How often does this compound?

Monthly. The annual rate is divided by 12 and applied each month, with contributions added at the end of the month — the convention used by most savings and investment accounts.

Is the return guaranteed?

No. Investment returns fluctuate and can be negative in some years. The rate you enter is an assumed average for planning, not a promise.

The longer your money compounds, the larger the interest share of the final balance. Starting a decade earlier can roughly double the ending balance for the same contributions.

What is the difference between nominal and real return?

The nominal return is the headline rate your money grows at. The real return subtracts inflation, showing actual purchasing-power gain. A 7% nominal return with 3% inflation is roughly a 4% real return — the figure that matters for long-run goals.

Does the timing of contributions matter?

This calculator adds contributions at the end of each month (an ordinary annuity). Adding them at the start of each month (an annuity due) would raise the result slightly, but over long horizons the difference is small relative to the rate and time assumptions.

Finance Calculators

Compound Interest Calculator

Compound interest is the engine behind long-term saving: each period’s interest is added to the balance, so future interest is earned on a growing base. This calculator compounds monthly and adds a regular contribution at the end of each month, mirroring how most savings and retirement accounts grow.

Enter a starting deposit, a monthly contribution, an expected annual return and a time horizon. The result shows the projected balance, how much you actually contributed, and how much of the balance is pure interest.

It is equally useful for projecting a retirement balance, comparing two savings plans, or simply grasping how much of a long-term balance is earned rather than contributed.

Initial deposit

Monthly contribution

Annual interest rate%

Time horizonyrs

Currency

Enter your values to see the result.

How this is calculated

Compound interest is interest earned on a growing base: each period’s interest is added to the balance, so future interest is calculated on principal plus all prior interest. This calculator compounds monthly and adds a regular contribution at the end of each month, mirroring how most savings and retirement accounts behave.

The balance is the sum of two parts. The initial deposit grows to P × (1 + r)^n, where P is the starting amount, r is the monthly rate (annual rate ÷ 12) and n is the number of months. The monthly contributions form an ordinary annuity whose future value is PMT × ((1 + r)^n − 1) ÷ r, where PMT is the monthly contribution. Total contributed is P + PMT × n, and the interest earned is the final balance minus that.

The model assumes a constant average rate held for the whole period, contributions made at month-end, and no taxes, fees or inflation. Real investment returns fluctuate year to year and can be negative, so the rate you enter is an assumption for planning, not a promise. The figure also ignores tax on investment growth and fund fees, both of which reduce the real balance.

Worked example

A $10,000 initial deposit, $250 added each month, 7% annual return, 20-year horizon.

1Monthly rate r = 0.07 ÷ 12 = 0.005833; months n = 20 × 12 = 240.
2Growth factor (1 + r)^n = (1.005833)^240 ≈ 4.031.
3Initial deposit grows to 10,000 × 4.031 = $40,310.
4Contributions grow to 250 × (4.031 − 1) ÷ 0.005833 ≈ $129,940.
5Final balance ≈ $170,250. Total contributed = 10,000 + 250 × 240 = $70,000, so interest earned ≈ $100,250.

Frequently asked questions

How often does this compound?: Monthly. The annual rate is divided by 12 and applied each month, with contributions added at the end of the month — the convention used by most savings and investment accounts.
Is the return guaranteed?: No. Investment returns fluctuate and can be negative in some years. The rate you enter is an assumed average for planning, not a promise.
Why start early?: The longer your money compounds, the larger the interest share of the final balance. Starting a decade earlier can roughly double the ending balance for the same contributions.
What is the difference between nominal and real return?: The nominal return is the headline rate your money grows at. The real return subtracts inflation, showing actual purchasing-power gain. A 7% nominal return with 3% inflation is roughly a 4% real return — the figure that matters for long-run goals.
Does the timing of contributions matter?: This calculator adds contributions at the end of each month (an ordinary annuity). Adding them at the start of each month (an annuity due) would raise the result slightly, but over long horizons the difference is small relative to the rate and time assumptions.

Related calculators

Method: monthly compound interest with end-of-month contributions (ordinary annuity). No external data source. Last updated: September 2026.

These results are indicative estimates for planning only and do not constitute financial advice. Actual loan terms, tax rules, investment returns and product conditions vary by country, provider and your personal circumstances. Always confirm figures with your bank, tax authority or a qualified financial adviser before making decisions.