Atmospheric pressure 

The pressure exerted by the earth's atmosphere is called atmospheric pressure. Atmospheric pressure changes with weather and elevation. 

Normal atmospheric pressure at sea level (an average value) is 1.013 x 105 Pa.

                        1 atm = 1.013 x 105 Pa.

Absolute pressure and Gauge pressure

The excess pressure above atmospheric pressure is usually called gauge pressure and the total pressure is called absolute pressure.

Thus,

         Gauge pressure = absolute pressure - atmospheric pressure.

                                         

                                                https://youtu.be/UCHIvvAvekA