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Airgead uses a monthly compounding model to project investment growth. Rather than applying interest once per year, the engine divides the annual rate by 12 to derive a monthly rate, applies that rate to two running balances on every monthly cycle, and records a single snapshot row at the end of each year. This produces a year-by-year table that shows exactly how regular contributions amplify the effect of compounding over time.

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The Monthly Rate Conversion

Before the main loop runs, the annual interest rate entered by the user is converted to a monthly equivalent: 
monthly rate = annual rate / 100 / 12
Dividing by 100 converts the percentage to a decimal, and dividing by 12 spreads it evenly across the twelve monthly periods in a year. For example, a 6% annual rate becomes a monthly rate of 0.005 (0.5%).

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Two Parallel Balances

Every calculation tracks two balances in lockstep, both initialised to startingBalance. A third accumulator, totalDeposits, tracks the total principal contributed so that interest earned can be isolated at the end. Each month, the inner loop performs these three operations in order:
  1. Both balances are multiplied by (1 + monthlyRate) — this is the compounding step
  2. monthlyDeposit is added only to withDeposits — the contributions scenario
  3. monthlyDeposit is added to totalDeposits — the principal tracker
Here is the exact inner loop extracted from js/lib/calculator.js: 
for (let m = 0; m < 12; m++) {
  withDeposits = withDeposits * (1 + monthlyRate) + monthlyDeposit;
  withoutDeposits = withoutDeposits * (1 + monthlyRate);
  totalDeposits += monthlyDeposit;
}
After all 12 months have been processed, a result row is pushed into the output array for that year.
The results array always begins with a year-zero row that represents the initial state before any compounding or deposits have occurred. All four values at year 0 reflect the raw startingBalance exactly as entered, and interestEarned is always 0. This is why the results table runs from Year 0 through Year N rather than Year 1 through Year N.

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The Result Row Shape

Each entry in the returned array has the following fields:
​
year
number
The year index. 0 represents the initial state before compounding begins; subsequent integers correspond to each completed year of the projection.
​
withDeposits
number
The projected balance at the end of the year, assuming the specified monthly deposit is made every month. Rounded to the nearest whole dollar.
​
withoutDeposits
number
The projected balance at the end of the year if no additional deposits are ever made — only the startingBalance compounds. Rounded to the nearest whole dollar.
​
totalDeposits
number
The cumulative principal contributed: the original startingBalance plus every monthly deposit made up to and including this year. Rounded to the nearest whole dollar. This value is used to calculate interestEarned.
​
interestEarned
number
The growth attributable purely to compound interest: withDeposits − totalDeposits. This isolates the interest component from contributions. Rounded to the nearest whole dollar. Always 0 on the year-zero row.

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Input Clamping

Before any calculation runs, all four inputs are sanitised to prevent invalid or extreme values from producing nonsensical output:
  • startingBalance — clamped to Math.max(0, ...): cannot be negative
  • monthlyDeposit — clamped to Math.max(0, ...): cannot be negative
  • annualRate — clamped to Math.max(0, ...): cannot be negative
  • years — clamped to Math.min(80, Math.max(0, Math.round(...))): must be a non-negative whole number, capped at 80 years
Non-numeric inputs are coerced to 0 via Number(value) || 0 before clamping is applied.

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What This Model Does Not Include

The calculation is intentionally simplified for educational purposes. It does not account for real-world factors that would affect an actual investment portfolio.
Airgead’s compound interest model does not account for any of the following:
  • Changing interest rates — the annual rate is held constant for the full projection period
  • Inflation — future values are expressed in nominal terms, not real (inflation-adjusted) terms
  • Account fees or management charges — no expense ratios or advisory fees are deducted
  • Taxes — no capital gains, dividend, or income tax is applied to growth
  • Contribution timing differences — deposits are assumed to occur at the same point within each month
  • Market volatility — returns are treated as perfectly smooth and consistent year over year
  • Withdrawal activity — no partial withdrawals or required minimum distributions are modelled
For real financial planning, consult a licensed financial adviser.

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