If you take off with an altimeter setting of 29.80 and later read 30.00 at 5,000 feet, what altitude are you actually at?

When flying, barometric pressure is crucial for determining altitude. An altimeter measures the air pressure and translates it into altitude based on a standard atmospheric pressure setting. When you take off using an altimeter setting of 29.80 inches of mercury (Hg) and then find that the pressure at 5,000 feet is actually 30.00 inches Hg, it indicates that the air pressure is higher than what was initially set in your altimeter.

To determine the true altitude you are at when reading 5,000 feet, you must correct for the difference in atmospheric pressure. In this scenario, the altimeter was set for a lower pressure than what is currently present at your altitude. The difference between 30.00 and 29.80 indicates that the pressure has increased; you would have to subtract the difference in altitude that corresponds to that change in pressure.

The difference between settings of 29.80 and 30.00 is 0.20 inches Hg. Generally speaking, each inch of Hg represents about 1,000 feet of altitude. Therefore, the 0.20 inch difference equates to approximately 200 feet. Since you're currently flying with the altimeter set at a lower pressure, this means you are actually