How to Read a Tachymeter

A tachymeter measures average speed over a known distance using the chronograph seconds hand. You start the chronograph as an object passes a marker, stop it exactly one kilometre or one mile later, and read the number the seconds hand points to on the tachymeter scale. That number is the speed in units per hour. No calculation, no phone, just the scale doing the arithmetic for you.

• What it needs: a chronograph and a tachymeter scale, usually printed round the bezel.
• What it measures: speed over a fixed, pre-measured distance.
• The one rule: the distance you time must match the unit you want, one kilometre for km/h, one mile for mph.

How the scale works

The tachymeter scale is not linear and it is not magic; it is just the number 3600 divided by the elapsed seconds, printed around the dial. There are 3,600 seconds in an hour, so if something takes 36 seconds to cover a kilometre, it is doing 3600 ÷ 36 = 100 km/h. The scale has that division baked in, so the 36-second mark simply reads "100". Because the maths only works cleanly within a single sweep of the seconds hand, most tachymeters run from around the 7-second mark (about 500 units/hour) down to the 60-second mark (60 units/hour).

Reading one, step by step

1. Pick a course of known length. A kilometre marker on a road, or a measured mile, works.
2. As the object passes the start, press the top pusher to start the chronograph.
3. The moment it reaches the end of that measured distance, press to stop.
4. Look at where the chronograph seconds hand has stopped, then read the number printed against it on the tachymeter scale.
5. That number is the average speed: units per hour, in whatever unit your distance was measured in.

Worked examples

Notice the pattern: the faster the object, the fewer seconds it takes, and the higher the number on the scale. Anything slower than 60 units an hour takes more than a full minute and runs off the end of the scale, which is why a tachymeter is no use for a walking pace.

Measuring things other than speed

Flip the idea around and a tachymeter can estimate rate of production or frequency of events per hour. Time how long a single unit takes to come off a line, and the scale reads how many that line makes in an hour. Time one repetition of anything and you get its hourly rate. It is a niche use, but it is where the scale came from, before speedometers were standard.

Is it actually useful now?

Honestly, rarely. A car has a speedometer and everyone has a phone. The tachymeter survives mostly as a design signature of the classic racing chronograph, and it is worn far more for that look than for timing anything. That is a fair reason to like it. Knowing how to read one is part of understanding the watch on your wrist, even if you never clock a car with it. For the wider function it sits on, see our chronograph explainer, and for the other scales a dial can carry, the complications overview.

Chronographs with a tachymeter bezel are in the chronograph collection, and the wider range is in the shop.

Common questions

Do I need to time exactly one kilometre?

For a direct reading, yes, one full unit of distance. Time a shorter or longer stretch and the scale no longer reads true without extra maths, which defeats the point.

Why does the scale stop at 60?

Below 60 units an hour, the event takes more than a full minute, so the seconds hand laps past 60 and the reading becomes ambiguous. The scale only works within one sweep.

Can a tachymeter measure a slow runner?

Not usefully. A person covering a kilometre takes several minutes, far off the end of the scale. Tachymeters are built for things moving well above 60 units an hour.