How is it calculated?
The calculation assumes a fixed monthly deposit and an average annual return, compounded monthly: each month the gain is computed on the principal plus all gains accumulated so far. That is the difference between simple and compound interest — and what produces the famous exponential curve.
The return you enter is an assumption, not a promise: markets are volatile, and long-term average returns differ from any single year's actual return. The calculation excludes capital-gains tax, management fees and inflation linkage — past returns do not guarantee future returns.