$100,000 invested at 10% annual return compounded monthly grows to $1,983,740 in 30 years — $1,883,740 of that is compound interest. Adding $200 monthly contributions would grow the total to $2,435,838.

Key Numbers

• •Starting amount: $100,000
• •Value after 30 years at 10%: $1,983,740
• •Interest earned: $1,883,740
• •With $200/month added: $2,435,838
• •At 10%, money doubles roughly every 7.2 years (rule of 72)