How regular saving compounds
Compound growth means each period's return is applied to the opening amount and previous growth. Adding money regularly increases the balance on which later returns are calculated. The result separates what you contributed from the growth generated by the model.
This page uses monthly compounding. Each month earns the annual rate divided by 12, then receives the contribution at month-end. At 0%, future value is simply the starting amount plus all contributions.
Contribution frequency is a budget conversion
A weekly amount is multiplied by 52, a fortnightly amount by 26 and a yearly amount by one. The annual total is then divided into 12 monthly-equivalent deposits. This makes cash-flow comparisons consistent without claiming to reproduce the exact day each deposit reaches an account.
For example, A$100 per week becomes A$5,200 per year or about A$433.33 per modelled month. An account receiving actual weekly deposits will differ slightly because each deposit earns interest for a different number of days.
Future dollars versus today's dollars
Future value is nominal: it is the number of dollars produced by the return assumption. Today's dollars reduce that number by the inflation assumption, helping show estimated purchasing power. Neither future returns nor inflation stay constant in practice.
Testing useful scenarios
Try changing one input at a time. Increasing the regular contribution shows the effect of saving more; extending the time shows the effect of more compounding periods; changing the rate shows sensitivity to an uncertain return. A result that only works at an optimistic rate may not be a resilient plan.
For cash savings, use an after-fee interest rate and consider whether interest tax applies. For investments, allow for fund fees, brokerage, tax and the possibility of negative years. Those items are outside this intentionally simple static formula.
Formula and limits
The model iterates month by month rather than using one closed-form expression, so it can produce the yearly table and handle negative or zero rates consistently. It does not model variable returns, contribution increases, withdrawals, tax, fees or deposit-protection rules.
ASIC Moneysmart also provides a compound interest calculator. Comparing disclosed assumptions is more useful than expecting two tools with different timing conventions to match exactly.