Compound interest: when your contributions arrive matters

Identify the money that earns each period’s growth

A starting balance is present from the beginning. A contribution paid later cannot earn growth for the time before it arrives. That simple distinction explains why a formula for one lump sum is not automatically the right formula for a series of monthly deposits.

MoneySmart describes compound interest as earning on the starting amount and on interest already added. For a real account, the provider’s calculation and crediting terms matter. A calculator may approximate them using equal monthly periods.

Write down the starting date, deposit dates and final measurement date. A comparison that gives one scenario an extra month of growth can look like an investment advantage when it is only a timing difference. Keep the same calendar before comparing the numbers.

Source: MoneySmart: compound interest

A checked beginning-versus-end example

Start with a fictional $1,000, add $100 each month for twelve months and assume a 6% nominal annual rate divided by twelve. The monthly rate is 0.5%. Ignore tax and fees.

With deposits at the end of each month, the ending balance is about $2,295.23. With the same twelve deposits at the beginning of each month, it is about $2,301.40. The difference is about $6.17. Both paths contribute $2,200 of external money in total; the timing changes how long the regular deposits receive the assumed growth.

The example is a deterministic calculation, not an available savings rate or investment forecast. It illustrates why two calculators can disagree slightly while both correctly implement different stated assumptions. Check the method before deciding that the larger displayed figure is more accurate.

Separate contributions from growth

In the end-of-month example, $1,000 of starting money plus $1,200 of deposits equals $2,200. The remaining approximately $95.23 is modelled growth. Calling the whole $2,295.23 “interest earned” would overstate the result dramatically.

The same discipline matters in longer investment projections. A balance can grow mainly because more money was added, even when the return is modest or negative over part of the period. Keep cumulative contributions and investment outcome in separate columns.

If a distribution is reinvested, record it as investment value retained rather than another personal contribution from wages. Otherwise progress can be counted twice. A clear cash-flow record helps explain whether the plan is succeeding through consistent saving, market outcomes or a combination.

Annual rates need a defined conversion

A nominal annual rate divided into monthly periods is not the same wording as an effective annual rate. At a nominal 6% compounded monthly, the effective annual growth factor is 1.005 to the power of twelve, giving approximately 6.17% over a year before other effects.

If 6% is instead an effective annual return assumption, the consistent monthly rate is the twelfth root of 1.06 minus one. Do not use the nominal division and then also claim the result has an effective annual rate of exactly 6%.

Real savings products may calculate daily and credit monthly, while investment returns do not arrive as fixed interest. Read the rate definition and calculator assumptions. Small conversion differences can accumulate, but they do not justify pretending an uncertain investment return will be delivered evenly.

Sources: MoneySmart: compound interest calculator and assumptions · MoneySmart: savings accounts and account conditions

More frequent payments can change the annual total

A fictional $200 a month is $2,400 over twelve payments. A $100 fortnightly transfer is $2,600 over twenty-six payments. The fortnightly plan contributes $200 more a year. Much of any projected advantage can come from the additional saving rather than compounding frequency.

To keep the same $2,400 annual contribution, the fortnightly amount is about $92.31. Use unrounded calculations where needed and check the actual number of payment dates in the planning period. Calendar years and fixed payday schedules do not always align neatly with a simplified model.

Compare equal annual contributions first if the question is timing. Then show the larger-contribution scenario separately if it is affordable. This keeps the reason for the result visible and prevents a payment-frequency change from disguising an increase in the saving commitment.

Missed deposits matter more than a perfect-looking formula

Suppose a planned $100 monthly deposit is missed three times. That removes $300 of contributions before considering any growth effect. In a one-year illustration, it can matter much more than the $6.17 beginning-versus-end difference shown earlier.

Choose a contribution the actual budget can sustain. Include annual bills and income changes rather than assuming every month is identical. If a transfer repeatedly causes a cash shortage elsewhere, revise the plan instead of preserving the calculator input while borrowing to fund it.

A useful review compares planned and actual deposits. Record any missed amount and the reason, then decide whether the target date, contribution or goal should change. The model should follow the household’s real capacity rather than become a number that is maintained only on screen.

Keep tax, fees and inflation on a consistent basis

A savings rate before tax may not equal the return retained by the household. An investment return may already include certain fees. Inflation affects purchasing power even when the nominal balance rises. Each factor needs a clear place in the calculation.

Do not deduct a fee twice or reduce a return for inflation when the model already adjusts the output. Likewise, do not compare today’s spending target with a future nominal balance without understanding the conversion. These are method questions, not evidence that a higher assumed return is needed.

Use current product documents and your actual tax circumstances where a decision depends on them. This article does not calculate individual tax. A registered tax agent can help establish the appropriate after-tax assumption, while the calculator remains an illustration of a stated path.

Read the Elaris result as a model

The Elaris savings tool converts its annual growth assumption using a twelfth root and adds contributions after each month’s growth. Its annual assumption therefore differs from the nominal-rate-divided-by-twelve teaching example above. Check the tool’s explanation before comparing it with a daily-interest bank account or an irregular investment schedule. It does not retrieve rates or guarantee that a product will earn the entered amount.

Save the starting balance, deposit amount, timing, rate basis, goal and date. Run zero growth as a useful contribution-only reference, then test other clearly labelled scenarios. Do not call chosen lower and higher rates probabilities unless the model actually estimates those probabilities.

A good result lets you explain where the balance came from. If the numbers are close across calculators, inspect timing and rounding first. If they are far apart, look for different annual contributions, missing fees or a real-versus-nominal mismatch before assuming compounding itself is mysterious.

For a near-term goal, also calculate the result without growth. In the worked example, the same starting $1,000 and twelve $100 deposits produce $2,200 at zero interest. That reference shows how much depends on the saving commitment and how much depends on the rate assumption. If a deadline is only achievable by selecting a much higher rate, consider the target, contribution or timing rather than treating an unverified return as the solution.

Variable returns can also make an arithmetic average misleading. A fictional $1,000 that rises 10% becomes $1,100; a subsequent 10% fall leaves $990. The simple average of the two percentages is zero, but the compounded outcome is a 1% loss. This is why a market investment should not be described as earning its long-run average evenly each year. A fixed-rate savings illustration and an uncertain investment path answer different questions even when a calculator uses similar-looking growth fields.

Source: MoneySmart: compound interest calculator and assumptions

FAQ: Is daily compounding always better than monthly?

Under the same nominal rate and other identical assumptions, frequency can affect the effective result. Real products can quote different rate bases and conditions, so compare the actual after-cost outcome. A frequency label alone does not identify the better account or investment.

FAQ: Why is a fortnightly projection larger?

First count the annual deposits. Half a monthly amount paid twenty-six times contributes more than the full amount paid twelve times. Separate that extra contribution from the timing effect before attributing the entire difference to compound interest.

FAQ: Do shares compound like a savings account?

Investment growth can be reinvested, but market returns are uncertain and can be negative. They are not fixed interest credited evenly each month. A smooth compounding model is a scenario for understanding inputs, not a description of how every asset actually performs.

FAQ: What should I verify before changing my transfer?

Check the affordable amount, actual payment dates, known bills and the calculator’s timing assumption. A sustainable transfer and complete budget matter more than choosing a date that adds a small modelled gain while creating a real cash shortage.

Sources and editorial notes

Sources checked 24 October 2026. Worked examples are fictional illustrations. Rules, eligibility and product terms can change; use the official source for your circumstances.

• MoneySmart: compound interest ↗
• MoneySmart: compound interest calculator and assumptions ↗
• MoneySmart: savings accounts and account conditions ↗

General information, not an individual financial or legal assessment. AI-assisted drafting with source checks; independent professional review has not been completed. How we prepare our guides · Suggest a correction