Monthly contributions, returns compounded monthly. Everything runs in your browser — nothing is sent anywhere.
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A flat annual return applied every month, contributions at the end of each month, no taxes, no fees, no withdrawals. Real markets do not pay a flat rate: a 7% average can arrive as +18% one year and −12% the next, and the order of those years changes your ending balance. Sequence risk is the reason a portfolio you are withdrawing from needs more margin for error than one you are still adding to.
A 1% annual expense ratio is roughly a 1% annual haircut to your return, so subtract it from the rate above to see the honest figure. On a 7% gross return that is a 14% smaller ending balance after 20 years — one of the few effects in investing you can lock in by choosing a cheaper fund.
$10,000 start, $500 monthly, 7% annual, compounded monthly for 20 years: contributions total $130,000 and the ending balance is about $300,850. The other $170,850 is growth, and most of it lands in the last few years — which is why abandoning the plan early costs far more than the slow first decade suggests.