How is gauge pressure different from absolute pressure?

The primary difference between gauge pressure and absolute pressure lies in their reference points.

• Gauge pressure is the pressure at a point in a fluid expressed in excess of the surrounding atmospheric pressure. It is the value typically measured by common instruments like a tire-pressure gauge, which effectively ignores the ambient pressure of the atmosphere.123
• Absolute pressure represents the total pressure at a point in a fluid, including the contribution of atmospheric pressure. It is calculated as the sum of the gauge pressure and the atmospheric pressure. An absolute pressure of zero corresponds to a complete vacuum, or empty space.423

In summary, while gauge pressure measures the difference between a system's internal pressure and the external atmosphere, absolute pressure provides the total magnitude of pressure relative to a perfect vacuum.23