Compound Interest Calculator with Contributions

What this tool does

Calculate compound interest from a starting balance, an annual nominal rate, a duration and a compounding frequency. Previously earned interest becomes part of the balance used for future calculations. You can add regular contributions and compare your starting amount, new deposits and interest in a year-by-year table. The result follows the fixed rate and timing assumptions you enter.

How to use this tool

1. 1

   Step 1: Enter the starting amount and rate

   The rate is the nominal annual rate. Choose how often it compounds: daily, monthly, quarterly, semiannually or annually.

2. 2

   Step 2: Set the time horizon

   Enter the duration in years. The calculation uses whole compounding periods, so choose a duration that fits the selected frequency for an exact period count.

3. 3

   Step 3: Add recurring contributions if you make them

   Choose a contribution amount and frequency. Beginning-of-period timing adds each scheduled contribution before interest in its assigned compounding period; end-of-period timing adds it afterward.

4. 4

   Step 4: Read the yearly table and export it

   Each row shows the opening balance, contributions, interest earned and closing balance for that year. Download the whole table as CSV.

Worked example

Compound interest formula and worked example

For a starting balance without additional deposits, use A = P(1 + r/n)^(n × t). A is the ending balance, P is the starting amount, r is the nominal annual rate as a decimal, n is the compounding periods per year and t is the number of years. For example, 1,000 at 6% compounded monthly for five years gives 1,000 × (1 + 0.06/12)^60 = 1,348.85, including 348.85 of interest. With contributions, the calculator adds deposits and interest period by period.

periodic rate = annual rate ÷ 100 ÷ periods per year
each period:  balance = balance × (1 + periodic rate)
beginning-of-period contributions are added before interest is applied
end-of-period contributions are added after interest is applied
lump sum only:  A = P(1 + r/n)^(n × t)

• When contribution and compounding frequencies differ, contributions are grouped into the compounding period in which they fall. This is a period-based model, not a day-by-day bank balance calculation.
• A 0% rate is valid and simply returns the sum of the starting amount and the contributions.
• Periodic rates of −100% or lower are rejected. A negative rate above that threshold can reduce the projected balance.

Rate, compounding and contribution timing

Limitations and privacy

Balances, rates and contribution amounts are calculated in your browser and never transmitted.

Frequently asked questions