Compound Interest Calculator

The most powerful idea in saving

Compound interest means your interest earns interest. Each period's gain is added to your balance, so the next period you earn a return on a bigger number. Over years, the growth curve bends sharply upward — which is why starting early matters more than starting big.

How compound interest works

With plain (simple) interest, you earn a return only on the money you originally put in. If you deposit $1,000 at 5% simple interest, you earn $50 every year — no more, no less — because the calculation always uses that same $1,000. Compound interest works differently: at the end of each period the interest you just earned is added to your balance, and the next period's interest is calculated on that larger total. The first year you earn $50 on $1,000; the second year you earn interest on $1,050; the third year on $1,102.50, and so on. Each round of interest quietly raises the base that the next round is measured against, and that snowball is the whole point of compounding.

The standard formula for the future value of a lump sum is A = P(1 + r/n)^nt. In plain words: A is the amount you end up with, P is the principal (your starting amount), r is the annual interest rate written as a decimal (so 6% becomes 0.06), n is the number of times interest is added each year, and t is the number of years. The term r/n is the rate applied in each single period, and the exponent nt is the total number of periods over the whole timeframe. When you also add regular contributions — as this calculator does — each deposit gets its own share of compounding for however many periods remain, which is why steady monthly saving builds up so much more than the raw deposits alone.

What each input means

• Starting amount (principal) — the money you begin with today, before any growth or new deposits. It can be zero if you are starting from scratch and building entirely through contributions.
• Monthly contribution — the amount you add every month. These regular deposits are added to the balance and then compound alongside your principal.
• Annual return — the yearly growth rate you expect, entered as a percentage. This is an assumption, not a guarantee; real returns vary year to year and can be negative.
• Years — how long the money stays invested. Time is the input that does the heavy lifting, because it decides how many rounds of compounding your money goes through.
• Compounding — how often interest is added to the balance: monthly, quarterly or annually. More frequent compounding lets interest start earning its own interest sooner.

What the chart shows

The gold dashed line is the money you actually put in. The green line is your balance. The widening gap between them is compounding at work — interest stacking on interest. Early on the two lines hug each other; later, the green line pulls away dramatically.

Worked example

Here is an illustrative example — the numbers are hypothetical and rounded, chosen only to show the mechanics. Suppose you start with $10,000, add nothing more, and earn an example 6% annual return compounded monthly for 20 years. Using A = P(1 + r/n)^nt, the rate per period is 0.06 ÷ 12 = 0.005, and the number of periods is 12 × 20 = 240. So A = 10,000 × (1.005)²⁴⁰, which works out to roughly $33,000. Your original $10,000 more than tripled, and about $23,000 of the final total is growth you never deposited. If you had instead added $300 every month over those same 20 years, the ending balance would be far higher again, because each contribution gets its own years of compounding. Treat these figures as an example of how the formula behaves, not a promise of any particular result.

Why compounding frequency matters

The same annual rate produces slightly different results depending on how often interest is added. Compounding monthly means twelve smaller boosts a year, each one lifting the base for the next, while compounding annually applies the full rate just once. Over a single year the difference is tiny; over several decades it adds up to a noticeable amount. This is also why comparing accounts can be tricky — two products quoting the same headline rate may not grow your money equally if one compounds monthly and the other annually.

The rule of 72

The rule of 72 is a quick mental shortcut, not an exact calculation. Dividing 72 by your annual return gives a close estimate of how many years it takes for money to double. At 6% that is about 12 years, at 9% about 8 years, and at 12% about 6 years. It is handy for a fast gut check, but for real planning use the calculator above, which does the full period-by-period math.

The cost of starting late

Because compounding rewards time, the years you wait are the most expensive years of all — you lose the rounds of growth that would have compounded the longest. An amount invested in your twenties has decades to snowball, while the same amount invested in your forties has far fewer compounding cycles left, so it never catches up even if markets behave identically. Waiting to save more money later often loses to starting with a smaller amount sooner. If you can only do one thing, starting now usually beats starting bigger.

What moves the result most

• Time — the longest lever, and the one you can't buy back. An extra decade often matters more than an extra few percent.
• Rate of return — small differences compound into big gaps. Try 6% vs 8% above.
• Regular contributions — steady monthly deposits supercharge the curve.
• Compounding frequency — monthly slightly beats annual.

Key terms

• Principal — your starting amount, the money you put in before any growth.
• Interest rate — the percentage return applied to your balance over a year.
• Compounding frequency — how many times per year interest is calculated and added to the balance.
• APR vs APY — APR (annual percentage rate) is the plain yearly rate before compounding is counted; APY (annual percentage yield) is the effective rate after compounding is included, so APY is the better number for comparing how fast savings actually grow.
• Future value — what your balance is projected to be worth at the end of the timeframe.
• Contribution — money you add on a regular schedule, on top of your principal.

Common mistakes to avoid

• Assuming a high, steady return. Real returns bounce around and can be negative in some years. A modest, realistic rate gives a more honest projection than an optimistic flat number.
• Forgetting about inflation. A balance decades from now buys less than the same figure today. The projection is in future dollars, not today's spending power.
• Ignoring fees and taxes. Account fees and taxes on gains quietly reduce your real return, and this calculator does not subtract them.
• Waiting to start. Delaying while you save up a bigger amount usually costs more than starting small today, because you give up your most valuable compounding years.

For the intuition behind all this, read what is compound interest.

Frequently asked questions

How does a compound interest calculator work?

A compound interest calculator projects how your money grows when interest earns its own interest over time. You enter a starting amount, any regular contributions, an expected annual rate and a time period, and it shows your balance growing year by year. This calculator also breaks down how much of your final total came from your own money versus growth.

What is the difference between simple and compound interest?

Simple interest is earned only on your original amount, while compound interest is earned on your original amount plus all the interest already accumulated. Over long periods, compounding grows your money dramatically faster. For a fuller explanation, read what is compound interest.

How does compounding frequency affect growth?

Interest can compound annually, quarterly, monthly or daily, and the more often it compounds, the faster your balance grows, because each round of interest starts earning sooner. The difference is small over a year but becomes meaningful across decades.

How much should I contribute regularly?

Any consistent contribution helps, because regular deposits combined with compounding are what build wealth over time. Even modest monthly amounts can grow substantially over decades. Try different contribution amounts in the calculator to see how much difference small increases make to your final total.

Does starting early really matter that much?

Yes — time is the most powerful factor in compounding. Because interest builds on interest, money invested earlier has the longest runway to grow, so starting even a few years sooner can outweigh contributing larger amounts later.

What is the compound interest formula?

The standard formula for a lump sum is A = P(1 + r/n) raised to the power of nt, where A is the final amount, P is the principal, r is the annual rate as a decimal, n is how many times interest compounds per year, and t is the number of years. This calculator uses the same idea but also adds your regular contributions period by period.

What is the difference between APR and APY?

APR (annual percentage rate) is the plain yearly rate before compounding is taken into account, while APY (annual percentage yield) is the effective rate once compounding is included. APY is the more useful number when comparing how quickly savings will actually grow, because it reflects interest earning its own interest.

Does this calculator account for inflation, taxes and fees?

No. The projection shows growth in future dollars and does not subtract inflation, account fees or taxes on your gains, all of which reduce your real return. Treat the result as an estimate of nominal growth and remember that its future spending power will be lower than the figure shown.

This calculator provides estimates for general information only and is not financial or tax advice. See our disclaimer.