Due to the cost of production and transfer of natural gas, the industry has demanded a higher level of accuracy and an economical method for measurement. Orifice fittings and meter tubes satisfy this demand for most natural gas measurement applications today. The level of accuracy achieved in orifice measurement has been continually refined and improved upon since it was first put to use for measurement of petroleum products. The accuracy of orifice measurement is controlled by published standards currently AGA 3 Part 2 / API 14.3 April 2000 that define the requirements to achieve a known level of accuracy and eliminate error in measurement. The intent of this paper is to focus on the basics of orifice measurement