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How does an altimeter watch measure altitude?

At the end of the day, when using an altimeter watch we want it to spit out the correct information. When you get to the trail junction and have to make a navigation decision, or you are climbing a fourteener and you want to know how far you have until you get to the top - you want your altimeter watch to accurately display the current altitude and maybe some other information like rate of ascent, total ascent, etc. Knowing what factors go into calculating altitude and how environmental conditions effect that output is important when you need to quickly determine if the altitude given is accurate or not.

So a simple lesson on how an altimeter watch calculates altitude:

In laymans terms, an altimeter watch measures atmospheric pressure. The higher you go, the less atmospheric pressure there is (this is why airplanes have pressurized cabins and climbers on Everest wear oxygen). Essentially, what you have on your wrist is a barometer with a function that uses a simple calculation to spit out altitude.

HighGear's Ryan Hamsho said it like this:

An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. The altimeter uses the changes in atmospheric pressure to determine the changes in altitude. Why does pressure change with altitude? Simply put, the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). For this reason, the higher in the atmosphere you are, the less atmosphere you have above, and the less pressure exerted on you.

Again, the calculation is simple. What it means in the field is you need to calibrate at least daily from known positions if you want the altimeter watch to be accurate. With stable weather, once a day is probably enough. When weather changes, you need to recalibrate at known points as often as once an hour. Use your paper map or Google Maps to record and set known points on your altimeter watch and make sure to set your watch when you get there. You User Manual on your watch will show you how to do this.

Keep in mind as you purchase an altimeter watch that it is a tool to assist you with calculating altitude, but it is not an exact scientific instrument. Used correctly with appropriate supplemental information (maps, etc). your altimeter watch can be a valuable tool in getting you to your destination.

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