Compound Interest Calculator

About Compound Interest Calculator

A compound interest calculator with monthly contributions shows how your savings grow when interest earns interest. Enter a starting amount, annual rate, number of years, how often it compounds — daily, monthly, quarterly, or annually — and any regular contributions, and it projects your future balance along with how much you put in versus how much is interest. It updates instantly in your browser.

Simple versus compound, with numbers

Simple interest pays only on the original sum. Compound interest pays on the balance, so each period's interest joins the principal and earns interest itself. On 10,000 at 7% for 30 years, simple interest returns 21,000 in interest for a total of 31,000. Compounded annually, the same deposit reaches about 76,100.

The gap comes entirely from the curve. Growth is exponential rather than linear, so the early years look almost identical and the later ones diverge sharply. Roughly half the final balance in that example is generated in the last eight years — which is why time in the market matters more than the amount for anyone starting young.

Compounding frequency matters less than people expect

More frequent compounding does help, but with diminishing returns. 10,000 at 6% for a year gives 10,600 compounded annually, 10,613.64 monthly, and 10,618.31 daily. Going from annual to monthly is worth about 14 units; going from monthly to daily adds under 5. The mathematical ceiling is continuous compounding, which here would be 10,618.37.

This is why comparing accounts on headline rate alone can mislead slightly, and why the effective annual rate exists — it folds compounding frequency into one comparable number. If two accounts quote the same nominal rate and different frequencies, compare their effective rates, but do not expect the difference to be large.

Regular contributions do the heavy lifting

For most people the contribution schedule matters far more than the rate. Starting at zero and adding 200 a month at 7% for 30 years reaches roughly 244,000, of which 72,000 was deposited and the rest is growth. Adding the same 200 a month for only the last 20 years reaches about 104,000 — less than half, for two-thirds of the contributions.

The corollary is the rule of 72: divide 72 by the rate to estimate the doubling time. At 6% money doubles roughly every 12 years, at 9% every 8. It is an approximation, but it is accurate enough for mental arithmetic and it makes the cost of delay concrete — a decade of delay is a whole doubling forgone.

What the projection ignores

A compound interest projection assumes a constant rate, which no real investment provides. Markets deliver an average across volatile years, and the order of returns matters — poor years early hurt more than the same poor years late, particularly once you are drawing money out. Treat the output as a smooth model, not a forecast.

It also excludes inflation, tax, and fees, all of which compound against you. A 7% return with 3% inflation is about 4% in real purchasing power. An annual fee of 1% sounds small and consumes a substantial share of the final balance over decades, because you lose the compounding on every unit taken. Run the numbers again with a lower rate to see the realistic picture.

Frequently asked questions

How does compound interest work?

Compound interest is calculated on both your original amount and the interest already earned. Over time this snowballs, so the balance grows faster the longer you leave it invested.

Does compounding frequency matter?

Yes. More frequent compounding — daily or monthly versus annually — earns slightly more, because interest is added and starts earning sooner. This calculator lets you switch between daily, monthly, quarterly, and annual.

How are regular contributions handled?

Each contribution is added at the end of every compounding period and then earns interest for the remaining time — the standard future value of a series formula.

Is this financial advice?

No. It's a math tool for projections using the numbers you enter; real returns vary and aren't guaranteed.