Compound Interest Case Studies: 6 Worked Examples & Calculator

These examples show how time, deposits and the assumed rate affect a balance. Every result below uses a fixed 7% nominal annual rate, monthly compounding, end-of-month deposits and no taxes, fees or inflation adjustment unless the case says otherwise. A fixed rate is an arithmetic scenario, not a forecast or guarantee; investment returns vary and may be negative.

Case 1: $100/month for 40 years

Starting at $0, deposit $100 at each month-end for 40 years. Total deposits are $48,000 and the projected balance is about $262,481, with about $214,481 of arithmetic growth.

Case 2: $500/month for 30 years

Deposit $500 at each month-end for 30 years. Total deposits are $180,000 and the projected balance is about $609,985, with about $429,985 of arithmetic growth.

Case 3: $1,000/month for 25 years

Deposit $1,000 at each month-end for 25 years. Total deposits are $300,000 and the projected balance is about $810,072, with about $510,072 of arithmetic growth. The larger deposit does not remove the uncertainty of the assumed rate.

Case 4: $50,000 lump sum for 40 years

Leave a $50,000 starting balance untouched for 40 years at the same nominal rate. The projected balance is about $815,571. This illustrates compounding under the stated rate; it does not describe how an index fund will actually perform each year.

Case 5: Deposit for 10 years, then stop

Deposit $500 at each month-end for 10 years, then make no further deposits for 30 years. Total deposits are $60,000 and the projected balance at year 40 is about $702,421. A comparison case of $500 per month for 30 years has $180,000 of deposits and a projected balance of about $609,985 under the same rate, so the result depends on the exact timing and assumptions.

Case 6: A one-percentage-point rate difference

With $500 at each month-end for 30 years, a 6% nominal scenario ends near $502,258 while a 7% nominal scenario ends near $609,985. This is a rate-sensitivity illustration, not proof that a fee or product will reduce returns by exactly one percentage point.

What these cases share

The calculation is the starting balance grown by the selected periodic factor plus the future value of each end-of-month deposit. Change the rate, timing, fees, taxes or inflation assumption and the result changes. Investor.gov notes that investments do not have a set rate of return and involve risk.

How to use the examples

Open the compound investment calculator, match the starting amount and deposit schedule, then run at least a lower and higher rate. Keep nominal and inflation-adjusted scenarios separate and use the product disclosure for actual account terms.