The author defines the cumulative rate of return, which measures the change in the amount of capital when taken at discrete moments of time. The measure defined, treated as a function of time, at the limit becomes equivalent to the logarithmic rate of return. Thus we arrive at a measure applicable also to the discrete time case; a measure which meets many assumptions of an economic nature. The author provides examples of cumulative rate of return applied to various situations such as: stowing money away in a piggybank, regular payments into a non-interest bearing account, or capitalization at maturity. The paper also demonstrates how the measure defined can be used to compare the return on various investments (deposits with different maturities and different interest rates). The considerations presented lead to the conclusion that financial mathematics can also make use of harmonic numbers, which are common in mathematical applications across many areas of science and technology.