Compound Interest Calculator

How to use the compound interest calculator

Enter your starting principal, your annual interest rate as a percent, and the number of years the money has to grow. Pick a compounding frequency — monthly by default, or daily, quarterly, or annually — and, if you plan to keep adding money, a monthly contribution. The calculator returns the ending balance, the total you deposited along the way, and the interest earned on top. The defaults model a common case: $1,000.00 at 5% for 10 years, compounded monthly with no contributions, grows to $1,647.01 — that is $647.01 of pure interest. Change any input and the result updates right away.

The years input doubles as a slider you can drag from 1 to 50, which is the fastest way to see compounding in action. Slide the term out and watch the balance curve upward rather than climb in a straight line — early on the gains are modest, but each year the interest is calculated on a larger balance, so the line bends steeper the longer you let it run. That bend is the entire difference between compound interest and the flat, equal-step growth of simple interest, and it is why time in the market tends to matter more than almost anything else.

Use the frequency control to see how often compounding happens changes the outcome. At the same 5% rate over 10 years, your $1,000.00 grows to $1,628.89 compounded once a year, $1,647.01 compounded monthly, and a hair more compounded daily — each step up in frequency adds a little because interest starts earning its own interest sooner. The gaps are small at ordinary rates, but they are real, and they widen as the rate or the term grows. When you compare two accounts, check the frequency along with the rate.

To model steady saving, add a monthly contribution. This is where the balance really moves: keep the $1,000.00 starting amount at 5% for 10 years, compounded monthly, and add $100.00 a month, and the balance climbs to $17,175.23. Of that, $12,000.00 is money you deposited and $4,175.23 is interest — your contributions and the growth on them reinforce each other into a snowball that rolls faster every year. The total-deposits figure is broken out separately so you can always see how much of the ending balance you put in versus how much the account earned.

A quick mental shortcut pairs well with this tool: the Rule of 72. Divide 72 by your interest rate to estimate the years it takes an amount to double — at 5%, that is 72 ÷ 5 ≈ 14.4 years. It is an approximation, not a substitute for the exact math here, but it is handy for sanity-checking a result at a glance. If you want growth on a fixed amount that never compounds — interest figured on the original principal alone — use the simple interest calculator instead; this one is built for balances that compound.

The formula

Compound interest pays interest on your interest. Each period the balance is multiplied by a small growth factor, and because that factor applies to a balance that already includes past interest, the total accelerates over time. With a recurring deposit, each contribution gets its own stretch of compounding too:

n = compounds per year      (12 monthly, 365 daily, 4 quarterly, 1 annually)
balance = P × (1 + r/n)^(n × years)
with deposits: + PMT × [ ((1 + r/n)^(n × years) − 1) ÷ (r/n) ]
interest = balance − principal − total deposits

Worked example with the defaults — $1,000.00 at 5% for 10 years, compounded monthly with no deposits: here n = 12, r = 0.05, and the exponent n × years = 120, so balance = 1000 × (1 + 0.05 ÷ 12)^120 = $1,647.01. Subtract the $1,000.00 principal and the interest is $647.01. Switch the frequency to annually and the same principal grows to only $1,628.89, because the interest compounds 10 times instead of 120 — more frequent compounding earns slightly more at the same rate.

Now add a $100.00 monthly contribution and keep everything else the same. The starting $1,000.00 still grows to $1,647.01, and the deposits add the annuity term, PMT × [ ((1 + r/n)^120 − 1) ÷ (r/n) ], which carries the balance to $17,175.23. Of that, $12,000.00 is the total you deposited (120 payments of $100.00) and the remaining $4,175.23 is interest. To estimate a doubling time without the full formula, fall back on the Rule of 72: 72 ÷ 5 ≈ 14.4 years for money to double at 5%.

Frequently asked questions

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