A decision tool, not just an answer.
Use this calculator to explore long-term saving and investing scenarios when you have a starting balance, a regular monthly contribution, an assumed annual return, and a time horizon. It separates money deposited from growth generated by compounding.
- Comparing the effect of starting now versus waiting several years.
- Testing how a larger monthly contribution changes a long-term projection.
- Separating contributed principal from estimated interest or investment growth.
Formula and calculation method.
FV = P(1 + i)ⁿ + C × [((1 + i)ⁿ − 1) ÷ i]FV is future value, P is the starting balance, C is the month-end contribution, i is the monthly rate (annual rate ÷ 12 ÷ 100), and n is the number of months. The first term grows the opening balance; the second grows the contribution stream.
Zero-rate case: At 0%, no compounding occurs: future balance = starting balance + monthly contribution × number of months.
See the result in context.
$5,000 starting balance; $250 monthly; 6% annually; 10 years
Working: The opening balance and 120 month-end contributions compound monthly
Result: $50,066.82 projected balance; $35,000 deposited; $15,066.82 interest
Interpretation: About 30% of the ending balance comes from modeled growth rather than deposited money.
$10,000 starting balance; no contributions; 5% annually; 20 years
Working: $10,000 × (1 + 0.05 ÷ 12)²⁴⁰
Result: $27,126.40 projected balance; $17,126.40 interest
Interpretation: Time allows the original balance to more than double even without additional deposits, assuming the rate remains constant.
$0 starting balance; $500 monthly; 0% annually; 5 years
Working: $500 × 60 months
Result: $30,000 projected balance; $0 interest
Interpretation: This baseline isolates what contributions alone produce and makes the compounding contribution easier to compare.
$25,000 starting balance; $400 monthly; 7% annually; 15 years
Working: The starting balance and 180 month-end contributions compound at 7% ÷ 12
Result: $198,008.59 projected balance; $97,000 deposited; $101,008.59 interest
Interpretation: Under the constant-rate assumption, modeled growth becomes slightly larger than all deposited principal.
Assumptions and common mistakes.
What the estimate assumes
- The entered annual rate is constant and compounding occurs monthly.
- Recurring contributions are equal and arrive at the end of every month.
- Interest or returns remain invested instead of being withdrawn.
- The time horizon is converted to 12 equal compounding periods per year.
Mistakes to avoid
- Treating a hypothetical annual return as guaranteed, especially for market investments.
- Confusing the projected balance with interest earned; part of the balance is your own deposits.
- Ignoring fees, taxes, inflation, and periods with negative returns.
- Comparing this monthly-compounding result with an account that compounds or credits interest differently.
What unusual inputs mean.
The result is simply the starting balance plus all monthly contributions, with no interest earned.
The calculator can project growth from monthly contributions alone.
The formula reduces to compound growth of the starting lump sum.
Small rate changes produce large differences over decades, so test conservative as well as optimistic scenarios.
How to use this calculator.
- Enter the amount already saved or invested. Use 0 if you are starting with monthly contributions only.
- Enter the contribution made at the end of each month.
- Enter an assumed annual rate and the number of years. Treat the rate as a scenario, not a promise.
- Compare the projected balance with deposited principal to see how much of the result comes from compounding.
What the estimate leaves out.
This is a mathematical projection, not a forecast or investment recommendation. It excludes taxes, account fees, inflation, contribution limits, changing cash flows, market volatility, and sequence-of-returns risk. Savings accounts may use a stated APY rather than the nominal annual rate convention modeled here.
Questions people ask.
What is compound interest?
Compound interest means growth is calculated on both the original principal and previously accumulated interest. With contributions, earlier deposits also have more time to compound.
How often does this calculator compound?
It uses monthly compounding and assumes recurring contributions arrive at the end of each month.
What is the difference between principal and interest earned?
Deposited principal is the starting balance plus all monthly contributions. Interest earned is the projected ending balance minus that deposited principal.
Can I enter a zero starting balance?
Yes. A zero starting balance models a plan funded entirely by monthly contributions.
What happens at a 0% interest rate?
The calculator adds the starting balance and all contributions without growth. This provides a useful baseline for measuring the effect of compounding.
Is an annual return the same as APY?
Not necessarily. This calculator treats the entered rate as a nominal annual rate divided across 12 monthly periods. APY already reflects compounding, so entering an APY can produce a slightly different result from an account’s own projection.
Does this account for inflation or investment risk?
No. The result is a nominal, constant-rate projection. Inflation reduces future purchasing power, and real investment returns vary from month to month and can be negative.
Do beginning-of-month contributions produce the same result?
No. Beginning-of-month deposits receive one extra month of compounding and would finish slightly higher. This calculator assumes month-end contributions.