Question 1

What is compound interest?

Accepted Answer

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest — which is calculated only on the principal — compound interest causes your balance to grow at an accelerating rate over time. The longer the horizon and the more frequently interest compounds, the more powerful the effect becomes.

Question 2

How does compound frequency affect my final balance?

Accepted Answer

Compound frequency determines how often interest is calculated and added to your balance. At the same nominal annual rate, daily or monthly compounding produces a slightly higher effective annual return than annual compounding. The difference is small at lower rates, but grows as both the rate and the time horizon increase. For example, at 6% annual rate, the effective annual return is 6.00% with annual compounding and 6.168% with daily compounding — a meaningful difference over 20 or 30 years.

Question 3

What is the difference between nominal and effective interest rate?

Accepted Answer

The nominal annual rate is the stated rate before compounding is applied. The effective annual rate (EAR) is the actual return earned after compounding. For annual compounding, they are equal. For more frequent compounding, the EAR exceeds the nominal rate: EAR = (1 + nominal / n)^n − 1, where n is the number of compounding periods per year. This calculator uses the nominal rate you enter and converts it to an effective monthly rate based on your chosen compounding frequency.

Question 4

Why does time horizon matter so much in compound growth?

Accepted Answer

Compounding is non-linear — your balance grows faster in later years because interest is being earned on a larger base. The first $1,000 of interest takes years to accumulate; later, a similar amount may be earned in months. This is why doubling your investment horizon often more than doubles your final balance. It is also why starting earlier — even with smaller contributions — tends to produce better outcomes than starting later with larger contributions.

Question 5

What does the Target Amount feature do?

Accepted Answer

If you enter a target amount, the calculator shows your projected progress toward that goal and, if you are below target, the extra monthly contribution needed to reach it within your chosen time horizon. This uses the present value of a growing annuity formula to back-calculate the required contribution increase. It is an estimate — actual results depend on your realised return, contribution consistency, and compounding terms.

Question 6

What does Starting Age do?

Accepted Answer

Starting Age is optional and purely informational. It does not change any calculation. When entered, the AI Analysis adds age-based context — for example, showing your projected balance "by age X" — which some users find easier to interpret than a year count alone. The math is identical whether or not you enter a starting age.

Question 7

What assumptions does this calculator make?

Accepted Answer

Four main assumptions: (1) the nominal interest rate is constant throughout the entire investment horizon; (2) monthly contributions are made consistently on the same date each month; (3) no taxes, fees, or inflation adjustments are applied; and (4) the effective monthly rate is derived from the nominal annual rate and the chosen compounding frequency. This is an educational illustration only — actual savings and investment returns vary.

Variable	Meaning
r	Nominal annual rate (e.g. 0.06 for 6%)
n	Compounding periods per year (1 / 2 / 12 / 365)
EAR	Effective annual rate after compounding
r_monthly	Effective monthly rate applied to balance each month

Compound Interest Calculator

Growth Breakdown

Growth Over Time

How It Works

Effective Monthly Rate

Future Value Formula

Assumptions

Frequently Asked Questions