Educational guide · Updated 6 de October de 2026 · 8 min read
Compound interest describes what happens when returns remain in the balance and can themselves earn returns in later periods. Reading a projection requires more than looking at its final value. You need to separate your contributions from assumed growth, understand what the annual rate means and check how the time horizon changes the outcome. This guide explains those steps without treating a mathematical assumption as a promised investment return.
What compounding actually means
Start with 1,000 currency units and a constant 5% effective annual return, with no fees, taxes or further contributions. At the end of year one, the balance is 1,050. In year two, the same percentage applies to 1,050, producing 1,102.50. The extra 2.50 compared with two simple increases of 50 comes from earning a return on the previous return.
Without additional contributions, the formula is starting capital multiplied by (1 + the periodic rate) raised to the number of periods. The units must agree. Applying an annual rate directly to a number of months is a serious mistake: it can create large, impressive looking figures that do not describe the intended calculation.
Effective rates and nominal rates
The savings calculator in Simuily asks for an assumed effective annual rate. It obtains the equivalent monthly rate by raising (1 + the annual rate) to the power 1/12 and subtracting 1. Twelve monthly periods then reproduce the same annual growth when there are no intervening contributions. Dividing an effective annual rate by twelve gives a different convention.
A nominal annual rate compounded monthly is handled differently: divide the nominal rate by twelve and compound the monthly result. Neither label should be left implicit. Before comparing a real savings or investment product with a projection, check how its rate is defined, how frequently interest is credited and whether the displayed figure includes costs.
Why contributions need their own line
Saving 200 each month for ten years means adding 24,000. If you started with 2,000, total contributions are 26,000. Projected growth is the final balance minus that amount. A chart that calls the entire final balance profit would confuse your own money with the assumed return.
Contribution timing matters too. A beginning-of-month contribution participates in that month's growth; an end-of-month contribution begins participating in the next period. This calculator uses end-of-month contributions. A comparison with another calculator is only meaningful when the timing, rate convention and number of periods match.
Reading the chart
The balance line describes the total projected amount. The contributions line shows the cumulative money you supplied. Their distance represents assumed growth. A detailed calculator should also make the values available in a table, so that the chart is not the only way to understand them. Period labels and currency units should remain visible on small screens.
A smooth curve comes from a constant rate assumption. It is not a forecast of smooth monthly investment performance. Market investments can fall, and a long-term average can conceal large variations along the way. Comparing lower, middle and higher assumptions is useful, but those scenarios should not be assigned probabilities unless a suitable model supports them.
Nominal money and purchasing power
The nominal balance is stated in future currency units. To estimate its value in today's money, divide it by (1 + the assumed annual inflation rate) raised to the number of years. At constant 2% inflation, a balance ten years away is divided by approximately 1.219. This helps relate a future amount to a target expressed at today's prices.
The adjustment is an assumption about overall purchasing power. It does not predict the price of a particular home, education course or household expense. Taxes and fees are also separate: unless the model explicitly includes them, the inflation adjusted amount should not be described as a net spendable return. Selecting another currency changes the display unit; it does not perform foreign exchange conversion.
A practical way to compare scenarios
Begin with a zero-growth calculation to see what your contributions alone achieve. Then change one input at a time: monthly saving, time horizon and assumed rate. Keeping the other inputs constant reveals which change causes the difference. If all three change together, a larger final number does not tell you which decision mattered most.
Avoid choosing a rate merely because it makes your goal achievable on paper. If a target remains out of reach under assumptions you can justify, consider the variables you can control: contribution amount, deadline and target size. A projection is most useful when you can explain it, revise it and distinguish observed facts from expectations.
Turn the projection into a repeatable check
Record your starting balance, contribution schedule, horizon, rate convention and inflation assumption. On a later visit, update the actual balance before changing expectations. This separates a change in saving behaviour from a change in assumed performance. Use the savings-goal guide when you want to translate a target into a monthly contribution rather than simply explore an ending balance.
Investor.gov provides an official educational compound-interest calculator that can be used as a comparison reference. Match its contribution and compounding settings before interpreting any difference between its result and a Simuily calculation.
A small calculation you can check independently
Before interpreting a twenty year projection, try an example that you can reconstruct without a complicated spreadsheet. Enter a starting balance of 1,000, no monthly contributions, one year and an effective annual rate of 5%. The nominal result should be 1,050. Next, set the rate to zero and add 100 each month for one year. The balance should be 2,200: the original 1,000 plus twelve contributions of 100. These checks test different things. The first checks the rate conversion; the second checks the number of contributions.
Now combine growth with contributions. You cannot simply apply 5% to all the money you will have contributed by the end, because your last contribution has not been invested for twelve months. This explains why multiplying annual contributions by an annual return often overstates their first year earnings. Monthly calculations preserve the timing of each deposit. An institution that calculates daily interest or applies special conditions may produce a different statement from this educational approximation. Check its terms before treating a projection as an expected payment.
Reading one row of the annual table
Year zero is the starting position. In the first year row, cumulative contributions include the opening capital and twelve monthly deposits. The nominal balance also includes calculated growth. Cumulative growth is the balance minus your contributions. Growth during a particular year, where shown separately, helps distinguish that year's result from everything accumulated since the beginning. None of these columns automatically represents taxable gains, available withdrawal income or a promised payout from a financial provider.
Compare two consecutive years. Their balance difference combines new contributions and growth, so you cannot label all of it investment return. If the balance rises from 10,000 to 13,000 while you deposit 2,400, the growth component in the model is 600. Calculating a return percentage on real cash flows also requires their dates and an appropriate method. The table separates amounts, but it does not replace a money weighted or time weighted performance calculation. Keep the question you want to answer separate from the largest number on the screen.
What happens when contributions stop temporarily
A saving pause does not necessarily mean withdrawing your balance. You can leave accumulated money in place while making no deposits for several months. To investigate this with a constant contribution calculator, divide the exercise into stages. Calculate up to the pause, use the resulting balance as the starting capital of a second calculation without contributions, then calculate a third stage when deposits resume. Retain the same rate convention and keep a separate record of the new money deposited in each stage.
Do not add the three final balances together: money ending one stage is the same money starting the next. Adding them would count it repeatedly. Also avoid using an inflation adjusted balance as the next stage's starting capital while continuing to apply nominal rates. Use the nominal balance, then adjust the final result for inflation across the whole period. A simple supporting sheet with dates, opening balances, deposits and closing balances makes the process easier to review. If stage lengths include partial years, use a monthly spreadsheet rather than rounding them to whole years.
Frequently asked questions about growth
Does a zero return mean saving is pointless? No. Contributions still increase the balance, and the zero growth scenario shows how much depends on your own saving effort. Can this calculator reproduce market losses? This version does not model market downturns or negative rates. Its nonnegative rate inputs describe constant growth assumptions. Never treat the displayed curve as the worst possible investment outcome or as evidence that a risky asset cannot lose money before you need to withdraw it.
Should you increase the assumed rate when your target looks distant? Changing an assumption does not improve the opportunities actually available. First test a different deadline or an affordable contribution, leaving any shortfall visible. What should you save with the result? Record the starting capital, monthly contribution, rate, duration, inflation assumption and calculation date alongside the CSV export. You can then explain a revised projection without attributing changes in your own inputs to market performance. This record is especially useful when comparing plans several months apart or discussing a shared household goal.