Percent abundance can be calculated by finding the percentage of an isotope compared to another isotope of an element using its relative atomic mass. This is an algebraic process and depends upon several measurements.
Because there are various isotopes of an element, its atomic weight is a calculation of the atomic masses and relative abundances of each isotope of that element. Using algebra, the atomic weights and respective relative abundance percentages of each isotope are multiplied and then added together to find the atomic weight of the element in question.
To accurately measure the atomic mass of an element, a mass spectrometer is used. In this instrument, atoms are positively ionized, accelerated and then deflected by a magnetic field. The amount of deflection is partially determined by the element's atomic mass.