$10,000 invested at 10% annual return compounded monthly grows to $198,374 in 30 years — $188,374 of that is compound interest. Adding $200 monthly contributions would grow the total to $650,472.

Key Numbers

• •Starting amount: $10,000
• •Value after 30 years at 10%: $198,374
• •Interest earned: $188,374
• •With $200/month added: $650,472
• •At 10%, money doubles roughly every 7.2 years (rule of 72)