The Residence Time (t) of each element is the length of time it takes various input processes to add an amount of the element to the ocean that is equal to the amount of the element in the ocean:
t = Mean Concentration in Ocean x Ocean Volume / Input per year
where the Ocean Volume is (1.37×1021 L). The value used for the input should be the sum of all inputs to the ocean. However, the residence time is often calculated using only one, or more, of the major input processes. For many elements, the major input is from rivers and the Input per year is the Mean River Concentration x Continental Runoff Rate (3.6×1016 L/yr). If the concentration of an element is not changing, then the Input and Output of an element must be equal. The residence time can then be calculated using the estimated output, if that is known.