Compound interest

What will my savings be worth in 10, 20 or 30 years?

Your scenario

Sample numbers. Change any field to use yours.

Investment parameters

Initial investment*

Monthly contribution

Annual interest rate (%)*

Investment period (years)*

Final amountSample

$284,577

After 30 years from $5,000 plus $200 a month at 7%, before taxes, fees, and inflation.

Total contributedYour initial investment plus monthly contributions: $77,000
Interest earnedInterest earned before taxes, fees, and inflation: $207,577
Growth multipleHow many times your money has multiplied: 3.696

What the ending balance is made of

Your contributions$77,00027%
Interest earned$207,57773%

Effective annual return: 7.23%
Interest per dollar contributed: 2.70
Years to doubleRule of 72, approximate: 10.3
Interest per month, averaged over the term: $577

Investment growth over time

Total contributed
Total value

What this means

Growth above contributions
Interest adds 270% on top of what you put in over 30 years. Most of it comes from earnings on earlier earnings.
Effect of time
Over 30 years, 73% of the ending balance is interest.
Monthly contributions
Your monthly contributions of $200 add $72,000 over 30 years, before interest.

Written by Elizabeth Stein

How this calculator works

Your starting balance grows at the rate you set, and each contribution starts growing from the moment it lands.
Growth compounds: the return earned in year three earns its own return in year four, and that is where the curve comes from.
The result splits into what you put in and what the growth added, because the difference between those two is the entire point.
A real return subtracts inflation. A 7% return with 3% inflation leaves about 4% of actual buying power.

Why starting earlier beats contributing more

Someone who invests $200 a month from 25 to 35 and then stops contributing entirely ends up, at a 7% return, with more at 65 than someone who contributes the same $200 a month from 35 all the way to 65. The first person put in $24,000. The second put in $72,000. The first starts ten years earlier and leaves the balance invested for thirty years after contributions stop. Time in the market moves the result more than contribution size does in this example.

What this assumes

The return is a constant annual rate. Real markets do not deliver a steady 7%; they deliver a lumpy average.
Contributions arrive on schedule and are never withdrawn.
Taxes and fund fees are not deducted. A 1% expense ratio is roughly a seventh of a 7% return, every year.
Nothing here predicts a market. A rate you type in is a scenario, not a forecast.

Finance Quest is educational. It is not financial advice, and no calculator here knows your full situation.

The mistake: treating the projected number as a plan

A calculator that says $1.2 million at 8% is describing one smooth path that will not happen. Markets deliver that average through crashes and recoveries, and the order those arrive in matters enormously if you are drawing the money down. Use the number to compare choices, not to plan a retirement date. Run it again at 5% and at 9% and treat the range as the honest answer.

Common questions

What is a realistic rate of return to use?

For a diversified stock portfolio, long-run US returns have averaged roughly 10% before inflation and around 7% after. Many people model 6 to 7% to stay conservative. For bonds or cash, use something much lower. Whatever you pick, run a pessimistic version too.

How does compounding frequency change the result?

Less than people expect. Moving from annual to monthly compounding at the same nominal rate adds a fraction of a percent. Time in the market and the contribution amount move the number far more than the compounding interval does.

What is the rule of 72?

Divide 72 by your annual return to get the rough number of years for money to double. At 8%, about nine years. It is a mental shortcut for sanity-checking a projection, not a precise formula.

Should I invest a lump sum or spread it out?

Historically a lump sum wins more often, because markets rise more often than they fall and money invested sooner compounds longer. Spreading it out reduces the regret if you happen to buy right before a drop. It is a question about your nerves as much as the arithmetic.

Read the full answer

How does compound interest work?Returns on returns, shown with $180 a month at 0%, 4% and 7%.

Related labs

Sources and review

Calculation version: 2026-1.

Source links checked: 2026-10-18. This check confirms the linked material is available and relevant. It does not verify every figure or calculation in this lab.

Sampled calculation review on 2026-10-18 by Codex (internal AI-assisted verification), for version 2026-1. This is not independent or professional approval.

Review scope: End-of-month deposits against a closed-form annuity example, zero return and fractional-year rejection. Returns are fixed nominal samples; no fees, tax or inflation are modeled.

Investor.gov compound interest calculator

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