*This post may contain affiliate links. Buying something through these links doesn't cost you anything and helps support Know Prepare Survive. For some light reading, check out our affiliate disclosure.*

Table of Contents

Watches have all sorts of funny numbers on them, especially if they are an analog watch.

They tell the time, some tell the date, and some can even measure atmospheric pressure!

But did you know that watches can also be used to measure speed? Distance? Even the amount of work you’re getting done?

It’s true! Read on for how this mathematical wizardry works.

## What is a Tachymeter

That bezel around your fancy watch may have numbers inscribed around the outside. That bezel may rotate.

It may even say “TACHYMETER” on it.

Or “TACHYMETRE,” if it’s British.

If so, that’s the tachymeter part of your watch (you probably already guessed that).

A tachymeter is a tool for converting time, in seconds per distance units of your choice, to speed, which is your distance units per hour.

It is a tool which requires, *gasp,* math.

Put away your smartphones folks. This is math you can do in your head.

Besides, if you need to do this math, chances are batteries are drained.

A tachymeter is a scale that shows 3,600 divided by the elapsed time, in seconds.

3,600 is how many seconds there are in an hour and tachymeters can be used to measure things which take less than a minute to accomplish, see.

There is a way to measure things which take longer than a minute, but we’ll get to that later.

The tachymeter scale typically aligns with the watch’s face and both start at 0, though they display 60 (tachymeter) and 12 (watch).

Most tachymeters do not start measuring until about 7 to 10 seconds in because 3,600 divided by tiny numbers is huge and awkward.

The numbers on the scale descend as time goes on.

After a full revolution, the tachymeter scale stops at 60. 3,600 seconds in an hour, divided by the 60 seconds in one minute. If you continued for an hour, it’d stop at 1.

This seems like mathemagics rather than mathematics, but keep with us and it’ll click.

## How to Use a Tachymeter to Measure Speed

We can use that funny scale to measure the speed of something.

Remember how we mentioned that you choose the unit of distance? This is your time to shine.

Most people use tachymeters to measure miles or kilometers. Knots and meters are also commonly measured.

You can use any measure of distance you want to so long as you know pretty exactly that the distance between two points is equal to 1 of your units.

We’ll use miles, they are easy.

Send an object—we’ll use a car—toward something a mile away.

Start the second’s hand at the very top—this is where a chronograph (stop watch) or rotating bezel is useful—and once the car reaches the 1 mile mark, measure where the second’s hand is pointing on the tachymeter scale.

If it’s been less than a minute and you are using exactly 1 mile, then that reading is exactly how many miles per hour that object is traveling!

So if the object was a car that took 30 seconds to travel that mile, the second’s hand would be pointing at 120, giving us 120 mph.

Now, we add some more math into the mix.

If the object took more than a minute to travel that distance, you need to divide both until the measure is less than a minute and less than an hour.

So if it took the car 75 seconds to travel that mile, the tachymeter would read 240, which is obviously incorrect.

But if we divide the time in half, we get 37.5 seconds, which puts it under a minute.

37.5 seconds on the tachymeter would point at 100. Divide that in half as well, and we get 50 mph, which was the car’s true speed.

Similarly, if you are measuring something that is less than a full unit, you need to divide.

If we measured the time it took to travel a half mile then you’d divide the tachymeter scale by 2. Quarter miles get divided b 4, and so on.

Now that we know how to measure speed, let’s turn that into distance!

## How to Use a Tachymeter to Measure Distance

Measuring distance on a tachymeter is a fair amount easier than measuring speed.

In order to measure the distance, you need to know your speed.

When you know your speed, start the measurement with the second hand at noon, then let her go as you’re traveling.

Once the second hand reaches the tachymeter scale mark that corresponds to your speed, you have traveled one unit of distance.

So if you were moving at 80 miles per hour, once the second hand travels around to 80 on the tachymeter scale, you’ve gone forward a mile.

In this case that takes about 45 seconds.

Unfortunately, this doesn’t work if you are speeding up and slowing down. Your speed has to remain constant.

## How to Use a Tachymeter to Measure Work

Back to the speed measurement for a moment, please.

The unit you choose does not have to be a unit of distance!

You can measure anything that takes less than a minute to accomplish.

Something went wrong, and now you have a huge amount of knives to sharpen. Why does this always happen to you?

You can use the tachymeter to measure how long it takes you to sharpen one knife. 40 seconds. That means you can sharpen 90 knives per hour.

In this way you can use a tachymeter to measure almost anything per hour.

## Conclusion

At first glance, tachymeters look complicated.

The numbers go backward and work off of 3,600 divided by seconds.

But with just a little bit of thought the tachymeter turns into a useful tool for measuring speed, distance, and even how many blasted knives you have left to sharpen.