For the purpose of this calculation, we assume that your investment earns interest income that compounds at the end of the compounding period you choose. Any additional contributions are applied immediately at the beginning of the period you made them.
Results are displayed by compounding period. Where the contribution frequency is greater than the compounding period (as in the case of weekly and bi-weekly contributions), annual contributions are divided over the number of compounding periods. For example, if you make contributions weekly that compound monthly, the calculations divide the year's 52 weeks into 12 months x 4.33 weeks each.