rates such as items produced per hour.
Does a tachymeter measure mph or km/h?
A tachymeter scale shows units per hour, not a fixed unit by itself. If you time one mile, the result is miles per hour. If you time one kilometer, the result is kilometers per hour.
Can a watch have a tachymeter without a chronograph?
A tachymeter needs an elapsed-time reference to be useful. It is therefore commonly paired with a chronograph, although the timing function could be provided in another way.
How do you read a tachymeter?
Time exactly one known unit, then read the number on the tachymeter scale aligned with the elapsed-seconds hand. That number is the rate in units per hour.
Can a tachymeter measure slow speeds?
Many standard tachymeter scales do not directly show rates below about 60 units per hour because one unit takes longer than 60 seconds. A spiral scale or a manual calculation may be needed for slower rates.
The bottom line
A tachymeter is a practical rate scale for a chronograph watch. Time one known unit, stop the seconds hand, and the aligned number gives you units per hour. It is a useful analog tool for quick average-rate calculations—and a distinctive feature of many chronograph designs.
