Compound Interest & Wealth Growth Simulator

How the Mathematical Formulas Work

FinMetric Pro simulates compounding month by month rather than using a single closed-form estimate, which keeps the result accurate even when contributions change. The recurrence applied each month is:

Balance(m) = Balance(m−1) × (1 + r) + C

where:
  r = annual return ÷ 12 (monthly rate)
  C = monthly contribution

Real Balance(m) = Balance(m) / (1 + i)^m
  where i = annual inflation ÷ 12 (monthly inflation rate)

Nominal balance is the raw dollar figure the account would show. Real balance deflates that figure by compounding inflation over the same number of months, expressing the result in today's purchasing power. The gap between the two lines widens every year — a direct visualization of why a return rate that beats inflation still erodes in real terms if inflation is high enough, and why "seven percent for thirty years" means something very different after adjusting for a 3% inflation environment versus a 1% one.

Total interest earned is simply the final nominal balance minus the sum of every contribution made (including the initial investment) — isolating exactly how much of the ending balance came from growth versus money the investor actually deposited.

Key Terminology Defined

Compound Interest

Interest calculated on both the initial principal and the accumulated interest from prior periods — growth building on growth.

Nominal Return

The stated rate of return before adjusting for inflation.

Real Return

Return after subtracting the effect of inflation, reflecting actual purchasing-power growth.

Investment Horizon

The length of time capital remains invested before being withdrawn or needed.

Dollar-Cost Averaging

Investing a fixed amount on a regular schedule (like the monthly contribution here) regardless of market conditions.

Purchasing Power

What a given amount of money can actually buy, which erodes over time under positive inflation.

Practical Case Studies & Financial Strategies

Two savers each start with $15,000. Saver A begins contributing $500 monthly at age 30; Saver B waits ten years and then contributes $900 monthly to "catch up." Even though Saver B contributes more per month for a shorter stretch, Saver A typically finishes with a larger nominal balance — because the extra decade of compounding on early contributions outweighs the higher monthly amount contributed later. This is the core argument for starting early even with small amounts rather than waiting to invest larger sums later.

The real-vs-nominal split matters most for long horizons. Over 25–30 years, even moderate inflation (3%) compounds enough to cut purchasing power roughly in half, which is why retirement projections that ignore inflation tend to overstate how much a given nominal balance will actually be able to buy decades from now. Reviewing the real balance line, not just the nominal one, is the more honest planning number.