In weather reports, barometric pressure readings are used to describe a weather system as having either high or low pressure. Barometric pressure is atmospheric pressure that is measured by device known as a barometer. The most common barometers use mercury to measure pressure. A common unit for the measurement of barometric pressure is inches of mercury. Because Hg is the chemical symbol for mercury, barometric pressure is commonly reported in inches of mercury, or in Hg. Pounds per square inch, or psi, is another metric for reporting pressure. One psi is equivalent to 2.036 inches of mercury.
Make your barometric pressure reading in inches of mercury.
Multiply the barometric pressure in inches of mercury by 0.4911. This yields the barometric pressure in psi.
Check your work using an online inches-of-mercury to pounds-per-square-inch converter.
If your barometric pressure is reported in atmospheres instead of inches of mercury, you can convert your atmospheres reading to psi. One atmosphere equals 66.7 Kilogram per square inch, so multiply the value in atmospheres by 14.7 to convert to psi.