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What could happen if you regularly added a small amount to your savings instead of relying only on the money already there? The answer is more than a larger pile of deposits. Each new contribution can also begin earning interest, which changes how the account grows over time. Compound interest means that money earns interest on the original balance and on interest that has already accumulated. The Airgead Investment Calculator makes that process easier to see by running two projections at once — one that leaves the starting balance untouched, and one that uses the same starting balance, rate, and timeframe but includes a monthly deposit. Looking at both paths together helps answer a practical question: how much of the difference comes from regularly adding money, and how much comes from the growth earned on those contributions?

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One Starting Point, Two Different Paths

Both projections begin with the same initial investment. They also use the same annual rate and the same number of years. The only difference is whether a monthly deposit is included. This single variable is enough to separate the two projections significantly over time, and understanding why requires looking at what each scenario actually does to the balance.

Without Monthly Deposits

The original balance grows only through the interest it earns. No new principal enters the account after the initial investment. Every dollar of growth comes entirely from the rate applied to that fixed starting amount.

With Monthly Deposits

The balance receives new principal every month. Those contributions immediately increase the base on which interest is calculated, and each contribution then has time to earn interest during the remainder of the projection period.
When the initial investment is left untouched, growth comes entirely from interest applied to a fixed base. Now imagine adding money every month. Each contribution immediately increases the principal and then has time to earn interest during the rest of the projection. Over several years, the distance between those two balances can become substantial. The important point is that the difference is not created by interest alone. The larger balance includes the money contributed each month plus the growth earned on those contributions — and those are two separate things worth understanding independently.

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Why Contribution Timing Matters

Not every monthly deposit has the same effect on the final balance. A contribution made near the beginning of a projection has more time to grow than one made during the final year. Later deposits still increase the balance, but they have fewer monthly compounding periods available to accumulate interest before the projection ends. This is why the sequence and consistency of contributions matter, not just the total amount deposited. Airgead handles this in monthly steps, applying the rate twelve times per year to reflect how compounding actually accumulates. The sequence for each month follows the same repeating pattern:
1

Existing balance earns one month of interest

The current balance — whatever it is at that point in the projection — is multiplied by the monthly rate. The resulting interest is added to the balance. This happens before any new contribution is applied.
2

Monthly contribution is added

After interest is applied, the monthly deposit is added to the balance. This means each new contribution enters an account that has just grown, and it immediately becomes part of the base for the following month’s interest calculation.
3

New balance moves into the next month

The updated balance — original balance plus interest plus new contribution — becomes the starting point for the next monthly cycle. Nothing resets; each month builds directly on the one before it.
4

Cycle repeats throughout the selected timeframe

Steps one through three repeat every month for the full duration of the projection. After twelve repetitions, the yearly result is recorded. The process then continues for each subsequent year.
This monthly schedule is useful because many people already manage income, bills, and savings on a monthly basis. The calculator connects a familiar monthly contribution amount to a long-term result without requiring manual calculation of every compounding step.

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Understanding Principal and Interest Separately

A large final balance does not tell the whole story on its own. To interpret the result correctly, it matters to know how much of that balance came from personal contributions and how much came from investment growth. That is why the Results page in Airgead reports total principal and earned interest as separate values rather than combining them into a single figure. Principal is the sum of the initial investment plus every monthly contribution made throughout the projection. Earned interest is the portion of the final balance that remains after subtracting that total principal — in other words, the growth generated by compounding alone. Consider a straightforward example. Someone starts with 1,000anddeposits1,000 and deposits 100 per month for one year. Before considering interest, the total principal at the end of that year is $2,200:
  • $1,000 — initial investment
  • 1,200—contributedthroughtwelvemonthlydeposits(1,200 — contributed through twelve monthly deposits (100 × 12)
  • $2,200 — total principal before interest
If the final projected balance is higher than $2,200, the amount above it represents earned interest. The exact result depends on the annual rate and how the calculator applies each monthly step. The example is not a promise of growth; it simply illustrates how to separate deposited money from investment earnings, which is the same separation Airgead surfaces in its results.

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The Monthly Rate Formula

The interest calculation at each monthly step uses the following formula to convert an annual percentage rate into the rate applied each month: 
monthly rate = annual rate / 100 / 12
Dividing by 100 converts the percentage to a decimal. Dividing by 12 produces a monthly decimal rate from the annual figure. This rate is then applied to the current balance twelve times per year — once per month — which means the balance compounds monthly rather than annually. A higher annual rate produces a larger monthly rate at each step, which in turn produces more interest per month on an equal balance.

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Using the Comparison Tool

The Airgead comparison feature lets users explore two plans side by side without treating either one as a definitive answer. The calculator demonstrates how the inputs relate mathematically; it does not prescribe how much anyone should contribute. A sustainable monthly amount depends on income, required expenses, debt, emergency savings, and other financial goals — all of which exist outside the calculator. Change one input at a time and ask questions such as:
  • What happens if I contribute less each month but continue for longer?
  • How does increasing the deposit affect principal and earned interest?
  • What changes when both plans use the same deposit but different assumed rates?
  • How much of each final balance came from deposits rather than interest?
These are hypothetical comparisons, not predictions or guarantees. The calculator uses the values entered and assumes the same rate throughout the entire projection. It does not model taxes, fees, inflation, withdrawals, market volatility, or returns that change over time.

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Why Year-by-Year Data Matters

A final number provides a destination, but the yearly breakdown shows the path. During the first few years, the difference between two scenarios may appear small. As contributions continue and more of the balance has time to earn interest, the gap can become easier to see. The chart provides a quick visual comparison. The table provides the exact balance, principal, and interest recorded for each year. Together, they make it possible to recognize the overall trend and inspect a specific point in the projection.

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The Practical Takeaway

Long-term growth depends on several connected factors working together: the starting balance, the contribution amount, the time available, and the assumed rate. Monthly deposits matter because they add principal consistently and give at least some of that new money time to earn interest across multiple compounding periods. A contribution made early has the most time to compound; a contribution made in the final months still adds to the balance even if compounding has little time to work on it. Even so, a larger input is not automatically a better real-life plan if the amount cannot be maintained. The most useful projection starts with a contribution that fits the person’s actual budget. From there, Airgead can show how that amount behaves over time and how a different contribution — higher or lower — would change the outcome. The relationship between what you put in and what growth adds on top of it becomes much clearer when principal and earned interest are visible as separate values year by year.
Use the year-by-year table on the Results page to see the precise balance, total principal, and earned interest recorded at the end of each year — not just the final projected figure.

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