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How the balance grows
Your contributions (blue) plus compound growth (green), year by year.
Estimates only. Interest compounds monthly; real value discounts the future balance by your inflation rate. Not investment advice.
Compound interest is interest earned on your interest — the reason a modest, steady investment can end up dwarfing what you actually paid in. At a 7% return, a single $10,000 grows to about $20,097 in 10 years and $81,165 in 30 years untouched; add $500 a month and it reaches roughly $462,000 in 25 years, of which about $302,000 is pure growth. This tool shows that split, plus the effective yield, the time to double, and — the step most calculators skip — what the result is actually worth after inflation.
The short answer
How a lump sum grows on its own
Start with the simplest case: a single deposit left untouched. The table below compounds $10,000 at 7% monthly. Notice how the interest earned isn't linear — it accelerates, because each year's growth is calculated on a bigger balance. Over 30 years the interest alone is more than seven times the original deposit.
| Years | Balance | Interest earned | Growth multiple |
|---|---|---|---|
| 5 years | $14,176 | $4,176 | 1.4× |
| 10 years | $20,097 | $10,097 | 2.0× |
| 20 years | $40,387 | $30,387 | 4.0× |
| 30 years | $81,165 | $71,165 | 8.1× |
| 40 years | $163,114 | $153,114 | 16.3× |
$10,000 lump sum, 7% annual return compounded monthly, no contributions. The balance doubles roughly every 10 years — compare the shortcut with BeCoin's live compound-interest tool.
The real power: regular contributions
Adding money consistently is where compounding gets dramatic. The table shows $500 a month at 7% starting from zero. Over 40 years you contribute $240,000 — but end with over $1.3 million, because compounding does more than four-fifths of the work. Every dollar you add early compounds the longest, so starting sooner beats saving more later.
| Years | Ending balance | You contribute | From compounding |
|---|---|---|---|
| 10 years | $86,542 | $60,000 | $26,542 |
| 20 years | $260,463 | $120,000 | $140,463 |
| 30 years | $609,985 | $180,000 | $429,985 |
| 40 years | $1,312,407 | $240,000 | $1,072,407 |
$0 starting balance, $500/month, 7% compounded monthly. By year 40, about 82% of the balance is compound growth. For broader wealth context, see the evidence behind millionaire statistics.
Does compounding frequency matter?
It matters — but far less than the return or the time horizon. The table compounds $10,000 at 5% for 10 years at four frequencies. Going from annual to monthly is worth a real bump; going from monthly to daily adds only a few dollars. What frequency really changes is the effective annual yield (APY), the true rate after compounding is baked in.
| Compounding | Ending balance | Effective APY |
|---|---|---|
| Annually | $16,288.95 | 5.000% |
| Quarterly | $16,436.19 | 5.095% |
| Monthly | $16,470.09 | 5.116% |
| Daily | $16,486.65 | 5.127% |
$10,000 at a 5% nominal rate for 10 years. The difference between annual and daily compounding is about $198 — meaningful, but dwarfed by the effect of a higher return or a longer horizon.
What return should you assume?
The rate you type in decides everything, so it should reflect where the money actually sits. A savings account barely keeps up with inflation; a diversified stock index has historically done the heavy lifting. Use the preset buttons in the calculator to jump between these anchors, or enter your own.
| Where the money sits | Typical annual return | Source / note |
|---|---|---|
| Savings account | ~0.38% | FDIC national average, Jul 2026 |
| High-yield savings | ~4% | Online banks, mid-2026 (rate-sensitive) |
| Balanced portfolio | ~6–7% | Stock/bond mix, long-run planning figure |
| US stock index (nominal) | ~10% | S&P 500, 1928–2025, dividends reinvested (NYU Stern / Damodaran) |
| US stock index (real) | ~7% | After ~3% inflation |
Past performance does not guarantee future results. Higher-return assets like crypto can compound faster but swing far more and carry no guarantee — model scenarios rather than plugging in a single optimistic rate.
The catch nobody mentions: inflation
Compounding builds a bigger number, but only the return above inflation builds real wealth. With prices rising about 3.5% a year (June 2026), a savings account at 0.38% actually loses purchasing power, while a 10% nominal stock return is closer to 7% once inflation is stripped out. That is why this calculator also shows the inflation-adjusted real value — the honest measure of what your future balance will buy. To see how fast inflation erodes a fixed sum, use our inflation calculator.
Your compounding depends on what markets do next
A calculator assumes one steady rate. BeCoin's models turn that assumption into bull, base & bear scenarios across 100+ assets — all in one plan.
Explore BeCoin PremiumMethodology & data sources
All figures are computed client-side with monthly compounding; no data is stored. The engine uses the future-value identity FV = P·(1+i)N + PMT·(((1+i)N − 1) ÷ i), where i is the annual return ÷ 12 and N is the number of months. Total contributed is your starting balance plus every monthly deposit; interest earned is the final balance minus that. Effective APY is (1 + rate ÷ 12)12 − 1. Time to double solves ln(2) ÷ (12 · ln(1 + rate ÷ 12)). The real value discounts the final balance to today's dollars by (1 + inflation)years. Return anchors: FDIC national savings average 0.38% (Jul 2026); S&P 500 ~10% nominal / ~7% real since 1928 (NYU Stern / Damodaran). Current inflation 3.5% for the 12 months to June 2026 (BLS CPI-U). Projections assume a constant return and are estimates, not guarantees.
Sources: FDIC National Rates, NYU Stern (Damodaran) historical returns, BLS Consumer Price Index. For education only — not investment advice. See our disclaimer.